Instructor Solutions Manual For Physics Volume 1

Instructor%20Solutions%20Manual%20for%20Physics%20Volume%201

Instructor%20Solutions%20Manual%20for%20Physics%20Volume%201

Instructor%20Solutions%20Manual%20for%20Physics%20Volume%201

User Manual: Pdf

Open the PDF directly: View PDF PDF.
Page Count: 313 [warning: Documents this large are best viewed by clicking the View PDF Link!]

Instructor Solutions Manual
for
Physics
by
Halliday, Resnick, and Krane
Paul Stanley
Beloit College
Volume 1: Chapters 1-24
A Note To The Instructor...
The solutions here are somewhat brief, as they are designed for the instructor, not for the student.
Check with the publishers before electronically posting any part of these solutions; website, ftp, or
server access must be restricted to your students.
I have been somewhat casual about subscripts whenever it is obvious that a problem is one
dimensional, or that the choice of the coordinate system is irrelevant to the numerical solution.
Although this does not change the validity of the answer, it will sometimes obfuscate the approach
if viewed by a novice.
There are some traditional formula, such as
v2
x=v2
0x+ 2axx,
which are not used in the text. The worked solutions use only material from the text, so there may
be times when the solution here seems unnecessarily convoluted and drawn out. Yes, I know an
easier approach existed. But if it was not in the text, I did not use it here.
I also tried to avoid reinventing the wheel. There are some exercises and problems in the text
which build upon previous exercises and problems. Instead of rederiving expressions, I simply refer
you to the previous solution.
I adopt a different approach for rounding of significant figures than previous authors; in partic-
ular, I usually round intermediate answers. As such, some of my answers will differ from those in
the back of the book.
Exercises and Problems which are enclosed in a box also appear in the Student’s Solution Manual
with considerably more detail and, when appropriate, include discussion on any physical implications
of the answer. These student solutions carefully discuss the steps required for solving problems, point
out the relevant equation numbers, or even specify where in the text additional information can be
found. When two almost equivalent methods of solution exist, often both are presented. You are
encouraged to refer students to the Student’s Solution Manual for these exercises and problems.
However, the material from the Student’s Solution Manual must not be copied.
Paul Stanley
Beloit College
stanley@clunet.edu
1
E1-1 (a) Megaphones; (b) Microphones; (c) Decacards (Deck of Cards); (d) Gigalows (Gigolos);
(e) Terabulls (Terribles); (f) Decimates; (g) Centipedes; (h) Nanonanettes (?); (i) Picoboos (Peek-a-
Boo); (j) Attoboys (’atta boy); (k) Two Hectowithits (To Heck With It); (l) Two Kilomockingbirds
(To Kill A Mockingbird, or Tequila Mockingbird).
E1-2 (a) $36,000/52 week = $692/week. (b) $10,000,000/(20 ×12 month) = $41,700/month. (c)
30 ×109/8 = 3.75 ×109.
E1-3 Multiply out the factors which make up a century.
1 century = 100 years 365 days
1 year 24 hours
1 day 60 minutes
1 hour
This gives 5.256 ×107minutes in a century, so a microcentury is 52.56 minutes.
The percentage difference from Fermi’s approximation is (2.56 min)/(50 min) ×100% or 5.12%.
E1-4 (3000 mi)/(3 hr) = 1000 mi/timezone-hour. There are 24 time-zones, so the circumference
is approximately 24 ×1000 mi = 24,000 miles.
E1-5 Actual number of seconds in a year is
(365.25 days) 24 hr
1 day 60 min
1 hr 60 s
1 min= 3.1558 ×107s.
The percentage error of the approximation is then
3.1416 ×107s3.1558 ×107s
3.1558 ×107s=0.45 %.
E1-6 (a) 108seconds per shake means 108shakes per second. There are
365 days
1 year 24 hr
1 day 60 min
1 hr 60 s
1 min= 3.1536 ×107s/year.
This means there are more shakes in a second.
(b) Humans have existed for a fraction of
106years/1010 years = 104.
That fraction of a day is
104(24 hr) 60 min
1 hr 60 s
1 min= 8.64 s.
E1-7 We’ll assume, for convenience only, that the runner with the longer time ran exactly one
mile. Let the speed of the runner with the shorter time be given by v1, and call the distance actually
ran by this runner d1. Then v1=d1/t1. Similarly, v2=d2/t2for the other runner, and d2= 1 mile.
We want to know when v1> v2. Substitute our expressions for speed, and get d1/t1> d2/t2.
Rearrange, and d1/d2> t1/t2or d1/d2>0.99937. Then d1>0.99937 mile ×(5280 feet/1 mile) or
d1>5276.7 feet is the condition that the first runner was indeed faster. The first track can be no
more than 3.3 feet too short to guarantee that the first runner was faster.
2
E1-8 We will wait until a day’s worth of minutes have been gained. That would be
(24 hr) 60 min
1 hr = 1440 min.
The clock gains one minute per day, so we need to wait 1,440 days, or almost four years. Of course,
if it is an older clock with hands that only read 12 hours (instead of 24), then after only 720 days
the clock would be correct.
E1-9 First find the “logarithmic average” by
log tav =1
2log(5 ×1017) + log(6 ×1015),
=1
2log 5×1017 ×6×1015,
=1
2log 3000 = log 3000.
Solve, and tav = 54.8 seconds.
E1-10 After 20 centuries the day would have increased in length by a total of 20 ×0.001 s = 0.02 s.
The cumulative effect would by the product of the average increase and the number of days; that
average is half of the maximum, so the cumulative effect is 1
2(2000)(365)(0.02 s) = 7300 s. That’s
about 2 hours.
E1-11 Lunar months are based on the Earth’s position, and as the Earth moves around the orbit
the Moon has farther to go to complete a phase. In 27.3 days the Moon may have orbited through
360, but since the Earth moved through (27.3/365) ×360= 27the Moon needs to move 27
farther to catch up. That will take (27/360)×27.3 days = 2.05 days, but in that time the Earth
would have moved on yet farther, and the moon will need to catch up again. How much farther?
(2.05/365) ×360= 2.02which means (2.02/360)×27.3 days = 0.153 days. The total so far is
2.2 days longer; we could go farther, but at our accuracy level, it isn’t worth it.
E1-12 (1.9 m)(3.281 ft/1.000 m) = 6.2 ft, or just under 6 feet, 3 inches.
E1-13 (a) 100 meters = 328.1 feet (Appendix G), or 328.1/3 = 10.9 yards. This is 28 feet longer
than 100 yards, or (28 ft)(0.3048 m/ft) = 8.5 m. (b) A metric mile is (1500 m)(6.214×104mi/m) =
0.932 mi.I’d rather run the metric mile.
E1-14 There are
300,000 years 365.25 days
1 year 24 hr
1 day 60 min
1 hr 60 s
1 min= 9.5×1012s
that will elapse before the cesium clock is in error by 1 s. This is almost 1 part in 1013. This kind
of accuracy with respect to 2572 miles is
1013(2572 mi) 1609 m
1 mi = 413 nm.
3
E1-15 The volume of Antarctica is approximated by the area of the base time the height; the
area of the base is the area of a semicircle. Then
V=Ah =1
2πr2h.
The volume is
V=1
2(3.14)(2000 ×1000 m)2(3000 m) = 1.88 ×1016 m3
= 1.88 ×1016 m3×100 cm
1 m 3
= 1.88 ×1022 cm3.
E1-16 The volume is (77×104m2)(26 m) = 2.00×107m3. This is equivalent to
(2.00×107m3)(103km/m)3= 0.02 km3.
E1-17 (a) C= 2πr = 2π(6.37 ×103km) = 4.00 ×104km. (b) A= 4πr2= 4π(6.37 ×103km)2=
5.10 ×108km. (c) V=4
3π(6.37 ×103km)3= 1.08 ×1012 km3.
E1-18 The conversions: squirrel, 19 km/hr(1000 m/km)/(3600 s/hr) = 5.3 m/s;
rabbit, 30 knots(1.688ft/s/knot)(0.3048 m/ft) = 15 m/s;
snail, 0.030 mi/hr(1609 m/mi)/(3600 s/hr) = 0.013 m/s;
spider, 1.8 ft/s(0.3048 m/ft) = 0.55 m/s;
cheetah, 1.9 km/min(1000 m/km)/(60 s/min) = 32 m/s;
human, 1000 cm/s/(100 cm/m) = 10 m/s;
fox, 1100 m/min/(60 s/min) = 18 m/s;
lion, 1900 km/day(1000 m/km)/(86,400 s/day) = 22 m/s.
The order is snail, spider, squirrel, human, rabbit, fox, lion, cheetah.
E1-19 One light-year is the distance traveled by light in one year, or (3 ×108m/s) ×(1 year).
Then
19,200mi
hr light-year
(3 ×108m/s) ×(1 year) 1609 m
1 mi 1 hr
3600 s100 year
1 century ,
which is equal to 0.00286 light-year/century.
E1-20 Start with the British units inverted,
gal
30.0 mi 231 in3
gal 1.639 ×102L
in3mi
1.609 km= 7.84 ×102L/km.
E1-21 (b) A light-year is
(3.00 ×105km/s) 3600 s
1 hr 24 hr
1 day (365 days) = 9.46 ×1012 km.
A parsec is
1.50 ×108km
000100 360
2πrad=1.50 ×108km
(1/3600)360
2πrad= 3.09 ×1013 km.
(a) (1.5×108km)/(3.09 ×1013 km/pc = 4.85 ×106pc. (1.5×108km)/(9.46 ×1012 km/ly) =
1.59 ×105ly.
4
E1-22 First find the “logarithmic average” by
log dav =1
2log(2 ×1026) + log(1 ×1015),
=1
2log 2×1026 ×1×1015,
=1
2log 2 ×1011 = log p2×1011.
Solve, and dav = 450 km.
E1-23 The number of atoms is given by (1 kg)/(1.00783 ×1.661 ×1027 kg), or 5.974 ×1026
atoms.
E1-24 (a) (2 ×1.0 + 16)u(1.661 ×1027kg) = 3.0×1026kg.
(b) (1.4×1021kg)/(3.0×1026kg) = 4.7×1046 molecules.
E1-25 The coffee in Paris costs $18.00 per kilogram, or
$18.00 kg10.4536 kg
1 lb = $8.16 lb1.
It is cheaper to buy coffee in New York (at least according to the physics textbook, that is.)
E1-26 The room volume is (21 ×13 ×12)ft3(0.3048 m/ft)3= 92.8 m3. The mass contained in the
room is
(92.8 m3)(1.21 kg/m3) = 112 kg.
E1-27 One mole of sugar cubes would have a volume of NA×1.0 cm3, where NAis the Avogadro
constant. Since the volume of a cube is equal to the length cubed, V=l3, then l=3
NAcm
= 8.4×107cm.
E1-28 The number of seconds in a week is 60 ×60 ×24 ×7 = 6.05 ×105. The “weight” loss per
second is then
(0.23 kg)/(6.05 ×105s) = 3.80 ×101mg/s.
E1-29 The definition of the meter was wavelengths per meter; the question asks for meters per
wavelength, so we want to take the reciprocal. The definition is accurate to 9 figures, so the reciprocal
should be written as 1/1,650,763.73 = 6.05780211 ×107m = 605.780211 nm.
E1-30 (a) 37.76 + 0.132 = 37.89. (b) 16.264 16.26325 = 0.001.
E1-31 The easiest approach is to first solve Darcy’s Law for K, and then substitute the known
SI units for the other quantities. Then
K=V L
AHt has units of m3(m)
(m2) (m) (s)
which can be simplified to m/s.
5
E1-32 The Planck length, lP, is found from
[lP] = [ci][Gj][hk],
L = (LT1)i(L3T2M1)j(ML2T1)k,
= Li+3j+2kTi2jkMj+k.
Equate powers on each side,
L: 1 = i+ 3j+ 2k,
T: 0 = i2jk,
M: 0 = j+k.
Then j=k, and i=3k, and 1 = 2k; so k= 1/2, j= 1/2, and i=3/2. Then
[lP] = [c3/2][G1/2][h1/2],
= (3.00 ×108m/s)3/2(6.67 ×1011 m3/s2·kg)1/2(6.63 ×1034 kg ·m2/s)1/2,
= 4.05 ×1035 m.
E1-33 The Planck mass, mP, is found from
[mP] = [ci][Gj][hk],
M = (LT1)i(L3T2M1)j(ML2T1)k,
= Li+3j+2kTi2jkMj+k.
Equate powers on each side,
L: 0 = i+ 3j+ 2k,
T: 0 = i2jk,
M: 1 = j+k.
Then k=j+ 1, and i=3j1, and 0 = 1+2k; so k= 1/2, and j=1/2, and i= 1/2. Then
[mP] = [c1/2][G1/2][h1/2],
= (3.00 ×108m/s)1/2(6.67 ×1011 m3/s2·kg)1/2(6.63 ×1034 kg ·m2/s)1/2,
= 5.46 ×108kg.
P1-1 There are 24 ×60 = 1440 traditional minutes in a day. The conversion plan is then fairly
straightforward
822.8 dec. min 1440 trad. min
1000 dec. min = 1184.8 trad. min.
This is traditional minutes since midnight, the time in traditional hours can be found by dividing
by 60 min/hr, the integer part of the quotient is the hours, while the remainder is the minutes. So
the time is 19 hours, 45 minutes, which would be 7:45 pm.
P1-2 (a) By similar triangles, the ratio of the distances is the same as the ratio of the diameters—
390:1.
(b) Volume is proportional to the radius (diameter) cubed, or 3903= 5.93 ×107.
(c) 0.52(2π/360) = 9.1×103rad.The diameter is then (9.1×103rad)(3.82 ×105km) =
3500 km.
6
P1-3 (a) The circumference of the Earth is approximately 40,000 km; 0.5 seconds of an arc is
0.5/(60 ×60 ×360) = 3.9×107of a circumference, so the north-south error is ±(3.9×107)(4 ×
107m) = ±15.6 m. This is a range of 31 m.
(b) The east-west range is smaller, because the distance measured along a latitude is smaller than
the circumference by a factor of the cosine of the latitude. Then the range is 31 cos 43.6= 22 m.
(c) The tanker is in Lake Ontario, some 20 km off the coast of Hamlin?
P1-4 Your position is determined by the time it takes for your longitude to rotate ”underneath”
the sun (in fact, that’s the way longitude was measured originally as in 5 hours west of the Azores...)
the rate the sun sweep over at equator is 25,000 miles/86,400 s = 0.29 miles/second. The correction
factor because of latitude is the cosine of the latitude, so the sun sweeps overhead near England at
approximately 0.19 mi/s. Consequently a 30 mile accuracy requires an error in time of no more than
(30 mi)/(0.19 mi/s) = 158 seconds.
Trip takes about 6 months, so clock accuracy needs to be within (158 s)/(180 day) = 1.2 sec-
onds/day.
(b) Same, except 0.5 miles accuracy requires 2.6 s accuracy, so clock needs to be within 0.007
s/day!
P1-5 Let Bbe breaths/minute while sleeping. Each breath takes in (1.43 g/L)(0.3 L) = 0.429 g;
and lets out (1.96 g/L)(0.3 L) = 0.288 g. The net loss is 0.141 g. Multiply by the number of breaths,
(8 hr)(60 min./hr)B(0.141 g) = B(67.68 g). I’ll take a short nap, and count my breaths, then finish
the problem.
I’m back now, and I found my breaths to be 8/minute. So I lose 541 g/night, or about 1 pound.
P1-6 The mass of the water is (1000 kg/m3)(5700 m3) = 5.7×106kg. The rate that water leaks
drains out is (5.7×106kg)
(12 hr)(3600 s/hr) = 132 kg/s.
P1-7 Let the radius of the grain be given by rg. Then the surface area of the grain is Ag= 4πr2
g,
and the volume is given by Vg= (4/3)πr3
g.
If Ngrains of sand have a total surface area equal to that of a cube 1 m on a edge, then
NAg= 6 m2. The total volume Vtof this number of grains of sand is NVg. Eliminate Nfrom these
two expressions and get
Vt=NVg=(6 m2)
Ag
Vg=(6 m2)rg
3.
Then Vt= (2 m2)(50 ×106m) = 1 ×104m3.
The mass of a volume Vtis given by
1×104m32600 kg
1 m3= 0.26 kg.
P1-8 For a cylinder V=πr2h, and A= 2πr2+ 2πrh. We want to minimize Awith respect to
changes in r, so
dA
dr =d
dr 2πr2+ 2πr V
πr2,
= 4πr 2V
r2.
Set this equal to zero; then V= 2πr3. Notice that h= 2rin this expression.
7
P1-9 (a) The volume per particle is
(9.27 ×1026kg)/(7870 kg/m3) = 1.178 ×1028m3.
The radius of the corresponding sphere is
r=3
r3(1.178 ×1028m3)
4π= 1.41 ×1010m.
Double this, and the spacing is 282 pm.
(b) The volume per particle is
(3.82 ×1026kg)/(1013 kg/m3) = 3.77 ×1029m3.
The radius of the corresponding sphere is
r=3
r3(3.77 ×1029m3)
4π= 2.08 ×1010m.
Double this, and the spacing is 416 pm.
P1-10 (a) The area of the plate is (8.43 cm)(5.12 cm) = 43.2 cm2. (b) (3.14)(3.7 cm)2= 43 cm2.
8
E2-1 Add the vectors as is shown in Fig. 2-4. If ~
ahas length a= 4 m and ~
bhas length b= 3 m
then the sum is given by ~
s. The cosine law can be used to find the magnitude sof ~
s,
s2=a2+b22ab cos θ,
where θis the angle between sides aand bin the figure.
(a) (7 m)2= (4 m)2+ (3 m)22(4 m)(3 m) cos θ, so cos θ=1.0, and θ= 180. This means
that ~
aand ~
bare pointing in the same direction.
(b) (1 m)2= (4 m)2+ (3 m)22(4 m)(3 m) cos θ, so cos θ= 1.0, and θ= 0. This means that
~
aand ~
bare pointing in the opposite direction.
(c) (5 m)2= (4 m)2+ (3 m)22(4 m)(3 m) cos θ, so cos θ= 0, and θ= 90. This means that ~
a
and ~
bare pointing at right angles to each other.
E2-2 (a) Consider the figures below.
(b) Net displacement is 2.4 km west, (5.23.1 = 2.1) km south. A bird would fly
p2.42+ 2.12km = 3.2 km.
E2-3 Consider the figure below.
a
ba+b
a
-b
a-b
E2-4 (a) The components are (7.34) cos(252) = 2.27ˆ
iand (7.34) sin(252) = 6.98ˆ
j.
(b) The magnitude is p(25)2+ (43)2= 50; the direction is θ= tan1(43/25) = 120. We
did need to choose the correct quadrant.
E2-5 The components are given by the trigonometry relations
O=Hsin θ= (3.42 km) sin 35.0= 1.96 km
and
A=Hcos θ= (3.42 km) cos 35.0= 2.80 km.
9
The stated angle is measured from the east-west axis, counter clockwise from east. So Ois measured
against the north-south axis, with north being positive; Ais measured against east-west with east
being positive.
Since her individual steps are displacement vectors which are only north-south or east-west, she
must eventually take enough north-south steps to equal 1.96 km, and enough east-west steps to
equal 2.80 km. Any individual step can only be along one or the other direction, so the minimum
total will be 4.76 km.
E2-6 Let ~
rf= 124ˆ
ikm and ~
ri= (72.6ˆ
i+ 31.4ˆ
j) km. Then the ship needs to travel
~
r=~
rf~
ri= (51.4ˆ
i+ 31.4ˆ
j) km.
Ship needs to travel 51.42+ 31.42km = 60.2 km in a direction θ= tan1(31.4/51.4) = 31.4west
of north.
E2-7 (a) In unit vector notation we need only add the components; ~
a+~
b= (5ˆ
i+3ˆ
j)+(3ˆ
i+2ˆ
j) =
(5 3)ˆ
i+ (3 + 2)ˆ
j= 2ˆ
i+ 5ˆ
j.
(b) If we define ~
c=~
a+~
band write the magnitude of ~
cas c, then c=qc2
x+c2
y=22+ 52=
5.39. The direction is given by tan θ=cy/cxwhich gives an angle of 68.2, measured counterclock-
wise from the positive x-axis.
E2-8 (a) ~
a+~
b= (4 1)ˆ
i+ (3 + 1)ˆ
j+ (1 + 4)ˆ
k= 3ˆ
i2ˆ
j+ 5ˆ
k.
(b) ~
a~
b= (4 − −1)ˆ
i+ (31)ˆ
j+ (1 4)ˆ
k= 5ˆ
i4ˆ
j3ˆ
k.
(c) Rearrange, and ~
c=~
b~
a, or ~
b~
a= (14)ˆ
i+ (1 − −3)ˆ
j+ (4 1)ˆ
k=5ˆ
i+ 4ˆ
j+ 3ˆ
k.
E2-9 (a) The magnitude of ~
ais p4.02+ (3.0)2= 5.0; the direction is θ= tan1(3.0/4.0) =
323.
(b) The magnitude of ~
bis 6.02+ 8.03= 10.0; the direction is θ= tan1(6.0/8.0) = 36.9.
(c) The resultant vector is ~
a+~
b= (4.0 + 6.0)ˆ
i+ (3.0 + 8.0)ˆ
j. The magnitude of ~
a+~
bis
p(10.0)2+ (5.0)2= 11.2; the direction is θ= tan1(5.0/10.0) = 26.6.
(d) The resultant vector is ~
a~
b= (4.06.0)ˆ
i+ (3.08.0)ˆ
j. The magnitude of ~
a~
bis
p(2.0)2+ (11.0)2= 11.2; the direction is θ= tan1(11.0/2.0) = 260.
(e) The resultant vector is ~
b~
a= (6.04.0)ˆ
i+ (8.0− −3.0)ˆ
j. The magnitude of ~
b~
ais
p(2.0)2+ (11.0)2= 11.2; the direction is θ= tan1(11.0/2.0) = 79.7.
E2-10 (a) Find components of ~
a;ax= (12.7) cos(28.2) = 11.2, ay= (12.7) sin(28.2)=6.00.
Find components of ~
b;bx= (12.7) cos(133) = 8.66, by= (12.7) sin(133) = 9.29. Then
~
r=~
a+~
b= (11.28.66)ˆ
i+ (6.00 + 9.29)ˆ
j= 2.54ˆ
i+ 15.29ˆ
j.
(b) The magnitude of ~
ris 2.542+ 15.292= 15.5.
(c) The angle is θ= tan1(15.29/2.54) = 80.6.
E2-11 Consider the figure below.
10
E2-12 Consider the figure below.
E2-13 Our axes will be chosen so that ˆ
ipoints toward 3 O’clock and ˆ
jpoints toward 12 O’clock.
(a)
The two relevant positions are ~
ri= (11.3 cm)ˆ
iand ~
rf= (11.3 cm)ˆ
j. Then
~
r=~
rf~
ri
= (11.3 cm)ˆ
j(11.3 cm)ˆ
i.
(b)
The two relevant positions are now ~
ri= (11.3 cm)ˆ
jand ~
rf= (11.3 cm)ˆ
j. Then
~
r=~
rf~
ri
= (11.3 cm)ˆ
j(11.3 cm)ˆ
j
= (22.6 cm)ˆ
j.
(c)
The two relevant positions are now ~
ri= (11.3 cm)ˆ
jand ~
rf= (11.3 cm)ˆ
j. Then
~
r=~
rf~
ri
= (11.3 cm)ˆ
j(11.3 cm)ˆ
j
= (0 cm)ˆ
j.
E2-14 (a) The components of ~
r1are
r1x= (4.13 m) cos(225) = 2.92 m
and
r1y= (4.13 m) sin(225) = 2.92 m.
11
The components of ~
r2are
r1x= (5.26 m) cos(0) = 5.26 m
and
r1y= (5.26 m) sin(0) = 0 m.
The components of ~
r3are
r1x= (5.94 m) cos(64.0) = 2.60 m
and
r1y= (5.94 m) sin(64.0) = 5.34 m.
(b) The resulting displacement is
h(2.92 + 5.26 + 2.60)ˆ
i+ (2.92+0+5.34)ˆ
jim = (4.94ˆ
i+ 2.42ˆ
j) m.
(c) The magnitude of the resulting displacement is 4.942+ 2.422m=5.5 m. The direction of
the resulting displacement is θ= tan1(2.42/4.94) = 26.1. (d) To bring the particle back to the
starting point we need only reverse the answer to (c); the magnitude will be the same, but the angle
will be 206.
E2-15 The components of the initial position are
r1x= (12,000 ft) cos(40) = 9200 ft
and
r1y= (12,000 ft) sin(40) = 7700 ft.
The components of the final position are
r2x= (25,8000 ft) cos(163) = 24,700 ft
and
r2y= (25,800 ft) sin(163) = 7540 ft.
The displacement is
~
r=~
r2~
r1=h(24,700 9,200)ˆ
i+ (7,540 9,200)ˆ
j)i= (33900ˆ
i1660ˆ
j) ft.
E2-16 (a) The displacement vector is ~
r= (410ˆ
i820ˆ
j) mi, where positive xis east and positive
yis north. The magnitude of the displacement is p(410)2+ (820)2mi = 920 mi. The direction is
θ= tan1(820/410) = 300.
(b) The average velocity is the displacement divided by the total time, 2.25 hours. Then
~
vav = (180ˆ
i360ˆ
j) mi/hr.
(c) The average speed is total distance over total time, or (410 + 820)/(2.25) mi/hr = 550 mi/hr.
12
E2-17 (a) Evaluate ~
rwhen t= 2 s.
~
r= [(2 m/s3)t3(5 m/s)t]ˆ
i+ [(6 m) (7 m/s4)t4]ˆ
j
= [(2 m/s3)(2 s)3(5 m/s)(2 s)]ˆ
i+ [(6 m) (7 m/s4)(2 s)4]ˆ
j
= [(16 m) (10 m)]ˆ
i+ [(6 m) (112 m)]ˆ
j
= [(6 m)]ˆ
i+ [(106 m)]ˆ
j.
(b) Evaluate:
~
v=d~
r
dt = [(2 m/s3)3t2(5 m/s)]ˆ
i+ [(7 m/s4)4t3]ˆ
j
= [(6 m/s3)t2(5 m/s)]ˆ
i+ [(28 m/s4)t3]ˆ
j.
Into this last expression we now evaluate ~
v(t= 2 s) and get
~
v= [(6 m/s3)(2 s)2(5 m/s)]ˆ
i+ [(28 m/s4)(2 s)3]ˆ
j
= [(24 m/s) (5 m/s)]ˆ
i+ [(224 m/s)]ˆ
j
= [(19 m/s)]ˆ
i+ [(224 m/s)]ˆ
j,
for the velocity ~
vwhen t= 2 s.
(c) Evaluate
~
a=d~
v
dt = [(6 m/s3)2t]ˆ
i+ [(28 m/s4)3t2]ˆ
j
= [(12 m/s3)t]ˆ
i+ [(84 m/s4)t2]ˆ
j.
Into this last expression we now evaluate ~
a(t= 2 s) and get
~
a= [(12 m/s3)(2 s)]ˆ
i+ [(84 m/s4)(2 2)2]ˆ
j
= [(24 m/s2)]ˆ
i+ [(336 m/s2)]ˆ
j.
E2-18 (a) Let ui point north, ˆ
jpoint east, and ˆ
kpoint up. The displacement is (8.7ˆ
i+ 9.7ˆ
j+
2.9ˆ
k) km. The average velocity is found by dividing each term by 3.4 hr; then
~
vav = (2.6ˆ
i+ 2.9ˆ
j+ 0.85) km/hr.
The magnitude of the average velocity is 2.62+ 2.92+ 0.852km/hr = 4.0 km/hr.
(b) The horizontal velocity has a magnitude of 2.62+ 2.92km/hr = 3.9 km/hr. The angle with
the horizontal is given by θ= tan1(0.85/3.9) = 13.
E2-19 (a) The derivative of the velocity is
~
a= [(6.0 m/s2)(8.0 m/s3)t]ˆ
i
so the acceleration at t= 3 s is ~
a= (18.0 m/s2)ˆ
i. (b) The acceleration is zero when (6.0 m/s2)
(8.0 m/s3)t= 0, or t= 0.75 s. (c) The velocity is never zero; there is no way to “cancel” out the
ycomponent. (d) The speed equals 10 m/s when 10 = pv2
x+ 82, or vx=±6.0 m/s. This happens
when (6.0 m/s2)(8.0 m/s3)t=±6.0 m/s, or when t= 0 s.
13
E2-20 If vis constant then so is v2=v2
x+v2
y. Take the derivative;
2vx
d
dtvx+ 2vy
d
dtvy= 2(vxax+vyay).
But if the value is constant the derivative is zero.
E2-21 Let the actual flight time, as measured by the passengers, be T. There is some time
difference between the two cities, call it ∆T= Namulevu time - Los Angeles time. The ∆Twill be
positive if Namulevu is east of Los Angeles. The time in Los Angeles can then be found from the
time in Namulevu by subtracting ∆T.
The actual time of flight from Los Angeles to Namulevu is then the difference between when the
plane lands (LA times) and when the plane takes off (LA time):
T= (18:50 T)(12:50)
= 6:00 T,
where we have written times in 24 hour format to avoid the AM/PM issue. The return flight time
can be found from
T= (18:50) (1:50 T)
= 17:00 + ∆T,
where we have again changed to LA time for the purpose of the calculation.
(b) Now we just need to solve the two equations and two unknowns.
17:00 + ∆T= 6:00 T
2∆T= 6:00 17:00
T=5:30.
Since this is a negative number, Namulevu is located west of Los Angeles.
(a) T= 6:00 T= 11 : 30, or eleven and a half hours.
(c) The distance traveled by the plane is given by d=vt = (520 mi/hr)(11.5 hr) = 5980 mi.
We’ll draw a circle around Los Angeles with a radius of 5980 mi, and then we look for where it
intersects with longitudes that would belong to a time zone ∆Taway from Los Angeles. Since the
Earth rotates once every 24 hours and there are 360 longitude degrees, then each hour corresponds
to 15 longitude degrees, and then Namulevu must be located approximately 15×5.5 = 83west of
Los Angeles, or at about longitude 160 east. The location on the globe is then latitude 5, in the
vicinity of Vanuatu.
When this exercise was originally typeset the times for the outbound and the inbound flights were
inadvertently switched. I suppose that we could blame this on the airlines; nonetheless, when the answers
were prepared for the back of the book the reversed numbers put Namulevu east of Los Angeles. That would
put it in either the North Atlantic or Brazil.
E2-22 There is a three hour time zone difference. So the flight is seven hours long, but it takes
3 hr 51 min for the sun to travel same distance. Look for when the sunset distance has caught up
with plane:
dsunset =dplane,
vsunset(t1:35) = vplanet,
(t1:35)/3:51 = t/7:00,
so t= 3:31 into flight.
14
E2-23 The distance is
d=vt = (112 km/hr)(1 s)/(3600 s/hr) = 31 m.
E2-24 The time taken for the ball to reach the plate is
t=d
v=(18.4 m)
(160 km/hr)(3600 s/hr)/(1000 m/km) = 0.414 s.
E2-25 Speed is distance traveled divided by time taken; this is equivalent to the inverse of the
slope of the line in Fig. 2-32. The line appears to pass through the origin and through the point
(1600 km, 80 ×106y), so the speed is v= 1600 km/80 ×106y= 2 ×105km/y. Converting,
v= 2 ×105km/y 1000 m
1 km 100 cm
1 m = 2 cm/y
E2-26 (a) For Maurice Greene vav = (100 m)/(9.81 m) = 10.2 m/s. For Khalid Khannouchi,
vav =(26.219 mi)
(2.0950 hr) 1609 m
1 mi 1hr
3600 s= 5.594 m/s.
(b) If Maurice Greene ran the marathon with an average speed equal to his average sprint speed
then it would take him
t=(26.219 mi)
10.2 m/s1609 m
1 mi 1hr
3600 s= 1.149 hr,
or 1 hour, 9 minutes.
E2-27 The time saved is the difference,
t=(700 km)
(88.5 km/hr) (700 km)
(104.6 km/hr) = 1.22 hr,
which is about 1 hour 13 minutes.
E2-28 The ground elevation will increase by 35 m in a horizontal distance of
x= (35.0 m)/tan(4.3) = 465 m.
The plane will cover that distance in
t=(0.465 km)
(1300 km/hr) 3600 s
1hr = 1.3 s.
E2-29 Let v1= 40 km/hr be the speed up the hill, t1be the time taken, and d1be the distance
traveled in that time. We similarly define v2= 60 km/hr for the down hill trip, as well as t2and d2.
Note that d2=d1.
15
v1=d1/t1and v2=d2/t2.vav =d/t, where dtotal distance and tis the total time. The total
distance is d1+d2= 2d1. The total time tis just the sum of t1and t2, so
vav =d
t
=2d1
t1+t2
=2d1
d1/v1+d2/v2
=2
1/v1+ 1/v2
,
Take the reciprocal of both sides to get a simpler looking expression
2
vav
=1
v1
+1
v2
.
Then the average speed is 48 km/hr.
E2-30 (a) Average speed is total distance divided by total time. Then
vav =(240 ft) + (240 ft)
(240 ft)/(4.0 ft/s) + (240 ft)/(10 ft/s) = 5.7 ft/s.
(b) Same approach, but different information given, so
vav =(60 s)(4.0 ft/s) + (60 s)(10 ft/s)
(60 s) + (60 s) = 7.0 ft/s.
E2-31 The distance traveled is the total area under the curve. The “curve” has four regions: (I)
a triangle from 0 to 2 s; (II) a rectangle from 2 to 10 s; (III) a trapezoid from 10 to 12 s; and (IV)
a rectangle from 12 to 16 s.
The area underneath the curve is the sum of the areas of the four regions.
d=1
2(2 s)(8 m/s) + (8.0 s)(8 m/s) + 1
2(2 s)(8 m/s + 4 m/s) + (4.0 s)(4 m/s) = 100 m.
E2-32 The acceleration is the slope of a velocity-time curve,
a=(8 m/s) (4 m/s)
(10 s) (12 s) =2 m/s2.
E2-33 The initial velocity is ~
vi= (18 m/s)ˆ
i, the final velocity is ~
vf= (30 m/s)ˆ
i. The average
acceleration is then
~
aav =~
v
t=~
vf~
vi
t=(30 m/s)ˆ
i(18 m/s)ˆ
i
2.4 s ,
which gives ~
aav = (20.0 m/s2)ˆ
i.
16
E2-34 Consider the figure below.
12345
0
5
10
-5
-10
E2-35 (a) Up to A vx>0 and is constant. From Ato B vxis decreasing, but still positive. From
Bto C vx= 0. From Cto D vx<0, but |vx|is decreasing.
(b) No. Constant acceleration would appear as (part of) a parabola; but it would be challenging
to distinguish between a parabola and an almost parabola.
E2-36 (a) Up to A vx>0 and is decreasing. From Ato B vx= 0. From Bto C vx>0 and is
increasing. From Cto D vx>0 and is constant.
(b) No. Constant acceleration would appear as (part of) a parabola; but it would be challenging
to distinguish between a parabola and an almost parabola.
E2-37 Consider the figure below.
v
v
a
x
E2-38 Consider the figure below.
17
−4
0
4
8
12
16
0123456t(s)
−4
0
4
8
12
16
0123456t(s)
x(cm)
v(cm/s)
The acceleration
is a constant
2 cm/s/s during
the entire time
interval.
E2-39 (a) Amust have units of m/s2.Bmust have units of m/s3.
(b) The maximum positive xposition occurs when vx= 0, so
vx=dx
dt = 2At 3Bt2
implies vx= 0 when either t= 0 or t= 2A/3B= 2(3.0 m/s2)/3(1.0 m/s3) = 2.0 s.
(c) Particle starts from rest, then travels in positive direction until t= 2 s, a distance of
x= (3.0 m/s2)(2.0 s)2(1.0 m/s3)(2.0 s)3= 4.0 m.
Then the particle moves back to a final position of
x= (3.0 m/s2)(4.0 s)2(1.0 m/s3)(4.0 s)3=16.0 m.
The total path followed was 4.0 m + 4.0 m + 16.0 m = 24.0 m.
(d) The displacement is 16.0 m as was found in part (c).
(e) The velocity is vx= (6.0 m/s2)t(3.0 m/s3)t2. When t= 0, vx= 0.0 m/s. When t= 1.0 s,
vx= 3.0 m/s. When t= 2.0 s, vx= 0.0 m/s. When t= 3.0 s, vx=9.0 m/s. When t= 4.0 s,
vx=24.0 m/s.
(f) The acceleration is the time derivative of the velocity,
ax=dvx
dt = (6.0 m/s2)(6.0 m/s3)t.
When t= 0 s, ax= 6.0 m/s2. When t= 1.0 s, ax= 0.0 m/s2. When t= 2.0 s, ax=6.0 m/s2.
When t= 3.0 s, ax=12.0 m/s2. When t= 4.0 s, ax=18.0 m/s2.
(g) The distance traveled was found in part (a) to be 20 m The average speed during the time
interval is then vx,av = (20 m)/(2.0 s) = 10 m/s.
E2-40 v0x= 0, vx= 360 km/hr = 100 m/s. Assuming constant acceleration the average velocity
will be
vx,av =1
2(100 m/s + 0) = 50 m/s.
The time to travel the distance of the runway at this average velocity is
t= (1800 m)/(50 m/s) = 36 s.
The acceleration is
ax= 2x/t2= 2(1800 m)/(36.0 s)2= 2.78 m/s2.
18
E2-41 (a) Apply Eq. 2-26,
vx=v0x+axt,
(3.0×107m/s) = (0) + (9.8 m/s2)t,
3.1×106s = t.
(b) Apply Eq. 2-28 using an initial position of x0= 0,
x=x0+v0x+1
2axt2,
x= (0) + (0) + 1
2(9.8 m/s2)(3.1×106s)2,
x= 4.7×1013 m.
E2-42 v0x= 0 and vx= 27.8 m/s. Then
t= (vxv0x)/a = ((27.8 m/s) (0)) /(50 m/s2) = 0.56 s.
I want that car.
E2-43 The muon will travel for tseconds before it comes to a rest, where tis given by
t= (vxv0x)/a =(0) (5.20 ×106m/s)/(1.30 ×1014m/s2) = 4 ×108s.
The distance traveled will be
x=1
2axt2+v0xt=1
2(1.30 ×1014m/s2)(4 ×108s)2+ (5.20 ×106m/s)(4 ×108s) = 0.104 m.
E2-44 The average velocity of the electron was
vx,av =1
2(1.5×105m/s + 5.8×106m/s) = 3.0×106m/s.
The time to travel the distance of the runway at this average velocity is
t= (0.012 m)/(3.0×106m/s) = 4.0×109s.
The acceleration is
ax= (vxv0x)/t = ((5.8×106m/s) (1.5×105m/s))/(4.0×109s) = 1.4×1015m/s2.
E2-45 It will be easier to solve the problem if we change the units for the initial velocity,
v0x= 1020km
hr 1000 m
km hr
3600 s= 283 m
s,
and then applying Eq. 2-26,
vx=v0x+axt,
(0) = (283 m/s) + ax(1.4 s),
202 m/s2=ax.
The problem asks for this in terms of g, so
202 m/s2 g
9.8 m/s2!= 21g.
19
E2-46 Change miles to feet and hours to seconds. Then vx= 81 ft/s and v0x= 125 ft/s. The
time is then
t= ((81 ft/s) (125 ft/s)) /(17 ft/s2) = 2.6 s.
E2-47 (a) The time to stop is
t= ((0 m/s) (24.6 m/s)) /(4.92 m/s2) = 5.00 s.
(b) The distance traveled is
x=1
2axt2+v0xt=1
2(4.92 m/s2)(5.00 s)2+ (24.6 m/s)(5.00 s) = 62 m.
E2-48 Answer part (b) first. The average velocity of the arrow while decelerating is
vy,av =1
2((0) + (260 ft/s)) = 130 ft/s.
The time for the arrow to travel 9 inches (0.75 feet) is
t= (0.75 ft)/(130 ft/s) = 5.8×103s.
(a) The acceleration of the arrow is then
ay= (vyv0y)/t = ((0) (260 ft/s))/(5.8×103s) = 4.5×104ft/s2.
E2-49 The problem will be somewhat easier if the units are consistent, so we’ll write the maxi-
mum speed as
1000 ft
min min
60 s= 16.7ft
s.
(a) We can find the time required for the acceleration from Eq. 2-26,
vx=v0x+axt,
(16.7 ft/s) = (0) + (4.00 ft/s2)t,
4.18 s = t.
And from this and Eq 2-28 we can find the distance
x=x0+v0x+1
2axt2,
x= (0) + (0) + 1
2(4.00 ft/s2)(4.18 s)2,
x= 34.9 ft.
(b) The motion of the elevator is divided into three parts: acceleration from rest, constant speed
motion, and deceleration to a stop. The total distance is given at 624 ft and in part (a) we found
the distance covered during acceleration was 34.9 ft. By symmetry, the distance traveled during
deceleration should also be 34.9 ft. The distance traveled at constant speed is then (624 34.9
34.9) ft = 554 ft. The time required for the constant speed portion of the trip is found from Eq.
2-22, rewritten as
t=x
v=554 ft
16.7 ft/s = 33.2 s.
The total time for the trip is the sum of times for the three parts: accelerating (4.18 s), constant
speed (33.2 s), and decelerating (4.18 s). The total is 41.6 seconds.
20
E2-50 (a) The deceleration is found from
ax=2
t2(xv0t) = 2
(4.0 s)2((34 m) (16 m/s)(4.0 s)) = 3.75 m/s2.
(b) The impact speed is
vx=v0x+axt= (16 m/s) + (3.75 m/s2)(4.0 s) = 1.0 m/s.
E2-51 Assuming the drops fall from rest, the time to fall is
t=s2y
ay
=s2(1700 m)
(9.8 m/s2)= 19 s.
The velocity of the falling drops would be
vy=ayt= (9.8 m/s2)(19 s) = 190 m/s,
or about 2/3 the speed of sound.
E2-52 Solve the problem out of order.
(b) The time to fall is
t=s2y
ay
=s2(120 m)
(9.8 m/s2)= 4.9 s.
(a) The speed at which the elevator hits the ground is
vy=ayt= (9.8 m/s2)(4.9 s) = 48 m/s.
(d) The time to fall half-way is
t=s2y
ay
=s2(60 m)
(9.8 m/s2)= 3.5 s.
(c) The speed at the half-way point is
vy=ayt= (9.8 m/s2)(3.5 s) = 34 m/s.
E2-53 The initial velocity of the “dropped” wrench would be zero. I choose vertical to be along
the yaxis with up as positive, which is the convention of Eq. 2-29 and Eq. 2-30. It turns out that
it is much easier to solve part (b) before solving part (a).
(b) We solve Eq. 2-29 for the time of the fall.
vy=v0ygt,
(24.0 m/s) = (0) (9.8 m/s2)t,
2.45 s = t.
(a) Now we can use Eq. 2-30 to find the height from which the wrench fell.
y=y0+v0yt1
2gt2,
(0) = y0+ (0)(2.45 s) 1
2(9.8 m/s2)(2.45 s)2,
0 = y029.4 m
We have set y= 0 to correspond to the final position of the wrench: on the ground. This results in
an initial position of y0= 29.4 m; it is positive because the wrench was dropped from a point above
where it landed.
21
E2-54 (a) It is easier to solve the problem from the point of view of an object which falls from the
highest point. The time to fall from the highest point is
t=s2y
ay
=s2(53.7 m)
(9.81 m/s2)= 3.31 s.
The speed at which the object hits the ground is
vy=ayt= (9.81 m/s2)(3.31 s) = 32.5 m/s.
But the motion is symmetric, so the object must have been launched up with a velocity of vy=
32.5 m/s.
(b) Double the previous answer; the time of flight is 6.62 s.
E2-55 (a) The time to fall the first 50 meters is
t=s2y
ay
=s2(50 m)
(9.8 m/s2)= 3.2 s.
(b) The total time to fall 100 meters is
t=s2y
ay
=s2(100 m)
(9.8 m/s2)= 4.5 s.
The time to fall through the second 50 meters is the difference, 1.3 s.
E2-56 The rock returns to the ground with an equal, but opposite, velocity. The acceleration is
then
ay= ((14.6 m/s) (14.6 m/s))/(7.72 s) = 3.78 m/s2.
That would put them on Mercury.
E2-57 (a) Solve Eq. 2-30 for the initial velocity. Let the distances be measured from the ground
so that y0= 0.
y=y0+v0yt1
2gt2,
(36.8 m) = (0) + v0y(2.25 s) 1
2(9.8 m/s2)(2.25 s)2,
36.8 m = v0y(2.25 s) 24.8 m,
27.4 m/s = v0y.
(b) Solve Eq. 2-29 for the velocity, using the result from part (a).
vy=v0ygt,
vy= (27.4 m/s) (9.8 m/s2)(2.25 s),
vy= 5.4 m/s.
(c) We need to solve Eq. 2-30 to find the height to which the ball rises, but we don’t know how
long it takes to get there. So we first solve Eq. 2-29, because we do know the velocity at the highest
point (vy= 0).
vy=v0ygt,
(0) = (27.4 m/s) (9.8 m/s2)t,
2.8 s = t.
22
And then we find the height to which the object rises,
y=y0+v0yt1
2gt2,
y= (0) + (27.4 m/s)(2.8 s) 1
2(9.8 m/s2)(2.8 s)2,
y= 38.3m.
This is the height as measured from the ground; so the ball rises 38.336.8 = 1.5 m above the point
specified in the problem.
E2-58 The time it takes for the ball to fall 2.2 m is
t=s2y
ay
=s2(2.2 m)
(9.8 m/s2)= 0.67 s.
The ball hits the ground with a velocity of
vy=ayt= (9.8 m/s2)(0.67 s) = 6.6 m/s.
The ball then bounces up to a height of 1.9 m. It is easier to solve the falling part of the motion,
and then apply symmetry. The time is would take to fall is
t=s2y
ay
=s2(1.9 m)
(9.8 m/s2)= 0.62 s.
The ball hits the ground with a velocity of
vy=ayt= (9.8 m/s2)(0.62 s) = 6.1 m/s.
But we are interested in when the ball moves up, so vy= 6.1 m/s.
The acceleration while in contact with the ground is
ay= ((6.1 m/s) (6.6 m/s))/(0.096 s) = 130 m/s2.
E2-59 The position as a function of time for the first object is
y1=1
2gt2,
The position as a function of time for the second object is
y2=1
2g(t1 s)2
The difference,
y=y2y1=frac12g((2 s)t1) ,
is the set equal to 10 m, so t= 1.52 s.
E2-60 Answer part (b) first.
(b) Use the quadratic equation to solve
(81.3 m) = 1
2(9.81 m/s2)t2+ (12.4 m/s)t
for time. Get t=3.0 s and t= 5.53 s. Keep the positive answer.
(a) Now find final velocity from
vy= (9.8 m/s2)(5.53 s) + (12.4 m/s) = 41.8 m/s.
23
E2-61 The total time the pot is visible is 0.54 s; the pot is visible for 0.27 s on the way down.
We’ll define the initial position as the highest point and make our measurements from there. Then
y0= 0 and v0y= 0. Define t1to be the time at which the falling pot passes the top of the window
y1, then t2=t1+ 0.27 s is the time the pot passes the bottom of the window y2=y11.1 m. We
have two equations we can write, both based on Eq. 2-30,
y1=y0+v0yt11
2gt2
1,
y1= (0) + (0)t11
2gt2
1,
and
y2=y0+v0yt21
2gt2
2,
y11.1 m = (0) + (0)t21
2g(t1+ 0.27 s)2,
Isolate y1in this last equation and then set the two expressions equal to each other so that we can
solve for t1,
1
2gt2
1= 1.1 m 1
2g(t1+ 0.27 s)2,
1
2gt2
1= 1.1 m 1
2g(t2
1+ [0.54 s]t1+ 0.073 s2),
0=1.1 m 1
2g([0.54 s]t1+ 0.073 s2).
This last line can be directly solved to yield t1= 0.28 s as the time when the falling pot passes the
top of the window. Use this value in the first equation above and we can find y1=1
2(9.8 m/s2)(0.28
s)2=0.38 m. The negative sign is because the top of the window is beneath the highest point, so
the pot must have risen to 0.38 m above the top of the window.
P2-1 (a) The net shift is p(22 m)2+ (17 m)2) = 28 m.
(b) The vertical displacement is (17 m) sin(52) = 13 m.
P2-2 Wheel “rolls” through half of a turn, or πr = 1.41 m. The vertical displacement is 2r=
0.90 m. The net displacement is
p(1.41 m)2+ (0.90 m)2= 1.67 m0.
The angle is
θ= tan1(0.90 m)/(1.41 m) = 33.
P2-3 We align the coordinate system so that the origin corresponds to the starting position of
the fly and that all positions inside the room are given by positive coordinates.
(a) The displacement vector can just be written,
~
r= (10 ft)ˆ
i+ (12 ft)ˆ
j+ (14 ft)ˆ
k.
(b) The magnitude of the displacement vector is |~
r|=102+ 122+ 142ft= 21 ft.
24
(c) The straight line distance between two points is the shortest possible distance, so the length
of the path taken by the fly must be greater than or equal to 21 ft.
(d) If the fly walks it will need to cross two faces. The shortest path will be the diagonal across
these two faces. If the lengths of sides of the room are l1,l2, and l3, then the diagonal length across
two faces will be given by
q(l1+l2)2+l2
3,
where we want to choose the lifrom the set of 10 ft, 12 ft, and 14 ft that will minimize the length.
The minimum distance is when l1= 10 ft, l2= 12 ft, and l3= 14. Then the minimal distance the
fly would walk is 26 ft.
P2-4 Choose vector ~
ato lie on the xaxis. Then ~
a=aˆ
iand ~
b=bxˆ
i+byˆ
jwhere bx=bcos θand
by=bsin θ. The sum then has components
rx=a+bcos θand ry=bsin θ.
Then
r2= (a+bcos θ)2+ (bsin θ)2,
=a2+ 2ab cos θ+b2.
P2-5 (a) Average speed is total distance divided by total time. Then
vav =(35.0 mi/hr)(t/2) + (55.0 mi/hr)(t/2)
(t/2) + (t/2) = 45.0 mi/hr.
(b) Average speed is total distance divided by total time. Then
vav =(d/2) + (d/2)
(d/2)/(35.0 mi/hr) + (d/2)/(55.0 mi/hr) = 42.8 mi/hr.
(c) Average speed is total distance divided by total time. Then
vav =d+d
(d)/(45.0 mi/hr) + (d)/(42.8 mi/hr) = 43.9 mi/hr
P2-6 (a) We’ll do just one together. How about t= 2.0 s?
x= (3.0 m/s)(2.0 s) + (4.0 m/s2)(2.0 s)2+ (1.0 m/s3)(2.0 s)3=2.0 m.
The rest of the values are, starting from t= 0, x= 0.0 m, 0.0 m, -2.0 m, 0.0 m, and 12.0 m.
(b) Always final minus initial. The answers are xfxi=2.0 m 0.0 m = 2.0 m and xfxi=
12.0 m 0.0 m = 12.0 m.
(c) Always displacement divided by (change in) time.
vav =(12.0 m) (2.0 m)
(4.0 s) (2.0 s) = 7.0 m/s,
and
vav =(0.0 m) (0.0 m)
(3.0 s) (0.0 s) = 0.0 m/s.
25
P2-7 (a) Assume the bird has no size, the trains have some separation, and the bird is just
leaving one of the trains. The bird will be able to fly from one train to the other before the two
trains collide, regardless of how close together the trains are. After doing so, the bird is now on the
other train, the trains are still separated, so once again the bird can fly between the trains before
they collide. This process can be repeated every time the bird touches one of the trains, so the bird
will make an infinite number of trips between the trains.
(b) The trains collide in the middle; therefore the trains collide after (51 km)/(34 km/hr) = 1.5
hr. The bird was flying with constant speed this entire time, so the distance flown by the bird is (58
km/hr)(1.5 hr) = 87 km.
P2-8 (a) Start with a perfect square:
(v1v2)2>0,
v2
1+v2
2>2v1v2,
(v2
1+v2
2)t1t2>2v1v2t1t2,
d2
1+d2
2+ (v2
1+v2
2)t1t2> d2
1+d2
2+ 2v1v2t1t2,
(v2
1t1+v2
2t2)(t1+t2)>(d1+d2)2,
v2
1t1+v2
2t2
d1+d2
>d1+d2
t1+t2
,
v1d1+v2d2
d1+d2
>v1t1+v2t2
t1+t2
Actually, it only works if d1+d2>0!
(b) If v1=v2.
P2-9 (a) The average velocity during the time interval is vav = ∆x/t, or
vav =(A+B(3 s)3)(A+B(2 s)3)
(3 s) (2 s) = (1.50 cm/s3)(19 s3)/(1 s) = 28.5 cm/s.
(b) v=dx/dt = 3Bt2= 3(1.50 cm/s3)(2 s)2= 18 cm/s.
(c) v=dx/dt = 3Bt2= 3(1.50 cm/s3)(3 s)2= 40.5 cm/s.
(d) v=dx/dt = 3Bt2= 3(1.50 cm/s3)(2.5 s)2= 28.1 cm/s.
(e) The midway position is (xf+xi)/2, or
xmid =A+B[(3 s)3+ (2 s)3)]/2 = A+ (17.5 s3)B.
This occurs when t=3
p(17.5 s3). The instantaneous velocity at this point is
v=dx/dt = 3Bt2= 3(1.50 cm/s3)( 3
p(17.5 s3))2= 30.3 cm/s.
P2-10 Consider the figure below.
1
x
t1
x
t1
x
t
1
x
t
(c) (d)(b)(a)
26
P2-11 (a) The average velocity is displacement divided by change in time,
vav =(2.0 m/s3)(2.0 s)3(2.0 m/s3)(1.0 s)3
(2.0 s) (1.0 s) =14.0 m
1.0 s = 14.0 m/s.
The average acceleration is the change in velocity. So we need an expression for the velocity, which
is the time derivative of the position,
v=dx
dt =d
dt(2.0 m/s3)t3= (6.0 m/s3)t2.
From this we find average acceleration
aav =(6.0 m/s3)(2.0 s)2(6.0 m/s3)(1.0 s)2
(2.0 s) (1.0 s) =18.0 m/s
1.0 s = 18.0 m/s2.
(b) The instantaneous velocities can be found directly from v= (6.0 m/s2)t2, so v(2.0 s) = 24.0
m/s and v(1.0 s) = 6.0 m/s. We can get an expression for the instantaneous acceleration by taking
the time derivative of the velocity
a=dv
dt =d
dt(6.0 m/s3)t2= (12.0 m/s3)t.
Then the instantaneous accelerations are a(2.0 s) = 24.0 m/s2and a(1.0 s) = 12.0 m/s2
(c) Since the motion is monotonic we expect the average quantities to be somewhere between
the instantaneous values at the endpoints of the time interval. Indeed, that is the case.
P2-12 Consider the figure below.
0246810
t(s)
0246810
t(s)
0246810
t(s)
0
5
10
15
a(m/s^2)
0
75
50
25
v(m/s) x(m)
P2-13 Start with vf=vi+at, but vf= 0, so vi=at, then
x=1
2at2+vit=1
2at2at2=1
2at2,
so t=p2x/a =q2(19.2 ft)/(32 ft/s2) = 1.10 s. Then vi=(32 ft/s2)(1.10 s) = 35.2 ft/s.
Converting,
35.2 ft/s(1/5280 mi/ft)(3600 s/h) = 24 mi/h.
27
P2-14 (b) The average speed during while traveling the 160 m is
vav = (33.0 m/s + 54.0 m/s)/2 = 43.5 m/s.
The time to travel the 160 m is t= (160 m)/(43.5 m/s) = 3.68 s.
(a) The acceleration is
a=2x
t22vi
t=2(160 m)
(3.68 s)22(33.0 m/s)
(3.68 s) = 5.69 m/s2.
(c) The time required to get up to a speed of 33 m/s is
t=v/a = (33.0 m/s)/(5.69 m/s2) = 5.80 s.
(d) The distance moved from start is
d=1
2at2=1
2(5.69 m/s2)(5.80 s)2= 95.7 m.
P2-15 (a) The distance traveled during the reaction time happens at constant speed; treac =d/v =
(15 m)/(20 m/s) = 0.75 s.
(b) The braking distance is proportional to the speed squared (look at the numbers!) and in
this case is dbrake =v2/(20 m/s2). Then dbrake = (25 m/s)2/(20 m/s2) = 31.25 m. The reaction time
distance is dreac = (25 m/s)(0.75 s) = 18.75 m. The stopping distance is then 50 m.
P2-16 (a) For the car xc=act2/2. For the truck xt=vtt. Set both xito the same value, and
then substitute the time from the truck expression:
x=act2/2 = ac(x/vt)2/2,
or
x= 2vt2/ac= 2(9.5 m/s)2/(2.2 m/s) = 82 m.
(b) The speed of the car will be given by vc=act, or
vc=act=acx/vt= (2.2 m/s)(82 m)/(9.5 m/s) = 19 m/s.
P2-17 The runner covered a distance d1in a time interval t1during the acceleration phase and
a distance d2in a time interval t2during the constant speed phase. Since the runner started from
rest we know that the constant speed is given by v=at1, where ais the runner’s acceleration.
The distance covered during the acceleration phase is given by
d1=1
2at2
1.
The distance covered during the constant speed phase can also be found from
d2=vt2=at1t2.
We want to use these two expressions, along with d1+d2= 100 m and t2= (12.2 s) t1, to get
100 m = d1+d2=1
2at2
1+at1(12.2 s t1),
=1
2at2
1+a(12.2 s)t1,
=(1.40 m/s2)t2
1+ (34.2 m/s)t1.
28
This last expression is quadratic in t1, and is solved to give t1= 3.40 s or t1= 21.0 s. Since the race
only lasted 12.2 s we can ignore the second answer.
(b) The distance traveled during the acceleration phase is then
d1=1
2at2
1= (1.40 m/s2)(3.40 s)2= 16.2 m.
P2-18 (a) The ball will return to the ground with the same speed it was launched. Then the total
time of flight is given by
t= (vfvi)/g = (25 m/s25 m/s)/(9.8 m/s2) = 5.1 s.
(b) For small quantities we can think in terms of derivatives, so
δt = (δvfδvi)/g,
or τ= 2/g.
P2-19 Use y=gt2/2, but only keep the absolute value. Then y50 = (9.8 m/s2)(0.05 s)2/2 =
1.2 cm; y100 = (9.8 m/s2)(0.10 s)2/2 = 4.9 cm; y150 = (9.8 m/s2)(0.15 s)2/2 = 11 cm; y200 =
(9.8 m/s2)(0.20 s)2/2 = 20 cm; y250 = (9.8 m/s2)(0.25 s)2/2 = 31 cm.
P2-20 The truck will move 12 m in (12 m)/(55 km/h) = 0.785 s. The apple will fall y=gt2/2 =
(9.81 m/s2)(0.785 s)2/2 = 3.02 m.
P2-21 The rocket travels a distance d1=1
2at2
1=1
2(20 m/s2)(60 s)2= 36,000 m during the accel-
eration phase; the rocket velocity at the end of the acceleration phase is v=at = (20 m/s2)(60 s) =
1200 m/s. The second half of the trajectory can be found from Eqs. 2-29 and 2-30, with y0= 36,000
m and v0y= 1200 m/s.
(a) The highest point of the trajectory occurs when vy= 0, so
vy=v0ygt,
(0) = (1200 m/s) (9.8 m/s2)t,
122 s = t.
This time is used to find the height to which the rocket rises,
y=y0+v0yt1
2gt2,
= (36000 m) + (1200 m/s)(122s) 1
2(9.8 m/s2)(122 s)2= 110000 m.
(b) The easiest way to find the total time of flight is to solve Eq. 2-30 for the time when the
rocket has returned to the ground. Then
y=y0+v0yt1
2gt2,
(0) = (36000 m) + (1200 m/s)t1
2(9.8 m/s2)t2.
This quadratic expression has two solutions for t; one is negative so we don’t need to worry about
it, the other is t= 270 s. This is the free-fall part of the problem, to find the total time we need to
add on the 60 seconds of accelerated motion. The total time is then 330 seconds.
29
P2-22 (a) The time required for the player to “fall” from the highest point a distance of y= 15 cm
is p2y/g; the total time spent in the top 15 cm is twice this, or 2p2y/g = 2p2(0.15 m)/(9.81 m/s) =
0.350 s.
(b) The time required for the player to “fall” from the highest point a distance of 76 cm is
p2(0.76 m)/(9.81 m/s) = 0.394 s, the time required for the player to fall from the highest point a
distance of (76 15 = 61) cm is p2(0.61 m)/g = 0.353 s. The time required to fall the bottom 15
cm is the difference, or 0.041 s. The time spent in the bottom 15 cm is twice this, or 0.081 s.
P2-23 (a) The average speed between Aand Bis vav = (v+v/2)/2=3v/4. We can also write
vav = (3.0 m)/t= 3v/4. Finally, v/2 = vgt. Rearranging, v/2 = gt. Combining all of the
above,
v
2=g4(3.0 m)
3vor v2= (8.0 m)g.
Then v=p(8.0 m)(9.8 m/s2) = 8.85 m/s.
(b) The time to the highest point above Bis v/2 = gt, so the distance above Bis
y=g
2t2+v
2t=g
2v
2g2
+v
2v
2g=v2
8g.
Then y= (8.85 m/s)2/(8(9.8 m/s2)) = 1.00 m.
P2-24 (a) The time in free fall is t=p2y/g =p2(145 m)/(9.81 m/s2) = 5.44 s.
(b) The speed at the bottom is v=gt =(9.81 m/s2)(5.44 s) = 53.4 m/s.
(c) The time for deceleration is given by v=25gt, or t=(53.4 m/s)/(25 ×9.81 /s2) =
0.218 s. The distance through which deceleration occurred is
y=25g
2t2+vt = (123 m/s2)(0.218 s)2+ (53.4 m/s)(0.218 s) = 5.80 m.
P2-25 Find the time she fell from Eq. 2-30,
(0 ft) = (144 ft) + (0)t1
2(32 ft/s2)t2,
which is a simple quadratic with solutions t=±3.0 s. Only the positive solution is of interest. Use
this time in Eq. 2-29 to find her speed when she hit the ventilator box,
vy= (0) (32 ft/s2)(3.0 s) = 96 ft/s.
This becomes the initial velocity for the deceleration motion, so her average speed during deceleration
is given by Eq. 2-27,
vav,y =1
2(vy+v0y) = 1
2((0) + (96 ft/s)) = 48 ft/s.
This average speed, used with the distance of 18 in (1.5 ft), can be used to find the time of deceleration
vav,y = ∆y/t,
and putting numbers into the expression gives ∆t= 0.031 s. We actually used ∆y=1.5 ft, where
the negative sign indicated that she was still moving downward. Finally, we use this in Eq. 2-26 to
find the acceleration,
(0) = (96 ft/s) + a(0.031 s),
which gives a= +3100 ft/s2. In terms of gthis is a= 97g, which can be found by multiplying
through by 1 = g/(32 ft/s2).
30
P2-26 Let the speed of the disk when it comes into contact with the ground be v1; then the
average speed during the deceleration process is v1/2; so the time taken for the deceleration process
is t1= 2d/v1, where d=2 mm. But dis also give by d=at2
1/2 + v1t1, so
d=100g
22d
v12
+v12d
v1= 200gd2
v2
1
+ 2d,
or v2
1=200gd. The negative signs are necessary!.
The disk was dropped from a height h=yand it first came into contact with the ground when
it had a speed of v1. Then the average speed is v1/2, and we can repeat most of the above (except
a=ginstead of 100g), and then the time to fall is t2= 2y/v1,
y=g
22y
v12
+v12y
v1= 2gy2
v2
1
+ 2y,
or v2
1=2gy. The negative signs are necessary!.
Equating, y= 100d= 100(2 mm) = 0.2 m, so h= 0.2 m. Note that although 100g’s sounds
like plenty, you still shouldn’t be dropping your hard disk drive!
P2-27 Measure from the feet! Jim is 2.8 cm tall in the photo, so 1 cm on the photo is 60.7
cm in real-life. Then Jim has fallen a distance y1=3.04 m while Clare has fallen a distance
y2=5.77 m. Clare jumped first, and the time she has been falling is t2; Jim jumped seconds, the
time he has been falling is t1=t2t. Then y2=gt2
2/2 and y1=gt2
1/2, or t2=p2y2/g =
p2(5.77 m)/(9.81 m/s2)=1.08 s and t1=p2y1/g =p2(3.04 m)/(9.81 m/s2)=0.79 s. So
Jim waited 0.29 s.
P2-28 (a) Assuming she starts from rest and has a speed of v1when she opens her chute, then
her average speed while falling freely is v1/2, and the time taken to fall y1=52.0 m is t1= 2y1/v1.
Her speed v1is given by v1=gt1, or v2
1=2gy1. Then v1=p2(9.81 m/s2)(52.0 m) =
31.9 m/s. We must use the negative answer, because she fall down! The time in the air is then
t1=p2y1/g =p2(52.0 m)/(9.81 m/s2) = 3.26 s.
Her final speed is v2=2.90 m/s, so the time for the deceleration is t2= (v2v1)/a, where
a= 2.10 m/s2. Then t2= (2.90 m/s− −31.9 m/s)/(2.10 m/s2) = 13.8 s.
Finally, the total time of flight is t=t1+t2= 3.26 s + 13.8 s = 17.1 s.
(b) The distance fallen during the deceleration phase is
y2=g
2t2
2+v1t2=(2.10 m/s2)
2(13.8 s)2+ (31.9 m/s)(13.8 s) = 240 m.
The total distance fallen is y=y1+y2=52.0 m 240 m = 292 m. It is negative because she was
falling down.
P2-29 Let the speed of the bearing be v1at the top of the windows and v2at the bottom.
These speeds are related by v2=v1gt12, where t12 = 0.125 s is the time between when the
bearing is at the top of the window and at the bottom of the window. The average speed is
vav = (v1+v2)/2 = v1gt12/2. The distance traveled in the time t12 is y12 =1.20 m, so
y12 =vavt12 =v1t12 gt2
12/2,
and expression that can be solved for v1to yield
v1=y12 +gt2
12/2
t12
=(1.20 m) + (9.81 m/s2)(0.125 s)2/2
(0.125 s) =8.99 m/s.
31
Now that we know v1we can find the height of the building above the top of the window. The time
the object has fallen to get to the top of the window is t1=v1/g =(8.99 m/s)/(9.81 m/s2) =
0.916 m.
The total time for falling is then (0.916 s) + (0.125 s) + (1.0 s) = 2.04 s. Note that we remembered
to divide the last time by two! The total distance from the top of the building to the bottom is then
y=gt2/2 = (9.81 m/s2)(2.04 s)2/2 = 20.4 m.
P2-30 Each ball falls from a highest point through a distance of 2.0 m in
t=p2(2.0 m)/(9.8 m/s2) = 0.639 s.
The time each ball spends in the air is twice this, or 1.28 s. The frequency of tosses per ball is the
reciprocal, f= 1/T = 0.781 s1. There are five ball, so we multiply this by 5, but there are two
hands, so we then divide that by 2. The tosses per hand per second then requires a factor 5/2, and
the tosses per hand per minute is 60 times this, or 117.
P2-31 Assume each hand can toss nobjects per second. Let τbe the amount of time that any
one object is in the air. Then 2is the number of objects that are in the air at any time, where
the “2” comes from the fact that (most?) jugglers have two hands. We’ll estimate n, but τcan be
found from Eq. 2-30 for an object which falls a distance hfrom rest:
0 = h+ (0)t1
2gt2,
solving, t=p2h/g. But τis twice this, because the object had to go up before it could come down.
So the number of objects that can be juggled is
4np2h/g
We estimate n= 2 tosses/second. So the maximum number of objects one could juggle to a height
hwould be
3.6ph/meters.
P2-32 (a) We need to look up the height of the leaning tower to solve this! If the height is h= 56 m,
then the time taken to fall a distance h=y1is t1=p2y1/g =p2(56 m)/(9.81 m/s2) = 3.4 s.
The second object, however, has only fallen a a time t2=t1t= 3.3 s, so the distance the second
object falls is y2=gt2
2/2 = (9.81 m/s2)(3.3 s)2/2 = 53.4. The difference is y1y2= 2.9 m.
(b) If the vertical separation is ∆y= 0.01 m, then we can approach this problem in terms of
differentials,
δy =at δt,
so δt = (0.01 m)/[((9.81 m/s2)(3.4 s)] = 3×104s.
P2-33 Use symmetry, and focus on the path from the highest point downward. Then ∆tU= 2tU,
where tUis the time from the highest point to the upper level. A similar expression exists for
the lower level, but replace Uwith L. The distance from the highest point to the upper level is
yU=gt2
U/2 = g(∆tU/2)2/2. The distance from the highest point to the lower level is yL=
gt2
L/2 = g(∆tL/2)2/2. Now H=yUyL=gt2
U/8− −gt2
L/8, which can be written as
g=8H
t2
Lt2
U
.
32
E3-1 The Earth orbits the sun with a speed of 29.8 km/s. The distance to Pluto is 5900×106km.
The time it would take the Earth to reach the orbit of Pluto is
t= (5900×106km)/(29.8 km/s) = 2.0×108s,
or 6.3 years!
E3-2 (a) a=F/m = (3.8 N)/(5.5 kg) = 0.69 m/s2.
(b) t=vf/a = (5.2 m/s)/(0.69 m/s2) = 7.5 s.
(c) x=at2/2 = (0.69 m/s2)(7.5 s)2/2 = 20 m.
E3-3 Assuming constant acceleration we can find the average speed during the interval from Eq.
2-27
vav,x =1
2(vx+v0x) = 1
2(5.8×106m/s) + (0)= 2.9×106m/s.
From this we can find the time spent accelerating from Eq. 2-22, since ∆x=vav,xt. Putting in
the numbers ∆t= 5.17×109s. The acceleration is then
ax=vx
t=(5.8×106m/s) (0)
(5.17×109s) = 1.1×1015m/s2.
The net force on the electron is from Eq. 3-5,
XFx=max= (9.11×1031kg)(1.1×1015m/s2) = 1.0×1015 N.
E3-4 The average speed while decelerating is vav = 0.7×107m/s. The time of deceleration is
t=x/vav = (1.0×1014m)/(0.7×107m/s) = 1.4×1021 s. The deceleration is a= ∆v/t = (1.4×
107m/s)/(1.4×1021 s) = 1.0×1028m/s2. The force is F=ma = (1.67×1027kg)(1.0×1028m/s2) =
17 N.
E3-5 The net force on the sled is 92 N90 N= 2 N; subtract because the forces are in opposite
directions. Then
ax=PFx
m=(2 N)
(25 kg) = 8.0×102m/s2.
E3-6 53 km/hr is 14.7 m/s. The average speed while decelerating is vav = 7.4 m/s. The time
of deceleration is t=x/vav = (0.65 m)/(7.4 m/s) = 8.8×102s. The deceleration is a= ∆v/t =
(14.7 m/s)/(8.8×102s) = 17×102m/s2. The force is F=ma = (39kg)(1.7×102m/s2) = 6600 N.
E3-7 Vertical acceleration is a=F/m = (4.5×1015N)/(9.11×1031kg) = 4.9×1015m/s2. The
electron moves horizontally 33 mm in a time t=x/vx= (0.033 m)/(1.2×107m/s) = 2.8×109s.
The vertical distance deflected is y=at2/2 = (4.9×1015m/s2)(2.8×109s)2/2 = 1.9×102m.
E3-8 (a) a=F/m = (29 N)/(930 kg) = 3.1×102m/s2.
(b) x=at2/2 = (3.1×102m/s2)(86400 s)2/2 = 1.2×108m.
(c) v=at = (3.1×102m/s2)(86400 s) = 2700 m/s.
33
E3-9 Write the expression for the motion of the first object as PFx=m1a1xand that of the
second object as PFx=m2a2x. In both cases there is only one force, F, on the object, so PFx=F.
We will solve these for the mass as m1=F/a1and m2=F/a2. Since a1> a2we can conclude that
m2> m1
(a) The acceleration of and object with mass m2m1under the influence of a single force of
magnitude Fwould be
a=F
m2m1
=F
F/a2F/a1
=1
1/(3.30 m/s2)1/(12.0 m/s2),
which has a numerical value of a= 4.55 m/s2.
(b) Similarly, the acceleration of an object of mass m2+m1under the influence of a force of
magnitude Fwould be
a=1
1/a2+ 1/a1
=1
1/(3.30 m/s2)+1/(12.0 m/s2),
which is the same as part (a) except for the sign change. Then a= 2.59 m/s2.
E3-10 (a) The required acceleration is a=v/t = 0.1c/t. The required force is F=ma = 0.1mc/t.
Then
F= 0.1(1200×103kg)(3.00×108m/s)/(2.59×105s) = 1.4×108N,
and
F= 0.1(1200×103kg)(3.00×108m/s)/(5.18×106s) = 6.9×106N,
(b) The distance traveled during the acceleration phase is x1=at2
1/2, the time required to
travel the remaining distance is t2=x2/v where x2=dx1.dis 5 light-months, or d= (3.00×
108m/s)(1.30×107s) = 3.90×1015m. Then
t=t1+t2=t1+dx1
v=t1+2dat2
1
2v=t1+2dvt1
2v.
If t1is 3 days, then
t= (2.59×105s) + 2(3.90×1015m) (3.00×107m/s)(2.59×105s)
2(3.00×107m/s) = 1.30×108s=4.12 yr,
if t1is 2 months, then
t= (5.18×106s) + 2(3.90×1015m) (3.00×107m/s)(5.18×106s)
2(3.00×107m/s) = 1.33×108s=4.20 yr,
E3-11 (a) The net force on the second block is given by
XFx=m2a2x= (3.8 kg)(2.6 m/s2) = 9.9 N.
There is only one (relevant) force on the block, the force of block 1 on block 2.
(b) There is only one (relevant) force on block 1, the force of block 2 on block 1. By Newton’s
third law this force has a magnitude of 9.9 N. Then Newton’s second law gives PFx=9.9
N= m1a1x= (4.6 kg)a1x. So a1x=2.2 m/s2at the instant that a2x= 2.6 m/s2.
E3-12 (a) W= (5.00 lb)(4.448 N/lb) = 22.2 N; m=W/g = (22.2 N)/(9.81 m/s2) = 2.26 kg.
(b) W= (240 lb)(4.448 N/lb) = 1070 N; m=W/g = (1070 N)/(9.81 m/s2) = 109 kg.
(c) W= (3600 lb)(4.448 N/lb) = 16000 N; m=W/g = (16000 N)/(9.81 m/s2) = 1630 kg.
34
E3-13 (a) W= (1420.00 lb)(4.448 N/lb) = 6320 N; m=W/g = (6320 N)/(9.81 m/s2) = 644 kg.
(b) m= 412 kg; W=mg = (412 kg)(9.81 m/s2) = 4040 N.
E3-14 (a) W=mg = (75.0 kg)(9.81 m/s2) = 736 N.
(b) W=mg = (75.0 kg)(3.72 m/s2) = 279 N.
(c) W=mg = (75.0 kg)(0 m/s2) = 0 N.
(d) The mass is 75.0 kg at all locations.
E3-15 If g= 9.81 m/s2, then m=W/g = (26.0 N)/(9.81 m/s2) = 2.65 kg.
(a) Apply W=mg again, but now g= 4.60 m/s2, so at this point W= (2.65 kg)(4.60 m/s2) =
12.2 N.
(b) If there is no gravitational force, there is no weight, because g= 0. There is still mass,
however, and that mass is still 2.65 kg.
E3-16 Upward force balances weight, so F=W=mg = (12000 kg)(9.81 m/s2) = 1.2×105N.
E3-17 Mass is m=W/g; net force is F=ma, or F=W a/g. Then
F= (3900 lb)(13 ft/s2)/(32 ft/s2) = 1600 lb.
E3-18 a= ∆v/t= (450 m/s)/(1.82 s) = 247 m/s2. Net force is F=ma = (523 kg)(247 m/s2) =
1.29×105N.
E3-19 PFx= 2(1.4×105N) = max. Then m= 1.22×105kg and
W=mg = (1.22×105kg)(9.81 m/s2) = 1.20×106N.
E3-20 Do part (b) first; there must be a 10 lb force to support the mass. Now do part (a), but
cover up the left hand side of both pictures. If you can’t tell which picture is which, then they must
both be 10 lb!
E3-21 (b) Average speed during deceleration is 40 km/h, or 11 m/s. The time taken to stop the
car is then t=x/vav = (61 m)/(11 m/s) = 5.6 s.
(a) The deceleration is a= ∆v/t= (22 m/s)/(5.6 s) = 3.9 m/s2. The braking force is F=
ma =W a/g = (13,000 N)(3.9 m/s2)/(9.81 m/s2) = 5200 N.
(d) The deceleration is same; the time to stop the car is then ∆t= ∆v/a = (11 m/s)/(3.9 m/s2) =
2.8s.
(c) The distance traveled during stopping is x=vavt= (5.6 m/s)(2.8 s) = 16 m.
E3-22 Assume acceleration of free fall is 9.81 m/s2at the altitude of the meteor. The net force is
Fnet =ma = (0.25 kg)(9.2m/s2) = 2.30 N. The weight is W=mg = (0.25 kg)(9.81 m/s2) = 2.45 N.
The retarding force is Fnet W= (2.3 N) (2.45 N) = 0.15 N.
E3-23 (a) Find the time during the “jump down” phase from Eq. 2-30.
(0 m) = (0.48 m) + (0)t1
2(9.8 m/s2)t2,
which is a simple quadratic with solutions t=±0.31 s. Use this time in Eq. 2-29 to find his speed
when he hit ground,
vy= (0) (9.8 m/s2)(0.31 s) = 3.1 m/s.
35
This becomes the initial velocity for the deceleration motion, so his average speed during deceleration
is given by Eq. 2-27,
vav,y =1
2(vy+v0y) = 1
2((0) + (3.1 m/s)) = 1.6 m/s.
This average speed, used with the distance of -2.2 cm (-0.022 m), can be used to find the time of
deceleration
vav,y = ∆y/t,
and putting numbers into the expression gives ∆t= 0.014 s. Finally, we use this in Eq. 2-26 to find
the acceleration,
(0) = (3.1 m/s) + a(0.014 s),
which gives a= 220 m/s2.
(b) The average net force on the man is
XFy=may= (83 kg)(220 m/s2) = 1.8×104N.
E3-24 The average speed of the salmon while decelerating is 4.6 ft/s. The time required to stop
the salmon is then t=x/vav = (0.38 ft)/(4.6 ft/s) = 8.3×102s. The deceleration of the salmon
is a= ∆v/t= (9.2 ft/s)/(8.2-2s) = 110 ft/s2. The force on the salmon is then F=W a/g =
(19 lb)(110 ft/s2)/(32 ft/s2) = 65 lb.
E3-25 From appendix G we find 1 lb = 4.448 N; so the weight is (100 lb)(4.448 N/1 lb) = 445
N; similarly the cord will break if it pulls upward on the object with a force greater than 387 N.
The mass of the object is m=W/g = (445 N)/(9.8 m/s2) = 45 kg.
There are two vertical forces on the 45 kg object, an upward force from the cord FOC (which has a
maximum value of 387 N) and a downward force from gravity FOG. Then PFy=FOC FOG = (387
N) (445 N) = 58 N. Since the net force is negative, the object must be accelerating downward
according to
ay=XFy/m = (58 N)/(45 kg) = 1.3 m/s2.
E3-26 (a) Constant speed means no acceleration, hence no net force; this means the weight is
balanced by the force from the scale, so the scale reads 65 N.
(b) Net force on mass is Fnet =ma =W a/g = (65 N)(2.4 m/s2)/(9.81 m/s2) = 16 N.. Since
the weight is 65 N, the scale must be exerting a force of (16 N) (65 N) = 49 N.
E3-27 The magnitude of the net force is WR= (1600 kg)(9.81 m/s2)(3700 N) = 12000 N.
The acceleration is then a=F/m = (12000 N)/(1600 kg) = 7.5 m/s2. The time to fall is
t=p2y/a =p2(72 m)/(7.5 m/s2) = 4.4 s.
The final speed is v=at = (7.5 m/s2)(4.4 s) = 33 m/s. Get better brakes, eh?
E3-28 The average speed during the acceleration is 140 ft/s. The time for the plane to travel 300
ft is
t=x/vav = (300 ft)/(140 ft/s) = 2.14 s.
The acceleration is then
a= ∆v/t= (280 ft/s)/(2.14 s) = 130 ft/s2.
The net force on the plane is F=ma =W a/g = (52000 lb)(130 ft/s2)/(32 ft/s2) = 2.1×105lb.
The force exerted by the catapult is then 2.1×105lb 2.4×104lb = 1.86×105lb.
36
E3-29 (a) The acceleration of a hovering rocket is 0, so the net force is zero; hence the thrust must
equal the weight. Then T=W=mg = (51000 kg)(9.81 m/s2) = 5.0×105N.
(b) If the rocket accelerates upward then the net force is F=ma = (51000 kg)(18 m/s2) =
9.2×105N. Now Fnet =TW, so T= 9.2×105N+5.0×105N = 1.42×106N.
E3-30 (a) Net force on parachute + person system is Fnet =ma = (77 kg + 5.2 kg)(2.5 s2) =
210 N. The weight of the system is W=mg = (77 kg+5.2 kg)(9.81 s2) = 810 N. If Pis the upward
force of the air on the system (parachute) then P=Fnet +W= (210 N) + (810 N) = 600 N.
(b) The net force on the parachute is Fnet =ma = (5.2 kg)(2.5 s2) = 13 N. The weight of the
parachute is W=mg = (5.2 kg)(9.81 m/s2) = 51 N. If Dis the downward force of the person on
the parachute then D=Fnet W+P=(13 N) (51 N) + 600 N = 560 N.
E3-31 (a) The total mass of the helicopter+car system is 19,500 kg; and the only other force
acting on the system is the force of gravity, which is
W=mg = (19,500 kg)(9.8 m/s2) = 1.91×105N.
The force of gravity is directed down, so the net force on the system is PFy=FBA (1.91×105
N). The net force can also be found from Newton’s second law: PFy=may= (19,500 kg)(1.4
m/s2)=2.7×104N. Equate the two expressions for the net force, FBA (1.91×105N) = 2.7×104
N, and solve; FBA = 2.2×105N.
(b) Repeat the above steps except: (1) the system will consist only of the car, and (2) the upward
force on the car comes from the supporting cable only FCC . The weight of the car is W=mg = (4500
kg)(9.8 m/s2)=4.4×104N. The net force is PFy=FCC (4.4×104N), it can also be written as
PFy=may= (4500 kg)(1.4 m/s2) = 6300 N. Equating, FCC = 50,000 N.
P3-1 (a) The acceleration is a=F/m = (2.7×105N)/(280 kg) = 9.64×108m/s2. The displace-
ment (from the original trajectory) is
y=at2/2 = (9.64×108m/s2)(2.4 s)2/2 = 2.8×107m.
(b) The acceleration is a=F/m = (2.7×105N)/(2.1 kg) = 1.3×105m/s2. The displacement
(from the original trajectory) is
y=at2/2 = (1.3×105m/s2)(2.4 s)2/2 = 3.7×105m.
P3-2 (a) The acceleration of the sled is a=F/m = (5.2 N)/(8.4 kg) = 0.62 m/s2.
(b) The acceleration of the girl is a=F/m = (5.2 N)/(40 kg) = 0.13 m/s2.
(c) The distance traveled by girl is x1=a1t2/2; the distance traveled by the sled is x2=a2t2/2.
The two meet when x1+x2= 15 m. This happens when (a1+a2)t2= 30 m. They then meet when
t=p(30 m)/(0.13 m/s2+ 0.62 m/s2) = 6.3 s. The girl moves x1= (0.13 m/s2)(6.3 s)2/2 = 2.6 m.
P3-3 (a) Start with block one. It starts from rest, accelerating through a distance of 16 m in a
time of 4.2 s. Applying Eq. 2-28,
x=x0+v0xt+1
2axt2,
16 m = (0) + (0)(4.2 s) + 1
2ax(4.2 s)2,
find the acceleration to be ax=1.8 m/s2.
37
Now for the second block. The acceleration of the second block is identical to the first for much
the same reason that all objects fall with approximately the same acceleration.
(b) The initial and final velocities are related by a sign, then vx=v0xand Eq. 2-26 becomes
vx=v0x+axt,
v0x=v0x+axt,
2v0x= (1.8 m/s2)(4.2 s).
which gives an initial velocity of v0x= 3.8 m/s.
(c) Half of the time is spent coming down from the highest point, so the time to “fall” is 2.1 s.
The distance traveled is found from Eq. 2-28,
x= (0) + (0)(2.1 s) + 1
2(1.8 m/s2)(2.1 s)2=4.0 m.
P3-4 (a) The weight of the engine is W=mg = (1400 kg)(9.81 m/s2)=1.37×104N. If each bolt
supports 1/3 of this, then the force on a bolt is 4600 N.
(b) If engine accelerates up at 2.60 m/s2, then net force on the engine is
Fnet =ma = (1400 kg)(2.60 m/s2) = 3.64×103N.
The upward force from the bolts must then be
B=Fnet +W= (3.64×103N) + (1.37×104N) = 1.73×104N.
The force per bolt is one third this, or 5800 N.
P3-5 (a) If craft descends with constant speed then net force is zero, so thrust balances weight.
The weight is then 3260 N.
(b) If the thrust is 2200 N the net force is 2200 N 3260 N = 1060 N. The mass is then
m=F/a = (1060 N)/(0.390 m/s2) = 2720 kg.
(c) The acceleration due to gravity is g=W/m = (3260 N)/(2720 kg) = 1.20 m/s2.
P3-6 The weight is originally Mg. The net force is originally Ma. The upward force is then
originally B=Mg Ma. The goal is for a net force of (Mm)aand a weight (Mm)g. Then
(Mm)a=B(Mm)g=Mg Ma Mg +mg =mg Ma,
or m= 2Ma/(a+g).
P3-7 (a) Consider all three carts as one system. Then
XFx=mtotalax,
6.5 N = (3.1 kg + 2.4 kg + 1.2 kg)ax,
0.97 m/s2=ax.
(b) Now choose your system so that it only contains the third car. Then
XFx=F23 =m3ax= (1.2 kg)(0.97 m/s2).
The unknown can be solved to give F23 = 1.2 N directed to the right.
(c) Consider a system involving the second and third carts. Then PFx=F12, so Newton’s law
applied to the system gives
F12 = (m2+m3)ax= (2.4 kg + 1.2 kg)(0.97 m/s2) = 3.5 N.
38
P3-8 (a) F=ma = (45.2 kg + 22.8 kg + 34.3 kg)(1.32 m/s2) = 135 N.
(b) Consider only m3. Then F=ma = (34.3 kg)(1.32 m/s2) = 45.3 N.
(c) Consider m2and m3. Then F=ma = (22.8 kg + 34.3 kg)(1.32 m/s2) = 75.4 N.
P3-9 (c) The net force on each link is the same, Fnet =ma = (0.100 kg)(2.50 m/s2) = 0.250 N.
(a) The weight of each link is W=mg = (0.100 kg)(9.81 m/s2) = 0.981 N. On each link (except
the top or bottom link) there is a weight, an upward force from the link above, and a downward force
from the link below. Then Fnet =UDW. Then U=Fnet +W+D= (0.250 N)+(0.981 N)+D=
1.231 N + D. For the bottom link D= 0. For the bottom link, U= 1.23 N. For the link above,
U= 1.23 N + 1.23 N = 2.46 N. For the link above, U= 1.23 N + 2.46 N = 3.69 N. For the link above,
U= 1.23 N + 3.69 N = 4.92 N.
(b) For the top link, the upward force is U= 1.23 N + 4.92 N = 6.15 N.
P3-10 (a) The acceleration of the two blocks is a=F/(m1+m2) The net force on block 2 is from
the force of contact, and is
P=m2a=F m2/(m1+m2) = (3.2 N)(1.2 kg)/(2.3 kg + 1.2 kg) = 1.1 N.
(b) The acceleration of the two blocks is a=F/(m1+m2) The net force on block 1 is from the
force of contact, and is
P=m1a=F m1/(m1+m2) = (3.2 N)(2.3 kg)/(2.3 kg + 1.2 kg) = 2.1 N.
Not a third law pair, eh?
P3-11 (a) Treat the system as including both the block and the rope, so that the mass of
the system is M+m. There is one (relevant) force which acts on the system, so PFx=P. Then
Newton’s second law would be written as P= (M+m)ax. Solve this for axand get ax=P/(M+m).
(b) Now consider only the block. The horizontal force doesn’t act on the block; instead, there is
the force of the rope on the block. We’ll assume that force has a magnitude R, and this is the only
(relevant) force on the block, so PFx=Rfor the net force on the block.. In this case Newton’s
second law would be written R=Max. Yes, axis the same in part (a) and (b); the acceleration of
the block is the same as the acceleration of the block + rope. Substituting in the results from part
(a) we find
R=M
M+mP.
39
E4-1 (a) The time to pass between the plates is t=x/vx= (2.3 cm)/(9.6×108cm/s) = 2.4×109s.
(b) The vertical displacement of the beam is then y=ayt2/2 == (9.4×1016 cm/s2)(2.4×
109s)2/2 = 0.27 cm.
(c) The velocity components are vx= 9.6×108cm/s and vy=ayt= (9.4×1016 cm/s2)(2.4×
109s) = 2.3×108cm/s.
E4-2 ~
a= ∆~
v/t=(6.30ˆ
i8.42ˆ
j)(m/s)/(3 s) = (2.10ˆ
i+ 2.81ˆ
j)(m/s2).
E4-3 (a) The velocity is given by
d~
r
dt =d
dt Aˆ
i+d
dt Bt2ˆ
j+d
dt Ctˆ
k,
~
v= (0) + 2Btˆ
j+Cˆ
k.
(b) The acceleration is given by
d~
v
dt =d
dt 2Btˆ
j+d
dt Cˆ
k,
~
a= (0) + 2Bˆ
j+ (0).
(c) Nothing exciting happens in the xdirection, so we will focus on the yz plane. The trajectory
in this plane is a parabola.
E4-4 (a) Maximum xis when vx= 0. Since vx=axt+vx,0,vx= 0 when t=vx,0/ax=
(3.6 m/s)/(1.2 m/s2) = 3.0 s.
(b) Since vx= 0 we have |~
v|=|vy|. But vy=ayt+vy,0=(1.4 m/s)(3.0 s) + (0) = 4.2 m/s.
Then |~
v|= 4.2 m/s.
(c) ~
r=~
at2/2 + ~
v0t, so
~
r= [(0.6 m/s2)ˆ
i(0.7 m/s2)ˆ
j](3.0 s)2+ [(3.6 m/s)ˆ
i](3.0 s) = (5.4 m)ˆ
i(6.3 m)ˆ
j.
E4-5 ~
F=~
F1+~
F2= (3.7 N)ˆ
j+ (4.3 N)ˆ
i. Then ~
a=~
F/m = (0.71 m/s2)ˆ
j+ (0.83 m/s2)ˆ
i.
E4-6 (a) The acceleration is ~
a=~
F/m = (2.2 m/s2)ˆ
j. The velocity after 15 seconds is ~
v=~
at+~
v0,
or
~
v= [(2.2 m/s2)ˆ
j](15 s) + [(42 m/s)ˆ
i] = (42 m/s)ˆ
i+ (33 m/s)ˆ
j.
(b) ~
r=~
at2/2 + ~
v0t, so
~
r= [(1.1 m/s2)ˆ
j](15 s)2+ [(42 m/s)ˆ
i](15 s) = (630 m)ˆ
i+ (250 m)ˆ
j.
E4-7 The block has a weight W=mg = (5.1 kg)(9.8 m/s2) = 50 N.
(a) Initially P= 12 N, so Py= (12 N) sin(25) = 5.1 N and Px= (12 N) cos(25) = 11 N. Since
the upward component is less than the weight, the block doesn’t leave the floor, and a normal force
will be present which will make PFy= 0. There is only one contribution to the horizontal force,
so PFx=Px. Newton’s second law then gives ax=Px/m = (11 N)/(5.1 kg) = 2.2 m/s2.
(b) As Pis increased, so is Py; eventually Pywill be large enough to overcome the weight of the
block. This happens just after Py=W= 50 N, which occurs when P=Py/sin θ= 120 N.
(c) Repeat part (a), except now P= 120 N. Then Px= 110 N, and the acceleration of the block
is ax=Px/m = 22 m/s2.
40
E4-8 (a) The block has weight W=mg = (96.0 kg)(9.81 m/s2) = 942 N. Px= (450 N) cos(38) =
355 N; Py= (450 N) sin(38) = 277 N. Since Py< W the crate stays on the floor and there is a
normal force N=WPy. The net force in the xdirection is Fx=Px(125 N) = 230 N. The
acceleration is ax=Fx/m = (230 N)/(96.0 kg) = 2.40 m/s2.
(b) The block has mass m=W/g = (96.0 N)/(9.81 m/s2)=9.79 kg. Px= (450 N) cos(38) =
355 N; Py= (450 N) sin(38) = 277 N. Since Py> W the crate lifts off of the floor! The net
force in the xdirection is Fx=Px(125 N) = 230 N. The xacceleration is ax=Fx/m =
(230 N)/(9.79 kg) = 23.5 m/s2. The net force in the ydirection is Fy=PyW= 181 N. The y
acceleration is ay=Fy/m = (181 N)/(9.79 kg) = 18.5 m/s2. Wow.
E4-9 Let ybe perpendicular and xbe parallel to the incline. Then P= 4600 N;
Px= (4600 N) cos(27) = 4100 N;
Py= (4600 N) sin(27) = 2090 N.
The weight of the car is W=mg = (1200 kg)(9.81 m/s2) = 11800 N;
Wx= (11800 N) sin(18) = 3650 N;
Wy= (11800 N) cos(18) = 11200 N.
Since Wy> Pythe car stays on the incline. The net force in the xdirection is Fx=PxWx= 450 N.
The acceleration in the xdirection is ax=Fx/m = (450 N)/(1200 kg) = 0.375 m/s2. The distance
traveled in 7.5 s is x=axt2/2 = (0.375 m/s2)(7.5 s)2/2 = 10.5 m.
E4-10 Constant speed means zero acceleration, so net force is zero. Let ybe perpendicular and xbe
parallel to the incline. The weight is W=mg = (110 kg)(9.81 m/s2) = 1080 N; Wx=Wsin(34);
Wy=Wcos(34). The push Fhas components Fx=Fcos(34) and Fy=Fsin(34). The
ycomponents will balance after a normal force is introduced; the xcomponents will balance if
Fx=Wx, or F=Wtan(34) = (1080 N) tan(34) = 730 N.
E4-11 If the xaxis is parallel to the river and the yaxis is perpendicular, then ~
a= 0.12ˆ
im/s2.
The net force on the barge is
X~
F=m~
a= (9500 kg)(0.12ˆ
im/s2) = 1100ˆ
iN.
The force exerted on the barge by the horse has components in both the xand ydirection. If
P= 7900 N is the magnitude of the pull and θ= 18is the direction, then ~
P=Pcos θˆ
i+Psin θˆ
j=
(7500ˆ
i+ 2400ˆ
j) N.
Let the force exerted on the barge by the water be ~
Fw=Fw,xˆ
i+Fw,yˆ
j. Then PFx= (7500
N) + Fw,x and PFy= (2400 N) + Fw,y . But we already found P~
F, so
Fx= 1100 N = 7500 N + Fw,x,
Fy= 0 = 2400 N + Fw,y.
Solving, Fw,x =6400 N and Fw,y =2400 N. The magnitude is found by Fw=qF2
w,x +F2
w,y =
6800 N.
41
E4-12 (a) Let ybe perpendicular and xbe parallel to the direction of motion of the plane.
Then Wx=mg sin θ;Wy=mg cos θ;m=W/g. The plane is accelerating in the xdirection, so
ax= 2.62 m/s2; the net force is in the xdirection, where Fx=max. But Fx=TWx, so
T=Fx+Wx=Wax
g+Wsin θ= (7.93×104N) (2.62 m/s2)
(9.81 m/s2)+ sin(27)= 5.72×104N.
(b) There is no motion in the ydirection, so
L=Wy= (7.93×104N) cos(27) = 7.07×104N.
E4-13 (a) The ball rolled off horizontally so v0y= 0. Then
y=v0yt1
2gt2,
(4.23 ft) = (0)t1
2(32.2 ft/s2)t2,
which can be solved to yield t= 0.514 s.
(b) The initial velocity in the xdirection can be found from x=v0xt; rearranging, v0x=x/t =
(5.11 ft)/(0.514 s) = 9.94 ft/s. Since there is no ycomponent to the velocity, then the initial speed
is v0= 9.94 ft/s.
E4-14 The electron travels for a time t=x/vx. The electron “falls” vertically through a distance
y=gt2/2 in that time. Then
y=g
2x
vx2
=(9.81 m/s2)
2(1.0 m)
(3.0×107m/s)2
=5.5×1015 m.
E4-15 (a) The dart “falls” vertically through a distance y=gt2/2 = (9.81 m/s2)(0.19 s)2/2 =
0.18 m.
(b) The dart travels horizontally x=vxt= (10 m/s)(0.19 s) = 1.9 m.
E4-16 The initial velocity components are
vx,0= (15 m/s) cos(20) = 14 m/s
and
vy,0=(15 m/s) sin(20) = 5.1 m/s.
(a) The horizontal displacement is x=vxt= (14 m/s)(2.3 s) = 32 m.
(b) The vertical displacement is
y=gt2/2 + vy,0t=(9.81 m/s2)(2.3 s)2/2+(5.1 m/s)(2.3 s) = 38 m.
E4-17 Find the time in terms of the the initial ycomponent of the velocity:
vy=v0ygt,
(0) = v0ygt,
t=v0y/g.
42
Use this time to find the highest point:
y=v0yt1
2gt2,
ymax =v0yv0y
g1
2gv0y
g2
,
=v2
0y
2g.
Finally, we know the initial ycomponent of the velocity from Eq. 2-6, so ymax = (v0sin φ0)2/2g.
E4-18 The horizontal displacement is x=vxt. The vertical displacement is y=gt2/2. Combin-
ing, y=g(x/vx)2/2. The edge of the nth step is located at y=nw,x=nw, where w= 2/3 ft.
If |y|> nw when x=nw then the ball hasn’t hit the step. Solving,
g(nw/vx)2/2< nw,
gnw/v2
x<2,
n < 2v2
x/(gw) = 2(5.0 ft/s)2/[(32 ft/s2)(2/3 ft)] = 2.34.
Then the ball lands on the third step.
E4-19 (a) Start from the observation point 9.1 m above the ground. The ball will reach the
highest point when vy= 0, this will happen at a time tafter the observation such that t=vy,0/g =
(6.1 m/s)/(9.81 m/s2) = 0.62 s. The vertical displacement (from the ground) will be
y=gt2/2 + vy,0t+y0=(9.81 m/s2)(0.62 s)2/2 + (6.1 m/s)(0.62 s) + (9.1 m) = 11 m.
(b) The time for the ball to return to the ground from the highest point is t=p2ymax/g =
p2(11 m)/(9.81 m/s2) = 1.5 s. The total time of flight is twice this, or 3.0 s. The horizontal distance
traveled is x=vxt= (7.6 m/s)(3.0 s) = 23 m.
(c) The velocity of the ball just prior to hitting the ground is
~
v=~
at+~
v0= (9.81 m/s2)ˆ
j(1.5 s) + (7.6 m/s)ˆ
i= 7.6 m/sˆ
i15 m/suj.
The magnitude is 7.62+ 152(m/s) = 17 m/s. The direction is
θ= arctan(15/7.6) = 63.
E4-20 Focus on the time it takes the ball to get to the plate, assuming it traveled in a straight
line. The ball has a “horizontal” velocity of 135 ft/s. Then t=x/vx= (60.5 ft)/(135 ft/s) = 0.448 s.
The ball will “fall” a vertical distance of y=gt2/2 = (32 ft/s2)(0.448 s)2/2 = 3.2 ft. That’s in
the strike zone.
E4-21 Since R1/g one can write R2/R1=g1/g2, or
R=R2R1=R11g1
g2= (8.09 m) 1(9.7999 m/s2)
(9.8128 m/s2)= 1.06 cm.
43
E4-22 If initial position is ~
r0= 0, then final position is ~
r= (13 ft)ˆ
i+ (3 ft)ˆ
j. The initial velocity
is ~
v0=vcos θˆ
i+vsin θˆ
j. The horizontal equation is (13 ft) = vcos θt; the vertical equation is
(3 ft) = (g/2)t2+vsin θt. Rearrange the vertical equation and then divide by the horizontal
equation to get
3 ft + (g/2)t2
(13 ft) = tan θ,
or
t2= [(13 ft) tan(55)(3 ft)][2/(32 m/s2)] = 0.973 s2,
or t= 0.986 s. Then v= (13 ft)/(cos(55)(0.986 s)) = 23 ft/s.
E4-23 vx=x/t = (150 ft)/(4.50 s) = 33.3 ft/s. The time to the highest point is half the hang
time, or 2.25 s. The vertical speed when the ball hits the ground is vy=gt =(32 ft/s2)(2.25 s) =
72.0 ft/s. Then the initial vertical velocity is 72.0 ft/s. The magnitude of the initial velocity is
722+ 332(ft/s) = 79 ft/s. The direction is
θ= arctan(72/33) = 65.
E4-24 (a) The magnitude of the initial velocity of the projectile is v= 264 ft/s. The projectile
was in the air for a time twhere
t=x
vx
=x
vcos θ=(2300 ft)
(264 ft/s) cos(27)= 9.8 s.
(b) The height of the plane was ywhere
y=gt2/2vy,0t= (32 ft/s)(9.8 s)2/2(264 ft/s) sin(27)(9.8 s) = 2700 ft.
E4-25 Define the point the ball leaves the racquet as ~
r= 0.
(a) The initial conditions are given as v0x= 23.6 m/s and v0y= 0. The time it takes for the ball
to reach the horizontal location of the net is found from
x=v0xt,
(12 m) = (23.6 m/s)t,
0.51 s = t,
Find how far the ball has moved horizontally in this time:
y=v0yt1
2gt2= (0)(0.51 s) 1
2(9.8 m/s2)(0.51 s)2=1.3 m.
Did the ball clear the net? The ball started 2.37 m above the ground and “fell” through a distance
of 1.3 m by the time it arrived at the net. So it is still 1.1 m above the ground and 0.2 m above the
net.
(b) The initial conditions are now given by v0x= (23.6 m/s)(cos[5.0]) = 23.5 m/s and v0y=
(23.6 m/s)(sin[5.0]) = 2.1 m/s. Now find the time to reach the net just as done in part (a):
t=x/v0x= (12.0 m)/(23.5 m/s) = 0.51 s.
Find the vertical position of the ball when it arrives at the net:
y=v0yt1
2gt2= (2.1 m/s)(0.51 s) 1
2(9.8 m/s2)(0.51 s)2=2.3 m.
Did the ball clear the net? Not this time; it started 2.37 m above the ground and then passed the
net 2.3 m lower, or only 0.07 m above the ground.
44
E4-26 The initial speed of the ball is given by v=gR =q(32 ft/s2)(350 ft) = 106 ft/s. The
time of flight from the batter to the wall is
t=x/vx= (320 ft)/[(106 ft/s) cos(45)] = 4.3 s.
The height of the ball at that time is given by y=y0+v0yt1
2gt2, or
y= (4 ft) + (106 ft/s) sin(45)(4.3 s) (16 ft/s2)(4.3 s)2= 31 ft,
clearing the fence by 7 feet.
E4-27 The ball lands x= (358 ft) + (39 ft) cos(28) = 392 ft from the initial position. The ball
lands y= (39 ft) sin(28)(4.60 ft) = 14 ft above the initial position. The horizontal equation
is (392 ft) = vcos θt; the vertical equation is (14 ft) = (g/2)t2+vsin θt. Rearrange the vertical
equation and then divide by the horizontal equation to get
14 ft + (g/2)t2
(392 ft) = tan θ,
or
t2= [(392 ft) tan(52)(14 ft)][2/(32 m/s2)] = 30.5 s2,
or t= 5.52 s. Then v= (392 ft)/(cos(52)(5.52 s)) = 115 ft/s.
E4-28 Since ball is traveling at 45when it returns to the same level from which it was thrown
for maximum range, then we can assume it actually traveled 1.6 m. farther than it would have
had it been launched from the ground level. This won’t make a big difference, but that means that
R= 60.0 m 1.6 m = 58.4 m. If v0is initial speed of ball thrown directly up, the ball rises to the
highest point in a time t=v0/g, and that point is ymax =gt2/2 = v2
0/(2g) above the launch point.
But v2
0=gR, so ymax =R/2 = (58.4 m)/2 = 29.2 m. To this we add the 1.60 m point of release to
get 30.8 m.
E4-29 The net force on the pebble is zero, so PFy= 0. There are only two forces on the
pebble, the force of gravity Wand the force of the water on the pebble FP W . These point in
opposite directions, so 0 = FP W W. But W=mg = (0.150 kg)(9.81 m/s2) = 1.47 N. Since
FP W =Win this problem, the force of the water on the pebble must also be 1.47 N.
E4-30 Terminal speed is when drag force equal weight, or mg =bvT2. Then vT=pmg/b.
E4-31 Eq. 4-22 is
vy(t) = vT1ebt/m,
where we have used Eq. 4-24 to substitute for the terminal speed. We want to solve this equation
for time when vy(t) = vT/2, so
1
2vT=vT1ebt/m,
1
2=1ebt/m,
ebt/m =1
2
bt/m =ln(1/2)
t=m
bln 2
45
E4-32 The terminal speed is 7 m/s for a raindrop with r= 0.15 cm. The mass of this drop is
m= 4πρr3/3, so
b=mg
vT
=4π(1.0×103kg/cm3)(0.15 cm)3(9.81 m/s2)
3(7 m/s) = 2.0×105kg/s.
E4-33 (a) The speed of the train is v= 9.58 m/s. The drag force on one car is f= 243(9.58) N =
2330 N. The total drag force is 23(2330 N) = 5.36×104N. The net force exerted on the cars (treated
as a single entity) is F=ma = 23(48.6×103kg)(0.182 m/s2) = 2.03×105N. The pull of the locomotive
is then P= 2.03×105N+5.36×104N = 2.57×105N.
(b) If the locomotive is pulling the cars at constant speed up an incline then it must exert a
force on the cars equal to the sum of the drag force and the parallel component of the weight. The
drag force is the same in each case, so the parallel component of the weight is W|| =Wsin θ=
2.03×105N = ma, where ais the acceleration from part (a). Then
θ= arcsin(a/g) = arcsin[(0.182 m/s2)/(9.81 m/s2)] = 1.06.
E4-34 (a) a=v2/r = (2.18×106m/s)2/(5.29×1011m) = 8.98×1022m/s2.
(b) F=ma = (9.11×1031kg)(8.98×1022m/s2) = 8.18×108N, toward the center.
E4-35 (a) v=rac=q(5.2 m)(6.8)(9.8 m/s2) = 19 m/s.
(b) Use the fact that one revolution corresponds to a length of 2πr:
19m
s1 rev
2π(5.2 m)60 s
1 min= 35 rev
min.
E4-36 (a) v= 2πr/T = 2π(15 m)/(12 s) = 7.85 m/s. Then a=v2/r = (7.85 m/s)2/(15 m) =
4.11 m/s2, directed toward center, which is down.
(b) Same arithmetic as in (a); direction is still toward center, which is now up.
(c) The magnitude of the net force in both (a) and (b) is F=ma = (75 kg)(4.11 m/s2) = 310 N.
The weight of the person is the same in both parts: W=mg = (75 kg)(9.81 m/s2) = 740 N. At
the top the net force is down, the weight is down, so the Ferris wheel is pushing up with a force of
P=WF= (740 N) (310 N) = 430 N. At the bottom the net force is up, the weight is down, so
the Ferris wheel is pushing up with a force of P=W+F= (740 N) + (310 N) = 1050 N.
E4-37 (a) v= 2πr/T = 2π(20×103m)/(1.0 s) = 1.26×105m/s.
(b) a=v2/r = (1.26×105m/s)2/(20×103m) = 7.9×105m/s2.
E4-38 (a) v= 2πr/T = 2π(6.37 ×106m)/(86400 s) = 463 m/s. a=v2/r = (463 m/s)2/(6.37 ×
106m) = 0.034 m/s2.
(b) The net force on the object is F=ma = (25.0 kg)(0.034 m/s2) = 0.85 N. There are two forces
on the object: a force up from the scale (S), and the weight down, W=mg = (25.0 kg)(9.80 m/s2) =
245 N. Then S=F+W= 245 N + 0.85 N = 246 N.
E4-39 Let ∆x= 15 m be the length; tw= 90 s, the time to walk the stalled Escalator; ts= 60
s, the time to ride the moving Escalator; and tm, the time to walk up the moving Escalator.
The walking speed of the person relative to a fixed Escalator is vwe = ∆x/tw; the speed of the
Escalator relative to the ground is veg = ∆x/ts; and the speed of the walking person relative to the
46
ground on a moving Escalator is vwg = ∆x/tm. But these three speeds are related by vwg =vwe +veg .
Combine all the above:
vwg =vwe +veg,
x
tm
=x
tw
+x
ts
,
1
tm
=1
tw
+1
ts
.
Putting in the numbers, tm= 36 s.
E4-40 Let vwbe the walking speed, vsbe the sidewalk speed, and vm=vw+vsbe Mary’s speed
while walking on the moving sidewalk. All three cover the same distance x, so vi=x/ti, where iis
one of w, s, or m. Put this into the Mary equation, and
1/tm= 1/tw+ 1/ts= 1/(150 s) + 1/(70 s) = 1/48 s.
E4-41 If it takes longer to fly westward then the speed of the plane (relative to the ground)
westward must be less than the speed of the plane eastward. We conclude that the jet-stream must
be blowing east. The speed of the plane relative to the ground is ve=vp+vjwhen going east and
vw=vpvjwhen going west. In either case the distance is the same, so x=viti, where iis e or
w. Since twteis given, we can write
twte=x
vpvjx
vp+vj
=x2vj
vp2vj2.
Solve the quadratic if you want, but since vjvpwe can neglect it in the denominator and
vj=vp2(0.83 h)/(2x) = (600 mi/h)2(0.417 h)/(2700 mi) = 56 mi/hr.
E4-42 The horizontal component of the rain drop’s velocity is 28 m/s. Since vx=vsin θ,v=
(28 m/s)/sin(64) = 31 m/s.
E4-43 (a) The position of the bolt relative to the elevator is ybe, the position of the bolt relative
to the shaft is ybs, and the position of the elevator relative to the shaft is yes. Zero all three positions
at t= 0; at this time v0,bs =v0,es = 8.0 ft/s.
The three equations describing the positions are
ybs =v0,bst1
2gt2,
yes =v0,est+1
2at2,
ybe +res =rbs,
where a= 4.0 m/s2is the upward acceleration of the elevator. Rearrange the last equation and
solve for ybe; get ybe =1
2(g+a)t2, where advantage was taken of the fact that the initial velocities
are the same.
Then
t=p2ybe/(g+a) = q2(9.0 ft)/(32 ft/s2+ 4 ft/s2) = 0.71 s
(b) Use the expression for ybs to find how the bolt moved relative to the shaft:
ybs =v0,bst1
2gt2= (8.0 ft)(0.71 s) 1
2(32 ft/s2)(0.71 s)2=2.4 ft.
47
E4-44 The speed of the plane relative to the ground is vpg = (810 km)/(1.9 h) = 426 km/h. The
velocity components of the plane relative to the air are vN= (480 km/h) cos(21) = 448 km/h and
vE= (480 km/h) sin(21) = 172 km/h. The wind must be blowing with a component of 172 km/h
to the west and a component of 448 426 = 22 km/h to the south.
E4-45 (a) Let ˆ
ipoint east and ˆ
jpoint north. The velocity of the torpedo is ~
v= (50 km/h)ˆ
isin θ+
(50 km/h)ˆ
jcos θ. The initial coordinates of the battleship are then ~
r0= (4.0 km)ˆ
isin(20) +
(4.0 km)ˆ
jcos(20) = (1.37 km)ˆ
i+ (3.76 km)ˆ
j. The final position of the battleship is ~
r= (1.37 km +
24 km/ht)ˆ
i+ (3.76 km)ˆ
j, where tis the time of impact. The final position of the torpedo is the
same, so
[(50 km/h)ˆ
isin θ+ (50 km/h)ˆ
jcos θ]t= (1.37 km + 24 km/ht)ˆ
i+ (3.76 km)ˆ
j,
or
[(50 km/h) sin θ]t24 km/ht= 1.37 km
and
[(50 km/h) cos θ]t= 3.76 km.
Dividing the top equation by the bottom and rearranging,
50 sin θ24 = 18.2 cos θ.
Use any trick you want to solve this. I used Maple and found θ= 46.8.
(b) The time to impact is then t= 3.76 km/[(50 km/h) cos(46.8)] = 0.110 h, or 6.6 minutes.
P4-1 Let ~
rAbe the position of particle of particle A, and ~
rBbe the position of particle B. The
equations for the motion of the two particles are then
~
rA=~
r0,A +~
vt,
=dˆ
j+vtˆ
i;
~
rB=1
2~
at2,
=1
2a(sin θˆ
i+ cos θˆ
j)t2.
A collision will occur if there is a time when ~
rA=~
rB. Then
dˆ
j+vtˆ
i=1
2a(sin θˆ
i+ cos θˆ
j)t2,
but this is really two equations: d=1
2at2cos θand vt =1
2at2sin θ.
Solve the second one for tand get t= 2v/(asin θ). Substitute that into the first equation, and
then rearrange,
d=1
2at2cos θ,
d=1
2a2v
asin θ2
cos θ,
sin2θ=2v
ad cos θ,
1cos2θ=2v2
ad cos θ,
0 = cos2θ+2v2
ad cos θ1.
48
This last expression is quadratic in cos θ. It simplifies the solution if we define b= 2v/(ad) = 2(3.0
m/s)2/([0.4 m/s2][30 m]) = 1.5, then
cos θ=b±b2+ 4
2=0.75 ±1.25.
Then cos θ= 0.5 and θ= 60.
P4-2 (a) The acceleration of the ball is ~
a= (1.20 m/s2)ˆ
i(9.81 m/s2)ˆ
j. Since ~
ais constant the
trajectory is given by ~
r=~
at2/2, since ~
v0= 0 and we choose ~
r0= 0. This is a straight line trajectory,
with a direction given by ~
a. Then
θ= arctan(9.81/1.20) = 83.0.
and R= (39.0 m)/tan(83.0) = 4.79 m. It will be useful to find H= (39.0 m)/sin(83.0) = 39.3 m.
(b) The magnitude of the acceleration of the ball is a=9.812+ 1.202(m/s2) = 9.88 m/s2. The
time for the ball to travel down the hypotenuse of the figure is then t=p2(39.3 m)/(9.88 m/s2) =
2.82 s.
(c) The magnitude of the speed of the ball at the bottom will then be
v=at = (9.88 m/s2)(2.82 s) = 27.9 m/s.
P4-3 (a) The rocket thrust is ~
T= (61.2 kN) cos(58.0)ˆ
i+ (61.2 kN) sin(58.0)ˆ
i= 32.4 kNˆ
i+
51.9 kNˆ
j. The net force on the rocket is the ~
F=~
T+~
W, or
~
F= 32.4 kNˆ
i+ 51.9 kNˆ
j(3030 kg)(9.81 m/s2)ˆ
j= 32.4 kNˆ
i+ 22.2 kNˆ
j.
The acceleration (until rocket cut-off) is this net force divided by the mass, or
~
a= 10.7m/s2ˆ
i+ 7.33m/s2ˆ
j.
The position at rocket cut-off is given by
~
r=~
at2/2 = (10.7m/s2ˆ
i+ 7.33m/s2ˆ
j)(48.0 s)2/2,
= 1.23×104mˆ
i+ 8.44×103mˆ
j.
The altitude at rocket cut-off is then 8.44 km.
(b) The velocity at rocket cut-off is
~
v=~
at= (10.7m/s2ˆ
i+ 7.33m/s2ˆ
j)(48.0 s) = 514 m/sˆ
i+ 352 m/sˆ
j,
this becomes the initial velocity for the “free fall” part of the journey. The rocket will hit the ground
after tseconds, where tis the solution to
0 = (9.81 m/s2)t2/2 + (352 m/s)t+ 8.44×103m.
The solution is t= 90.7 s. The rocket lands a horizontal distance of x=vxt= (514 m/s)(90.7 s) =
4.66×104m beyond the rocket cut-off; the total horizontal distance covered by the rocket is 46.6 km+
12.3 km = 58.9 km.
49
P4-4 (a) The horizontal speed of the ball is vx= 135 ft/s. It takes
t=x/vx= (30.0 ft)/(135 ft/s) = 0.222 s
to travel the 30 feet horizontally, whether the first 30 feet, the last 30 feet, or 30 feet somewhere in
the middle.
(b) The ball “falls” y=gt2/2 = (32 ft/s2)(0.222 s)2/2 = 0.789 ft while traveling the first
30 feet.
(c) The ball “falls” a total of y=gt2/2 = (32 ft/s2)(0.444 s)2/2 = 3.15 ft while traveling
the first 60 feet, so during the last 30 feet it must have fallen (3.15 ft) (0.789 ft) = 2.36 ft.
(d) The distance fallen because of acceleration is not linear in time; the distance moved horizon-
tally is linear in time.
P4-5 (a) The initial velocity of the ball has components
vx,0= (25.3 m/s) cos(42.0) = 18.8 m/s
and
vy,0= (25.3 m/s) sin(42.0) = 16.9 m/s.
The ball is in the air for t=x/vx= (21.8 m)/(18.8 m/s) = 1.16 s before it hits the wall.
(b) y=gt2/2 + vy,0t=(4.91 m/s2)(1.16 s)2+ (16.9 m/s)(1.16 s) = 13.0 m.
(c) vx=vx,0= 18.8 m/s. vy=gt +vy,0=(9.81 m/s2)(1.16 s) + (16.9 m/s) = 5.52 m/s.
(d) Since vy>0 the ball is still heading up.
P4-6 (a) The initial vertical velocity is vy,0=v0sin φ0. The time to the highest point is t=vy,0/g.
The highest point is H=gt2/2. Combining,
H=g(v0sin φ0/g)2/2 = v2
0sin2φ0/(2g).
The range is R= (v2
0/g) sin 2φ0= 2(v2
0/g) sin φ0cos φ0. Since tan θ=H/(R/2), we have
tan θ=2H
R=v2
0sin2φ0/g
2(v2
0/g) sin φ0cos φ0
=1
2tan φ0.
(b) θ= arctan(0.5 tan 45) = 26.6.
P4-7 The components of the initial velocity are given by v0x=v0cos θ= 56 ft/s and v0y=
v0sin θ= 106 ft/s where we used v0= 120 ft/s and θ= 62.
(a) To find hwe need only find out the vertical position of the stone when t= 5.5 s.
y=v0yt1
2gt2= (106 ft/s)(5.5 s) 1
2(32 ft/s2)(5.5 s)2= 99 ft.
(b) Look at this as a vector problem:
~
v=~
v0+~
at,
=v0xˆ
i+v0yˆ
jgˆ
jt,
=v0xˆ
i+ (v0ygt)ˆ
j,
= (56 ft/s)ˆ
i+(106 ft/s (32 ft/s2)(5.5 s)ˆ
j,
= (56 ft/s)ˆ
i+ (70.0 ft/s)ˆ
j.
50
The magnitude of this vector gives the speed when t= 5.5 s; v=p562+ (70)2ft/s= 90 ft/s.
(c) Highest point occurs when vy= 0. Solving Eq. 4-9(b) for time; vy=0=v0ygt =
(106 ft/s) (32 ft/s2)t;t= 3.31 s. Use this time in Eq. 4-10(b),
y=v0yt1
2gt2= (106 ft/s)(3.31 s) 1
2(32 ft/s2)(3.31 s)2= 176 ft.
P4-8 (a) Since R= (v2
0/g) sin 2φ0, it is sufficient to prove that sin 2φ0is the same for both
sin 2(45+α) and sin 2(45α).
sin 2(45±α) = sin(90±2α) = cos(±2α) = cos(2α).
Since the ±dropped out, the two quantities are equal.
(b) φ0= (1/2) arcsin(Rg/v2
0) = (1/2) arcsin((20.0 m)(9.81 m/s2)/(30.0 m/s)2) = 6.3. The other
choice is 906.3= 83.7.
P4-9 To score the ball must pass the horizontal distance of 50 m with an altitude of no less than
3.44 m. The initial velocity components are v0x=v0cos θand v0y=v0sin θwhere v0= 25 m/s,
and θis the unknown.
The time to the goal post is t=x/v0x=x/(v0cos θ).
The vertical motion is given by
y=v0yt1
2gt2= (v0sin θ)x
v0cos θ1
2gx
v0cos θ2
,
=xsin θ
cos θgx2
2v2
0
1
cos2θ.
In this last expression yneeds to be greater than 3.44 m. In this last expression use
1
cos2θ1 + 1 = 1
cos2θcos2θ
cos2θ+ 1 = 1cos2θ
cos2θ+ 1 = sin2θ
cos2θ+ 1 = tan2θ+ 1.
This gives for our yexpression
y=xtan θgx2
2v0tan2θ+ 1,
which can be combined with numbers and constraints to give
(3.44 m) (50 m) tan θ(9.8 m/s2)(50 m)2
2(25 m/s)2tan2θ+ 1,
3.44 50 tan θ20 tan2θ+ 1,
0≤ −20 tan2θ+ 50 tan θ23
Solve, and tan θ= 1.25 ±0.65, so the allowed kicking angles are between θ= 31and θ= 62.
P4-10 (a) The height of the projectile at the highest point is H=Lsin θ. The amount of time
before the projectile hits the ground is t=p2H/g =p2Lsin θ/g. The horizontal distance covered
by the projectile in this time is x=vxt=vp2Lsin θ/g. The horizontal distance to the projectile
when it is at the highest point is x0=Lcos θ. The projectile lands at
D=xx0=vp2Lsin θ/g Lcos θ.
(b) The projectile will pass overhead if D > 0.
51
P4-11 v2=v2
x+v2
y. For a projectile vxis constant, so we need only evaluate d2(v2
y)/dt2. The first
derivative is 2vydvy/dt =2vyg. The derivative of this (the second derivative) is 2g dvy/dt = 2g2.
P4-12 |~
r|is a maximum when r2is a maximum. r2=x2+y2, or
r2= (vx,0t)2+ (gt2/2 + vy,0t)2,
= (v0tcos φ0)2+v0tsin φ0gt2/22,
=v2
0t2v0gt3sin φ0+g2t4/4.
We want to look for the condition which will allow dr2/dt to vanish. Since
dr2/dt = 2v2
0t3v0gt2sin φ0+g2t3
we can focus on the quadratic discriminant, b24ac, which is
9v2
0g2sin2φ08v2
0g2,
a quantity which will only be greater than zero if 9 sin2φ0>8. The critical angle is then
φc= arcsin(p8/9) = 70.5.
P4-13 There is a downward force on the balloon of 10.8 kN from gravity and an upward force
of 10.3 kN from the buoyant force of the air. The resultant of these two forces is 500 N down, but
since the balloon is descending at constant speed so the net force on the balloon must be zero. This
is possible because there is a drag force on the balloon of D=bv2, this force is directed upward.
The magnitude must be 500 N, so the constant bis
b=(500 N)
(1.88 m/s)2= 141 kg/m.
If the crew drops 26.5 kg of ballast they are “lightening” the balloon by
(26.5 kg)(9.81 m/s2) = 260 N.
This reduced the weight, but not the buoyant force, so the drag force at constant speed will now be
500 N 260 N = 240 N.
The new constant downward speed will be
v=pD/b =p(240 N)/(141 kg/m) = 1.30 m/s.
P4-14 The constant bis
b= (500 N)/(1.88 m/s) = 266 N ·s/m.
The drag force after “lightening” the load will still be 240 N. The new downward speed will be
v=D/b = (240 N)/(266 N ·s/m) = 0.902 m/s.
P4-15 (a) Initially v0= 0, so D= 0, the only force is the weight, so a=g.
(b) After some time the acceleration is zero, then W=D, or bvT2=mg, or vT=pmg/b.
(c) When v=vT/2 the drag force is D=bvT2/4 = mg/4, so the net force is F=DW=
3mg/4. The acceleration is the a=3g/4.
52
P4-16 (a) The net force on the barge is F=D=bv, this results in a differential equation
m dv/dt =bv, which can be written as
dv/v =(b/m)dt,
Zdv/v =(b/m)Zdt,
ln(vf/vi) = bt/m.
Then t= (m/b) ln(vi/vf).
(b) t= [(970 kg)/(68 N ·s/m)] ln(32/8.3) = 19 s.
P4-17 (a) The acceleration is the time derivative of the velocity,
ay=dvy
dt =d
dt mg
b1ebt/m=mg
b
b
mebt/m,
which can be simplified as ay=gebt/m. For large tthis expression approaches 0; for small tthe
exponent can be expanded to give
ayg1bt
m=gvTt,
where in the last line we made use of Eq. 4-24.
(b) The position is the integral of the velocity,
Zt
0
vydt =Zt
0mg
b1ebt/m dt,
Zt
0
dy
dt dt =mg
bt(m/b)ebt/m
t
0,
Zy
0
dy =vTt+vT
gevTt/g 1,
y=vTt+vT
gevTt/g 1.
P4-18 (a) We have vy=vT(1 ebt/m) from Eq. 4-22; this can be substituted into the last line
of the solution for P4-17 to give
y95 =vTtvy
g.
We can also rearrange Eq. 4-22 to get t=(m/b) ln(1 vy/vT), so
y95 =vT2/g ln(1 vy/vT)vy
vT.
But vy/vT= 0.95, so
y95 =vT2/g (ln(0.05) 0.95) = vT2/g (ln 20 19/20) .
(b) y95 = (42 m/s)2/(9.81 m/s2)(2.05) = 370 m.
P4-19 (a) Convert units first. v= 86.1 m/s, a= 0.05(9.81 m/s2) = 0.491 m/s2. The minimum
radius is r=v2/a = (86.1 m/s)/(0.491 m/s2) = 15 km.
(b) v=ar =p(0.491 m/s2)(940 m) = 21.5 m/s. That’s 77 km/hr.
53
P4-20 (a) The position is given by ~
r=Rsin ωtˆ
i+R(1 cos ωt)ˆ
j, where ω= 2π/(20 s) = 0.314 s1
and R= 3.0 m. When t= 5.0 s ~
r= (3.0 m)ˆ
i+ (3.0 m)ˆ
j; when t= 7.5 s ~
r= (2.1 m)ˆ
i+ (5.1 m)ˆ
j;
when t= 10 s ~
r= (6.0 m)ˆ
j. These vectors have magnitude 4.3 m, 5.5 m and 6.0 m, respectively. The
vectors have direction 45, 68and 90respectively.
(b) ∆~
r= (3.0 m)ˆ
i+ (3.0 m)ˆ
j, which has magnitude 4.3 m and direction 135.
(c) vav = ∆r/t= (4.3 m)/(5.0 s) = 0.86 m/s. The direction is the same as ∆~
r.
(d) The velocity is given by ~
v=cos ωtˆ
i+sin ωtˆ
j. At t= 5.0 s ~
v= (0.94 m/s)ˆ
j; at t= 10 s
~
v= (0.94 m/s)ˆ
i.
(e) The acceleration is given by ~
a=2sin ωtˆ
i+2cos ωtˆ
j. At t= 5.0 s ~
a= (0.30 m/s2)ˆ
i;
at t= 10 s ~
a= (0.30 m/s2)ˆ
j.
P4-21 Start from where the stone lands; in order to get there the stone fell through a vertical
distance of 1.9 m while moving 11 m horizontally. Then
y=1
2gt2which can be written as t=r2y
g.
Putting in the numbers, t= 0.62 s is the time of flight from the moment the string breaks. From
this time find the horizontal velocity,
vx=x
t=(11 m)
(0.62 s) = 18 m/s.
Then the centripetal acceleration is
ac=v2
r=(18 m/s)2
(1.4 m) = 230 m/s2.
P4-22 (a) The path traced out by her feet has circumference c1= 2πr cos 50, where ris the
radius of the earth; the path traced out by her head has circumference c2= 2π(r+h) cos 50, where
his her height. The difference is ∆c= 2πh cos 50= 2π(1.6 m) cos 50= 6.46 m.
(b) a=v2/r = (2πr/T )2/r = 4π2r/T 2. Then ∆a= 4π2r/T 2. Note that ∆r=hcos θ! Then
a= 4π2(1.6 m) cos 50/(86400 s)2= 5.44×109m/s2.
P4-23 (a) A cycloid looks something like this:
(b) The position of the particle is given by
~
r= (Rsin ωt +ωRt)ˆ
i+ (Rcos ωt +R)ˆ
j.
The maximum value of yoccurs whenever cos ωt = 1. The minimum value of yoccurs whenever
cos ωt =1. At either of those times sin ωt = 0.
The velocity is the derivative of the displacement vector,
~
v= (cos ωt +ωR)ˆ
i+ (sin ωt)ˆ
j.
When yis a maximum the velocity simplifies to
~
v= (2ωR)ˆ
i+ (0)ˆ
j.
54
When yis a minimum the velocity simplifies to
~
v= (0)ˆ
i+ (0)ˆ
j.
The acceleration is the derivative of the velocity vector,
~
a= (2sin ωt)ˆ
i+ (2cos ωt)ˆ
j.
When yis a maximum the acceleration simplifies to
~
a= (0)ˆ
i+ (2)ˆ
j.
When yis a minimum the acceleration simplifies to
~
a= (0)ˆ
i+ (2)ˆ
j.
P4-24 (a) The speed of the car is vc= 15.3 m/s. The snow appears to fall with an angle θ=
arctan(15.3/7.8) = 63.
(b) The apparent speed is p(15.3)2+ (7.8)2(m/s) = 17.2 m/s.
P4-25 (a) The decimal angles are 89.994250and 89.994278. The earth moves in the orbit
around the sun with a speed of v= 2.98×104m/s (Appendix C). The speed of light is then between
c= (2.98×104m/s) tan(89.994250)=2.97×108m/s and c= (2.98 ×104m/s) tan(89.994278) =
2.98×108m/s. This method is highly sensitive to rounding. Calculating the orbital speed from the
radius and period of the Earth’s orbit will likely result in different answers!
P4-26 (a) Total distance is 2l, so t0= 2l/v.
(b) Assume wind blows east. Time to travel out is t1=l/(v+u), time to travel back is
t2=l/(vu). Total time is sum, or
tE=l
v+u+l
vu=2lv
v2u2=t0
1u2/v2.
If wind blows west the times reverse, but the result is otherwise the same.
(c) Assume wind blows north. The airplane will still have a speed of vrelative to the wind, but
it will need to fly with a heading away from east. The speed of the plane relative to the ground will
be v2u2. This will be the speed even when it flies west, so
tN=2l
v2u2=t0
p1u2/v2.
(d) If u>vthe wind sweeps the plane along in one general direction only; it can never fly back.
Sort of like a black hole event horizon.
P4-27 The velocity of the police car with respect to the ground is ~
vpg =76km/hˆ
i. The velocity
of the motorist with respect the ground is ~
vmg =62 km/hˆ
j.
The velocity of the motorist with respect to the police car is given by solving
~
vmg =~
vmp +~
vpg,
so ~
vmp = 76km/hˆ
i62 km/hˆ
j. This velocity has magnitude
vmp =p(76km/h)2+ (62 km/h)2= 98 km/h.
55
The direction is
θ= arctan(62 km/h)/(76km/h) = 39,
but that is relative to ˆ
i. We want to know the direction relative to the line of sight. The line of sight
is
α= arctan(57 m)/(41 m) = 54
relative to ˆ
i, so the answer must be 15.
P4-28 (a) The velocity of the plane with respect to the air is ~
vpa; the velocity of the air with
respect to the ground is ~
vag, the velocity of the plane with respect to the ground is ~
vpg. Then
~
vpg =~
vpa +~
vag. This can be represented by a triangle; since the sides are given we can find the
angle between ~
vag and ~
vpg (points north) using the cosine law
θ= arccos (135)2(135)2(70)2
2(135)(70) = 75.
(b) The direction of ~
vpa can also be found using the cosine law,
θ= arccos (70)2(135)2(135)2
2(135)(135) = 30.
56
E5-1 There are three forces which act on the charged sphere— an electric force, FE, the force of
gravity, W, and the tension in the string, T. All arranged as shown in the figure on the right below.
(a) Write the vectors so that they geometrically show that the sum is zero, as in the figure
on the left below. Now W=mg = (2.8×104kg)(9.8 m/s2)=2.7×103N. The magnitude
of the electric force can be found from the tangent relationship, so FE=Wtan θ= (2.7×103
N) tan(33) = 1.8×103N.
θ
(a)
W
T F Eθ
FE
W
T
(b)
(b) The tension can be found from the cosine relation, so
T=W/ cos θ= (2.7×103N)/cos(33) = 3.2×103N.
E5-2 (a) The net force on the elevator is F=ma =W a/g = (6200 lb)(3.8 ft/s2)/(32 ft/s2) =
740 lb. Positive means up. There are two force on the elevator: a weight Wdown and a tension
from the cable Tup. Then F=TWor T=F+W= (740 lb) + (6200 lb) = 6940 lb.
(b) If the elevator acceleration is down then F=740 lb; consequently T=F+W= (740 lb)+
(6200 lb) = 5460 lb.
E5-3 (a) The tension Tis up, the weight Wis down, and the net force Fis in the direction of the
acceleration (up). Then F=TW. But F=ma and W=mg, so
m=T/(a+g) = (89 N)/[(2.4 m/s2) + (9.8 m/s2)] = 7.3 kg.
(b) T= 89 N. The direction of velocity is unimportant. In both (a) and (b) the acceleration is
up.
E5-4 The average speed of the elevator during deceleration is vav = 6.0 m/s. The time to stop the
elevator is then t= (42.0 m)/(6.0 m/s) = 7.0 s. The deceleration is then a= (12.0 m/s)/(7.0 s) =
1.7 m/s2. Since the elevator is moving downward but slowing down, then the acceleration is up,
which will be positive.
The net force on the elevator is F=ma; this is equal to the tension Tminus the weight W.
Then
T=F+W=ma +mg = (1600 kg)[(1.7 m/s2) + (9.8 m/s2)] = 1.8×104N.
E5-5 (a) The magnitude of the man’s acceleration is given by
a=m2m1
m2+m1
g=(110 kg) (74 kg)
(110 kg) + (74 kg) g= 0.2g,
and is directed down. The time which elapses while he falls is found by solving y=v0yt+1
2ayt2,
or, with numbers, (12 m) = (0)t+1
2(0.2g)t2which has the solutions t=±3.5 s. The velocity
with which he hits the ground is then v=v0y+ayt= (0) + (0.2g)(3.5 s) = 6.9 m/s.
57
(b) Reducing the speed can be accomplished by reducing the acceleration. We can’t change Eq.
5-4 without also changing one of the assumptions that went into it. Since the man is hoping to
reduce the speed with which he hits the ground, it makes sense that he might want to climb up the
rope.
E5-6 (a) Although it might be the monkey which does the work, the upward force to lift him still
comes from the tension in the rope. The minimum tension to lift the log is T=Wl=mlg. The net
force on the monkey is TWm=mma. The acceleration of the monkey is then
a= (mlmm)g/mm= [(15 kg) (11 kg)](9.8 m/s2)/(11 kg) = 3.6 m/s2.
(b) Atwood’s machine!
a= (mlmm)g/(ml+mm) = [(15 kg) (11 kg)](9.8 m/s2)/[(15 kg) + (11 kg)] = 1.5 m/s.
(c) Atwood’s machine!
T= 2mlmmg/(ml+mm) = 2(15 kg)(11 kg)(9.8 m/s2)/[(15 kg) + (11 kg)] = 120 N.
E5-7 The weight of each car has two components: a component parallel to the cables W|| =Wsin θ
and a component normal to the cables W.. The normal component is “balanced” by the supporting
cable. The parallel component acts with the pull cable.
In order to accelerate a car up the incline there must be a net force up with magnitude F=ma.
Then F=Tabove Tbelow W||, or
T=ma +mg sin θ= (2800 kg)[(0.81 m/s2) + (9.8 m/s2) sin(35)] = 1.8×104N.
E5-8 The tension in the cable is T, the weight of the man + platform system is W=mg, and the
net force on the man + platform system is F=ma =W a/g =TW. Then
T=W a/g +W=W(a/g + 1) = (180 lb + 43 lb)[(1.2 ft/s2)/(32 ft/s2) + 1] = 231 lb.
E5-9 See Sample Problem 5-8. We need only apply the (unlabeled!) equation
µs= tan θ
to find the egg angle. In this case θ= tan1(0.04) = 2.3.
E5-10 (a) The maximum force of friction is F=µsN. If the rear wheels support half of the weight
of the automobile then N=W/2 = mg/2. The maximum acceleration is then
a=F/m =µsN/m =µsg/2.
(b) a= (0.56)(9.8 m/s2)/2 = 2.7 m/s2.
E5-11 The maximum force of friction is F=µsN. Since there is no motion in the ydirection the
magnitude of the normal force must equal the weight, N=W=mg. The maximum acceleration is
then
a=F/m =µsN/m =µsg= (0.95)(9.8 m/s2) = 9.3 m/s2.
E5-12 There is no motion in the vertical direction, so N=W=mg. Then µk=F/N =
(470 N)/[(9.8 m/s2)(79 kg)] = 0.61.
58
E5-13 A 75 kg mass has a weight of W= (75 kg)(9.8 m/s2) = 735 N, so the force of friction on
each end of the bar must be 368 N. Then
Ffs
µs
=(368 N)
(0.41) = 900 N.
E5-14 (a) There is no motion in the vertical direction, so N=W=mg.
To get the box moving you must overcome static friction and push with a force of PµsN=
(0.41)(240 N) = 98 N.
(b) To keep the box moving at constant speed you must push with a force equal to the kinetic
friction, P=µkN= (0.32)(240 N) = 77 N.
(c) If you push with a force of 98 N on a box that experiences a (kinetic) friction of 77 N, then
the net force on the box is 21 N. The box will accelerate at
a=F/m =F g/W = (21 N)(9.8 m/s2)/(240 N) = 0.86 m/s2.
E5-15 (a) The maximum braking force is F=µsN. There is no motion in the vertical direction,
so N=W=mg. Then F=µsmg = (0.62)(1500 kg)(9.8 m/s2) = 9100 N.
(b) Although we still use F=µsN,N6=Won an incline! The weight has two components;
one which is parallel to the surface and the other which is perpendicular. Since there is no motion
perpendicular to the surface we must have N=W=Wcos θ. Then
F=µsmg cos θ= (0.62)(1500 kg)(9.8 m/s2) cos(8.6) = 9000 N.
E5-16 µs= tan θis the condition for an object to sit without slipping on an incline. Then
θ= arctan(0.55) = 29. The angle should be reduced by 13.
E5-17 (a) The force of static friction is less than µsN, where Nis the normal force. Since the
crate isn’t moving up or down, PFy=0=NW. So in this case N=W=mg = (136 kg)(9.81
m/s2) = 1330 N. The force of static friction is less than or equal to (0.37)(1330 N) = 492 N; moving
the crate will require a force greater than or equal to 492 N.
(b) The second worker could lift upward with a force L, reducing the normal force, and hence
reducing the force of friction. If the first worker can move the block with a 412 N force, then
412 µsN. Solving for N, the normal force needs to be less than 1110 N. The crate doesn’t move
off the table, so then N+L=W, or L=WN= (1330 N) (1110 N) = 220 N.
(c) Or the second worker can help by adding a push so that the total force of both workers is
equal to 492 N. If the first worker pushes with a force of 412 N, the second would need to push with
a force of 80 N.
E5-18 The coefficient of static friction is µs= tan(28.0) = 0.532. The acceleration is a=
2(2.53 m)/(3.92 s)2=.329 m/s2. We will need to insert a negative sign since this is downward.
The weight has two components: a component parallel to the plane, W|| =mg sin θ; and a
component perpendicular to the plane, W=mg cos θ. There is no motion perpendicular to the
plane, so N=W. The kinetic friction is then f=µkN=µkmg cos θ. The net force parallel to
the plane is F=ma =fW|| =µkmg cos θmg sin θ. Solving this for µk,
µk= (a+gsin θ)/(gcos θ),
= [(0.329 m/s2) + (9.81 m/s2) sin(28.0)]/[((9.81 m/s2) cos(28.0)] = 0.494.
59
E5-19 The acceleration is a=2d/t2, where d= 203 m is the distance down the slope and tis
the time to make the run.
The weight has two components: a component parallel to the incline, W|| =mg sin θ; and a
component perpendicular to the incline, W=mg cos θ. There is no motion perpendicular tot he
plane, so N=W. The kinetic friction is then f=µkN=µkmg cos θ. The net force parallel to
the plane is F=ma =fW|| =µkmg cos θmg sin θ. Solving this for µk,
µk= (a+gsin θ)/(gcos θ),
= (gsin θ2d/t2)/(gcos θ).
If t= 61 s, then
µk=(9.81 m/s2) sin(3.0)2(203 m)/(61 s)2
(9.81 m/s2) cos(3.0)= 0.041;
if t= 42 s, then
µk=(9.81 m/s2) sin(3.0)2(203 m)/(42 s)2
(9.81 m/s2) cos(3.0)= 0.029.
E5-20 (a) If the block slides down with constant velocity then a= 0 and µk= tan θ. Not only
that, but the force of kinetic friction must be equal to the parallel component of the weight, f=W||.
If the block is projected up the ramp then the net force is now 2W|| = 2mg sin θ. The deceleration
is a= 2gsin θ; the block will travel a time t=v0/a before stopping, and travel a distance
d=at2/2 + v0t=a(v0/a)2/2 + v0(v0/a) = v2
0/(2a) = v2
0/(4gsin θ)
before stopping.
(b) Since µk< µs, the incline is not steep enough to get the block moving again once it stops.
E5-21 Let a1be acceleration down frictionless incline of length l, and t1the time taken. The a2
is acceleration down “rough” incline, and t2= 2t1is the time taken. Then
l=1
2a1t2
1and l=1
2a2(2t1)2.
Equate and find a1/a2= 4.
There are two force which act on the ice when it sits on the frictionless incline. The normal force
acts perpendicular to the surface, so it doesn’t contribute any components parallel to the surface.
The force of gravity has a component parallel to the surface, given by
W|| =mg sin θ,
and a component perpendicular to the surface given by
W=mg cos θ.
The acceleration down the frictionless ramp is then
a1=W||
m=gsin θ.
When friction is present the force of kinetic friction is fk=µkN; since the ice doesn’t move
perpendicular to the surface we also have N=W; and finally the acceleration down the ramp is
a2=W|| fk
m=g(sin θµcos θ).
60
Previously we found the ratio of a1/a2, so we now have
sin θ= 4 sin θ4µcos θ,
sin 33= 4 sin 334µcos 33,
µ= 0.49.
E5-22 (a) The static friction between Aand the table must be equal to the weight of block Bto
keep Afrom sliding. This means mBg=µs(mA+mC)g, or mc=mBsmA= (2.6 kg)/(0.18)
(4.4 kg) = 10 kg.
(b) There is no up/down motion for block A, so f=µkN=µkmAg. The net force on the system
containing blocks Aand Bis F=WBf=mBgµkmAg; the acceleration of this system is then
a=gmBµkmA
mA+mB
= (9.m/s2)(2.6 kg) (0.15)(4.4 kg)
(2.6 kg) + (4.4 kg) = 2.7 m/s2.
E5-23 There are four forces on the block— the force of gravity, W=mg; the normal force,
N; the horizontal push, P, and the force of friction, f. Choose the coordinate system so that
components are either parallel (x-axis) to the plane or perpendicular (y-axis) to it. θ= 39. Refer
to the figure below.
P
θ
W
f
N
The magnitudes of the xcomponents of the forces are Wx=Wsin θ,Px=Pcos θand f; the
magnitudes of the ycomponents of the forces are Wy=Wcos θ,Py=Psin θ.
(a) We consider the first the case of the block moving up the ramp; then fis directed down.
Newton’s second law for each set of components then reads as
XFx=PxfWx=Pcos θfWsin θ=max,
XFy=NPyWy=NPsin θWcos θ=may
Then the second equation is easy to solve for N
N=Psin θ+Wcos θ= (46 N) sin(39) + (4.8 kg)(9.8 m/s2) cos(39) = 66 N.
The force of friction is found from f=µkN= (0.33)(66 N) = 22 N. This is directed down the incline
while the block is moving up. We can now find the acceleration in the xdirection.
max=Pcos θfWsin θ,
= (46 N) cos(39)(22 N) (4.8 kg)(9.8 m/s2) sin(39) = 16 N.
So the block is slowing down, with an acceleration of magnitude 3.3 m/s2.
(b) The block has an initial speed of v0x= 4.3 m/s; it will rise until it stops; so we can use
vy= 0 = v0y+aytto find the time to the highest point. Then t= (vyv0y)/ay=(4.3 m/s)/(3.3
m/s2= 1.3 s. Now that we know the time we can use the other kinematic relation to find the
distance
y=v0yt+1
2ayt2= (4.3 m/s)(1.3 s) + 1
2(3.3 m/s2)(1.3 s)2= 2.8 m
61
(c) When the block gets to the top it might slide back down. But in order to do so the frictional
force, which is now directed up the ramp, must be sufficiently small so that f+PxWx. Solving
for fwe find fWxPxor, using our numbers from above, f≤ −6 N. Is this possible? No, so
the block will not slide back down the ramp, even if the ramp were frictionless, while the horizontal
force is applied.
E5-24 (a) The horizontal force needs to overcome the maximum static friction, so PµsN=
µsmg = (0.52)(12 kg)(9.8 m/s2) = 61 N.
(b) If the force acts upward from the horizontal then there are two components: a horizontal
component Px=Pcos θand a vertical component Py=Psin θ. The normal force is now given by
W=Py+N; consequently the maximum force of static friction is now µsN=µs(mg Psin θ).
The block will move only if Pcos θµs(mg Psin θ), or
Pµsmg
cos θ+µssin θ=(0.52)(12 kg)(9.8 m/s2)
cos(62) + (0.52) sin(62)= 66 N.
(c) If the force acts downward from the horizontal then θ=62, so
Pµsmg
cos θ+µssin θ=(0.52)(12 kg)(9.8 m/s2)
cos(62) + (0.52) sin(62)= 5900 N.
E5-25 (a) If the tension acts upward from the horizontal then there are two components: a hori-
zontal component Tx=Tcos θand a vertical component Ty=Tsin θ. The normal force is now given
by W=Ty+N; consequently the maximum force of static friction is now µsN=µs(WTsin θ).
The crate will move only if Tcos θµs(WTsin θ), or
PµsW
cos θ+µssin θ=(0.52)(150 lb)
cos(17) + (0.52) sin(17)= 70 lb.
(b) Once the crate starts to move then the net force on the crate is F=Txf. The acceleration
is then
a=g
W[Tcos θµk(WTsin θ)],
=(32 ft/s2)
(150 lb) {(70 lb) cos(17)(0.35)[(150 lb) (70 lb) sin(17)]},
= 4.6 ft/s2.
E5-26 If the tension acts upward from the horizontal then there are two components: a horizontal
component Tx=Tcos θand a vertical component Ty=Tsin θ. The normal force is now given by
W=Ty+N; consequently the maximum force of static friction is now µsN=µs(WTsin θ). The
crate will move only if Tcos θµs(WTsin θ), or
WTcos θs+Tsin θ.
We want the maximum, so we find dW/dθ,
dW/dθ =(Ts) sin θ+Tcos θ,
which equals zero when µs= tan θ. For this problem θ= arctan(0.35) = 19, so
W(1220 N) cos(19)/(0.35) + (1220 N) sin(19) = 3690 N.
62
E5-27 The three force on the know above Amust add to zero. Construct a vector diagram:
~
TA+~
TB+~
Td= 0, where ~
Tdrefers to the diagonal rope. TAand TBmust be related by TA=
TBtan θ, where θ= 41.
T
T
TA
B
d
There is no up/down motion of block B, so N=WBand f=µsWB. Since block Bis at rest
f=TB. Since block Ais at rest WA=TA. Then
WA=WB(µstan θ) = (712 N)(0.25) tan(41) = 155 N.
E5-28 (a) Block 2 doesn’t move up/down, so N=W2=m2gand the force of friction on block 2
is f=µkm2g. Block 1 is on a frictionless incline; only the component of the weight parallel to the
surface contributes to the motion, and W|| =m1gsin θ. There are two relevant forces on the two
mass system. The effective net force is the of magnitude W|| f, so the acceleration is
a=gm1sin θµkm2
m1+m2
= (9.81 m/s2)(4.20 kg) sin(27)(0.47)(2.30 kg)
(4.20 kg) + (2.30 kg) = 1.25 m/s2.
The blocks accelerate down the ramp.
(b) The net force on block 2 is F=m2a=Tf. The tension in the cable is then
T=m2a+µkm2g= (2.30 kg)[(1.25 m/s2) + (0.47)(9.81 m/s2)] = 13.5 N.
E5-29 This problem is similar to Sample Problem 5-7, except now there is friction which can act
on block B. The relevant equations are now for block B
NmBgcos θ= 0
and
TmBgsin θ±f=mBa,
where the sign in front of fdepends on the direction in which block Bis moving. If the block is
moving up the ramp then friction is directed down the ramp, and we would use the negative sign.
If the block is moving down the ramp then friction will be directed up the ramp, and then we will
use the positive sign. Finally, if the block is stationary then friction we be in such a direction as to
make a= 0.
For block Athe relevant equation is
mAgT=mAa.
Combine the first two equations with f=µN to get
TmBgsin θ±µmBgcos θ=mBa.
63
Take some care when interpreting friction for the static case, since the static value of µyields the
maximum possible static friction force, which is not necessarily the actual static frictional force.
Combine this last equation with the block Aequation,
mAgmAamBgsin θ±µmBgcos θ=mBa,
and then rearrange to get
a=gmAmBsin θ±µmBcos θ
mA+mB
.
For convenience we will use metric units; then the masses are mA= 13.2 kg and mB= 42.6 kg. In
addition, sin 42= 0.669 and cos 42= 0.743.
(a) If the blocks are originally at rest then
mAmBsin θ= (13.2 kg) (42.6 kg)(0.669) = 15.3 kg
where the negative sign indicates that block Bwould slide downhill if there were no friction.
If the blocks are originally at rest we need to consider static friction, so the last term can be as
large as
µmBcos θ= (.56)(42.6 kg)(0.743) = 17.7 kg.
Since this quantity is larger than the first static friction would be large enough to stop the blocks
from accelerating if they are at rest.
(b) If block Bis moving up the ramp we use the negative sign, and the acceleration is
a= (9.81 m/s2)(13.2 kg) (42.6 kg)(0.669) (.25)(42.6 kg)(0.743)
(13.2 kg) + (42.6 kg) =4.08 m/s2.
where the negative sign means down the ramp. The block, originally moving up the ramp, will
slow down and stop. Once it stops the static friction takes over and the results of part (a) become
relevant.
(c) If block Bis moving down the ramp we use the positive sign, and the acceleration is
a= (9.81 m/s2)(13.2 kg) (42.6 kg)(0.669) + (.25)(42.6 kg)(0.743)
(13.2 kg) + (42.6 kg) =1.30 m/s2.
where the negative sign means down the ramp. This means that if the block is moving down the
ramp it will continue to move down the ramp, faster and faster.
E5-30 The weight can be resolved into a component parallel to the incline, W|| =Wsin θand a
component that is perpendicular, W=Wcos θ. There are two normal forces on the crate, one from
each side of the trough. By symmetry we expect them to have equal magnitudes; since they both
act perpendicular to their respective surfaces we expect them to be perpendicular to each other.
They must add to equal the perpendicular component of the weight. Since they are at right angles
and equal in magnitude, this yields N2+N2=W2, or N=W/2.
Each surface contributes a frictional force f=µkN=µkW/2; the total frictional force is
then twice this, or 2µkW. The net force on the crate is F=Wsin θ2µkWcos θdown the
ramp. The acceleration is then
a=g(sin θ2µkcos θ).
64
E5-31 The normal force between the top slab and the bottom slab is N=Wt=mtg. The force
of friction between the top and the bottom slab is fµN =µmtg. We don’t yet know if the slabs
slip relative to each other, so we don’t yet know what kind of friction to consider.
The acceleration of the top slab is
at= (110 N)/(9.7 kg) µ(9.8 m/s2) = 11.3 m/s2µ(9.8 m/s2).
The acceleration of the bottom slab is
ab=µ(9.8 m/s2)(9.7,kg)/(42 kg) = µ(2.3 m/s2).
Can these two be equal? Only if µ0.93. Since the static coefficient is less than this, the block
slide. Then at= 7.6 m/s2and ab= 0.87 m/s2.
E5-32 (a) Convert the speed to ft/s: v= 88 ft/s. The acceleration is
a=v2/r = (88 ft/s)2/(25 ft) = 310 ft/s2.
(b) a= 310 ft/s2g/(32 ft/s2) = 9.7g.
E5-33 (a) The force required to keep the car in the turn is F=mv2/r =W v2/(rg), or
F= (10700 N)(13.4 m/s)2/[(61.0 m)(9.81 m/s2)] = 3210 N.
(b) The coefficient of friction required is µs=F/W = (3210 N)/(10700 N) = 0.300.
E5-34 (a) The proper banking angle is given by
θ= arctan v2
Rg = arctan (16.7 m/s)2
(150 m)(9.8 m/s2)= 11.
(b) If the road is not banked then the force required to keep the car in the turn is F=mv2/r =
W v2/(Rg) and the required coefficient of friction is
µs=F/W =v2
Rg =(16.7 m/s)2
(150 m)(9.8 m/s2)= 0.19.
E5-35 (a) This conical pendulum makes an angle θ= arcsin(0.25/1.4) = 10with the vertical.
The pebble has a speed of
v=pRg tan θ=p(0.25 m)(9.8 m/s2) tan(10) = 0.66 m/s.
(b) a=v2/r = (0.66 m/s)2/(0.25 m) = 1.7 m/s2.
(c) T=mg/cosθ = (0.053 kg)(9.8 m/s2)/cos(10) = 0.53 N.
E5-36 Ignoring air friction (there must be a forward component to the friction!), we have a nor-
mal force upward which is equal to the weight: N=mg = (85 kg)(9.8 m/s2) = 833 N. There
is a sideways component to the friction which is equal tot eh centripetal force, F=mv2/r =
(85 kg)(8.7 m/s)2/(25 m) = 257 N. The magnitude of the net force of the road on the person is
F=p(833 N)2+ (257 N)2= 870 N,
and the direction is θ= arctan(257/833) = 17off of vertical.
65
E5-37 (a) The speed is v= 2πrf = 2π(5.3×1011 m)(6.6×1015/s) = 2.2×106m/s.
(b) The acceleration is a=v2/r = (2.2×106m/s)2/(5.3×1011m) = 9.1×1022 m/s2.
(c) The net force is F=ma = (9.1×1031 kg)(9.1×1022 m/s2) = 8.3×108N.
E5-38 The basket has speed v= 2πr/t. The basket experiences a frictional force F=mv2/r =
m(2πr/t)2/r = 4π2mr/t2. The coefficient of static friction is µs=F/N =F/W . Combining,
µs=4π2r
gt2=4π2(4.6 m)
(9.8 m/s2)(24 s)2= 0.032.
E5-39 There are two forces on the hanging cylinder: the force of the cord pulling up Tand the
force of gravity W=Mg. The cylinder is at rest, so these two forces must balance, or T=W.
There are three forces on the disk, but only the force of the cord on the disk Tis relevant here, since
there is no friction or vertical motion.
The disk undergoes circular motion, so T=mv2/r. We want to solve this for vand then express
the answer in terms of m,M,r, and G.
v=rT r
m=rMgr
m.
E5-40 (a) The frictional force stopping the car is F=µsN=µsmg. The deceleration of the car
is then a=µsg. If the car is moving at v= 13.3 m/s then the average speed while decelerating
is half this, or vav = 6.7 m/s. The time required to stop is t=x/vav = (21 m)/(6.7 m/s) = 3.1 s.
The deceleration is a= (13.3 m/s)/(3.1 s) = 4.3 m/s2. The coefficient of friction is µs=a/g =
(4.3 m/s2)/(9.8 m/s2) = 0.44.
(b) The acceleration is the same as in part (a), so r=v2/a = (13.3 m/s)2/(4.3 m/s2) = 41 m.
E5-41 There are three forces to consider: the normal force of the road on the car N; the force of
gravity on the car W; and the frictional force on the car f. The acceleration of the car in circular
motion is toward the center of the circle; this means the net force on the car is horizontal, toward
the center. We will arrange our coordinate system so that ris horizontal and zis vertical. Then the
components of the normal force are Nr=Nsin θand Nz=Ncos θ; the components of the frictional
force are fr=fcos θand fz=fsin θ.
The direction of the friction depends on the speed of the car. The figure below shows the two
force diagrams.
W
f
N
θ
W
N
θ
f
The turn is designed for 95 km/hr, at this speed a car should require no friction to stay on the
road. Using Eq. 5-17 we find that the banking angle is given by
tan θb=v2
rg =(26 m/s)2
(210 m)(9.8 m/s2)= 0.33,
66
for a bank angle of θb= 18.
(a) On the rainy day traffic is moving at 14 m/s. This is slower than the rated speed, so any
frictional force must be directed up the incline. Newton’s second law is then
XFr=Nrfr=Nsin θfcos θ=mv2
r,
XFz=Nz+fzW=Ncos θ+fsin θmg = 0.
We can substitute f=µsNto find the minimum value of µswhich will keep the cars from slipping.
There will then be two equations and two unknowns, µsand N. Solving for N,
N(sin θµscos θ) = mv2
rand N(cos θ+µssin θ) = mg.
Combining,
(sin θµscos θ)mg = (cos θ+µssin θ)mv2
r
Rearrange,
µs=gr sin θv2cos θ
gr cos θ+v2sin θ.
We know all the numbers. Put them in and we’ll find µs= 0.22
(b) Now the frictional force will point the other way, so Newton’s second law is now
XFr=Nr+fr=Nsin θ+fcos θ=mv2
r,
XFz=NzfzW=Ncos θfsin θmg = 0.
The bottom equation can be rearranged to show that
N=mg
cos θµssin θ.
This can be combined with the top equation to give
mg sin θ+µscos θ
cos θµssin θ=mv2
r.
We can solve this final expression for vusing all our previous numbers and get v= 35 m/s. That’s
about 130 km/hr.
E5-42 (a) The net force on the person at the top of the Ferris wheel is mv2/r =WNt, pointing
down. The net force on the bottom is still mv2/r, but this quantity now equals NbWand is point
up. Then Nb= 2WNt= 2(150 lb) (125 lb) = 175 lb.
(b) Doubling the speed would quadruple the net force, so the new scale reading at the top would
be (150 lb) 4[(150 lb) (125 lb)] = 50 lb. Wee!
E5-43 The net force on the object when it is not sliding is F=mv2/r; the speed of the object is
v= 2πrf (fis rotational frequency here), so F= 4π2mrf2. The coefficient of friction is then at
least µs=F/W = 4π2rf2/g. If the object stays put when the table rotates at 33 1
3rev/min then
µs4π2(0.13 m)(33.3/60 /s)2/(9.8 m/s2) = 0.16.
If the object slips when the table rotates at 45.0 rev/min then
µs4π2(0.13 m)(45.0/60 /s)2/(9.8 m/s2) = 0.30.
67
E5-44 This is effectively a banked highway problem if the pilot is flying correctly.
r=v2
gtan θ=(134 m/s)2
(9.8 m/s2) tan(38.2)= 2330 m.
E5-45 (a) Assume that frigate bird flies as well as a pilot. Then this is a banked highway problem.
The speed of the bird is given by v2=gr tan θ. But there is also vt = 2πr, so 2πv2=gvt tan θ, or
v=gt tan θ
2π=(9.8 m/s2)(13 s) tan(25)
2π= 9.5 m/s.
(b) r=vt/(2π) = (9.5 m/s)(13 s)/(2π) = 20 m.
E5-46 (a) The radius of the turn is r=p(33 m)2(18 m)2= 28 m. The speed of the plane is v=
2πrf = 2π(28 m)(4.4/60 /s) = 13 m/s. The acceleration is a=v2/r = (13 m/s)2/(28 m) = 6.0m/s2.
(b) The tension has two components: Tx=Tcos θand Ty=Tsin θ. In this case θ=
arcsin(18/33) = 33. All of the centripetal force is provided for by Tx, so
T= (0.75 kg)(6.0 m/s2)/cos(33) = 5.4 N.
(c) The lift is balanced by the weight and Ty. The lift is then
Ty+W= (5.4 N) sin(33) + (0.75 kg)(9.8 m/s2) = 10 N.
E5-47 (a) The acceleration is a=v2/r = 4π2r/t2= 4π2(6.37×106m)/(8.64×104s)2= 3.37 ×
102m/s2. The centripetal force on the standard kilogram is F=ma = (1.00 kg)(3.37×102m/s2) =
0.0337 N.
(b) The tension in the balance would be T=WF= (9.80 N) (0.0337 N) = 9.77 N.
E5-48 (a) v= 4(0.179 m/s4)(7.18 s)32(2.08 m/s2)(7.18 s) = 235 m/s.
(b) a= 12(0.179 m/s4)(7.18 s)22(2.08 m/s2) = 107 m/s2.
(c) F=ma = (2.17 kg)(107 m/s2) = 232 N.
E5-49 The force only has an xcomponent, so we can use Eq. 5-19 to find the velocity.
vx=v0x+1
mZt
0
Fxdt =v0+F0
mZt
0
(1 t/T )dt
Integrating,
vx=v0+a0t1
2Tt2
Now put this expression into Eq. 5-20 to find the position as a function of time
x=x0+Zt
0
vxdt =Zt
0v0x+a0t1
2Tt2 dt
Integrating,
x=v0T+a01
2T21
6TT3=v0T+a0
T2
3.
Now we can put t=Tinto the expression for v.
vx=v0+a0T1
2TT2=v0+a0T/2.
68
P5-1 (a) There are two forces which accelerate block 1: the tension, T, and the parallel component
of the weight, W||,1=m1gsin θ1. Assuming the block accelerates to the right,
m1a=m1gsin θ1T.
There are two forces which accelerate block 2: the tension, T, and the parallel component of the
weight, W||,2=m2gsin θ2. Assuming the block 1 accelerates to the right, block 2 must also accelerate
to the right, and
m2a=Tm2gsin θ2.
Add these two equations,
(m1+m2)a=m1gsin θ1m2gsin θ2,
and then rearrange:
a=m1gsin θ1m2gsin θ2
m1+m2
.
Or, take the two net force equations, divide each side by the mass, and set them equal to each other:
gsin θ1T/m1=T/m2gsin θ2.
Rearrange,
T1
m1
+1
m2=gsin θ1+gsin θ2,
and then rearrange again:
T=m1m2g
m1+m2
(sin θ1+ sin θ2).
(b) The negative sign we get in the answer means that block 1 accelerates up the ramp.
a=(3.70 kg) sin(28)(4.86 kg) sin(42)
(3.70 kg) + (4.86 kg) (9.81 m/s2) = 1.74 m/s2.
T=(3.70 kg)(4.86 kg)(9.81 m/s2)
(3.70 kg) + (4.86 kg) [sin(28) + sin(42)] = 23.5 N.
(c) No acceleration happens when m2= (3.70 kg) sin(28)/sin(42) = 2.60 kg. If m2is more
massive than this m1will accelerate up the plane; if m2is less massive than this m1will accelerate
down the plane.
P5-2 (a) Since the pulley is massless, F= 2T. The largest value of Tthat will allow block 2 to
remain on the floor is TW2=m2g. So F2(1.9 kg)(9.8 m/s2) = 37 N.
(b) T=F/2 = (110 N)/2 = 55 N.
(c) The net force on block 1 is TW1= (55 N) (1.2 kg)(9.8 m/s2) = 43 N. This will result in
an acceleration of a= (43 N)/(1.2 kg) = 36 m/s2.
P5-3 As the string is pulled the two masses will move together so that the configuration will look
like the figure below. The point where the force is applied to the string is massless, so PF= 0 at
that point. We can take advantage of this fact and the figure below to find the tension in the cords,
F/2 = Tcos θ. The factor of 1/2 occurs because only 1/2 of Fis contained in the right triangle that
has Tas the hypotenuse. From the figure we can find the xcomponent of the force on one mass to
be Tx=Tsin θ. Combining,
Tx=F
2
sin θ
cos θ=F
2tan θ.
69
But the tangent is equal to
tan θ=Opposite
Adjacent =x
L2x2
And now we have the answer in the book.
F
T
θT
What happens when x=L? Well, axis infinite according to this expression. Since that could
only happen if the tension in the string were infinite, then there must be some other physics that
we had previously ignored.
P5-4 (a) The force of static friction can be as large as fµsN= (0.60)(12 lb) = 7.2 lb. That is
more than enough to hold the block up.
(b) The force of static friction is actually only large enough to hold up the block: f= 5.0 lb.
The magnitude of the force of the wall on the block is then Fbw =p(5.0)2+ (12.0)2lb = 13 lb.
P5-5 (a) The weight has two components: normal to the incline, W=mg cos θand parallel to the
incline, W|| =mg sin θ. There is no motion perpendicular to the incline, so N=W=mg cos θ. The
force of friction on the block is then f=µN =µmg cos θ, where we use whichever µis appropriate.
The net force on the block is FfW|| =F±µmg cos θmg sin θ.
To hold the block in place we use µsand friction will point up the ramp so the ±is +, and
F= (7.96 kg)(9.81 m/s2)[sin(22.0)(0.25) cos(22.0)] = 11.2 N.
(b) To find the minimum force to begin sliding the block up the ramp we still have static friction,
but now friction points down, so
F= (7.96 kg)(9.81 m/s2)[sin(22.0) + (0.25) cos(22.0)] = 47.4 N.
(c) To keep the block sliding up at constant speed we have kinetic friction, so
F= (7.96 kg)(9.81 m/s2)[sin(22.0) + (0.15) cos(22.0)] = 40.1 N.
P5-6 The sand will slide if the cone makes an angle greater than θwhere µs= tan θ. But
tan θ=h/R or h=Rtan θ. The volume of the cone is then
Ah/3 = πR2h/3 = πR3tan θ/3 = πµsR3/3.
P5-7 There are four forces on the broom: the force of gravity W=mg; the normal force of the
floor N; the force of friction f; and the applied force from the person P(the book calls it F). Then
XFx=Pxf=Psin θf=max,
XFy=NPyW=NPcos θmg =may= 0
Solve the second equation for N,
N=Pcos θ+mg.
70
(a) If the mop slides at constant speed f=µkN. Then
Psin θf=Psin θµk(Pcos θ+mg) = 0.
We can solve this for P(which was called Fin the book);
P=µmg
sin θµkcos θ.
This is the force required to push the broom at constant speed.
(b) Note that Pbecomes negative (or infinite) if sin θµkcos θ. This occurs when tan θcµk.
If this happens the mop stops moving, to get it started again you must overcome the static friction,
but this is impossible if tan θ0µs
P5-8 (a) The condition to slide is µstan θ. In this case, (0.63) >tan(24) = 0.445.
(b) The normal force on the slab is N=W=mg cos θ. There are three forces parallel to the
surface: friction, f=µsN=µsmg cos θ; the parallel component of the weight, W|| =mg sin θ, and
the force F. The block will slide if these don’t balance, or
F > µsmg cos θmg sin θ= (1.8×107kg)(9.8 m/s2)[(0.63) cos(24)sin(24)] = 3.0×107N.
P5-9 To hold up the smaller block the frictional force between the larger block and smaller block
must be as large as the weight of the smaller block. This can be written as f=mg. The normal
force of the larger block on the smaller block is N, and the frictional force is given by fµsN. So
the smaller block won’t fall if mg µsN.
There is only one horizontal force on the large block, which is the normal force of the small block
on the large block. Newton’s third law says this force has a magnitude N, so the acceleration of the
large block is N=Ma.
There is only one horizontal force on the two block system, the force F. So the acceleration of
this system is given by F= (M+m)a. The two accelerations are equal, otherwise the blocks won’t
stick together. Equating, then, gives N/M =F/(M+m).
We can combine this last expression with mg µsNand get
mg µsFM
M+m
or
Fg(M+m)m
µsM=(9.81 m/s2)(88 kg + 16 kg)(16 kg)
(0.38)(88 kg) = 490 N
P5-10 The normal force on the ith block is Ni=migcos θ; the force of friction on the ith block
is then fi=µimigcos θ. The parallel component of the weight on the ith block is W||,i =migsin θ.
(a) The net force on the system is
F=X
i
mig(sin θµicos θ).
Then
a= (9.81 m/s2)(1.65 kg)(sin 29.50.226 cos 29.5) + (3.22 kg)(sin 29.50.127 cos 29.5)
(1.65 kg) + (3.22 kg) ,
= 3.46 m/s2.
(b) The net force on the lower mass is m2a=W||,2f2T, so the tension is
T= (9.81 m/s2)(3.22 kg)(sin 29.50.127 cos 29.5)(3.22 kg)(3.46 m/s2) = 0.922 N.
71
(c) The acceleration will stay the same, since the system is still the same. Reversing the order
of the masses can only result in a reversing of the tension: it is still 0.992 N, but is now negative,
meaning compression.
P5-11 The rope wraps around the dowel and there is a contribution to the frictional force ∆f
from each small segment of the rope where it touches the dowel. There is also a normal force ∆Nat
each point where the contact occurs. We can find ∆Nmuch the same way that we solve the circular
motion problem.
In the figure on the left below we see that we can form a triangle with long side Tand short side
N. In the figure on the right below we see a triangle with long side rand short side rθ. These
triangles are similar, so rθ/r = ∆N/T .
T
T
r
r
r∆θN
Now ∆f=µNand T(θ) + ∆fT(θ+ ∆θ). Combining, and taking the limit as ∆θ0,
dT =df
Z1
µ
dT
T=Z
integrating both sides of this expression,
Z1
µ
dT
T=Z,
1
µln T|T2
T1=π,
T2=T1eπµ.
In this case T1is the weight and T2is the downward force.
P5-12 Answer this out of order!
(b) The maximum static friction between the blocks is 12.0 N; the maximum acceleration of the
top block is then a=F/m = (12.0 N)/(4.40 kg) = 2.73 m/s2.
(a) The net force on a system of two blocks that will accelerate them at 2.73 m/s2is F=
(4.40 kg + 5.50 kg)(2.73 m/s2) = 27.0 N.
(c) The coefficient of friction is µs=F/N =F/mg = (12.0 N)/[(4.40 kg)(9.81 m/s2)] = 0.278.
P5-13 The speed is v= 23.6 m/s.
(a) The average speed while stopping is half the initial speed, or vav = 11.8 m/s. The time to stop
is t= (62 m)/(11.8 m/s) = 5.25 s. The rate of deceleration is a= (23.6 m/s)/(5.25 s) = 4.50 m/s2.
The stopping force is F=ma; this is related to the frictional force by F=µsmg, so µs=a/g =
(4.50 m/s2)/(9.81 m/s2) = 0.46.
72
(b) Turning,
a=v2/r = (23.6 m/s)2/(62 m) = 8.98 m/s2.
Then µs=a/g = (8.98 m/s2)/(9.81 m/s2) = 0.92.
P5-14 (a) The net force on car as it travels at the top of a circular hill is Fnet =mv2/r =WN;
in this case we are told N=W/2, so Fnet =W/2 = (16000 N)/2 = 8000 N. When the car travels
through the bottom valley the net force at the bottom is Fnet =mv2/r =NW. Since the
magnitude of v,r, and hence Fnet is the same in both cases,
N=W/2 + W= 3W/2 = 3(16000 N)/2 = 24000 N
at the bottom of the valley.
(b) You leave the hill when N= 0, or
v=rg =p(250 m)(9.81 m/s2) = 50 m/s.
(c) At this speed Fnet =W, so at the bottom of the valley N= 2W= 32000 N.
P5-15 (a) v= 2πr/t = 2π(0.052 m)(3/3.3 s) = 0.30 m/s.
(b) a=v2/r = (0.30 m/s)2/(0.052 m) = 1.7 m/s2, toward center.
(c) F=ma = (0.0017 kg)(1.7 m/s2) = 2.9×103N.
(d) If the coin can be as far away as rbefore slipping, then
µs=F/mg = (2πr/t)2/(rg) = 4π2r/(t2g) = 4π2(0.12 m)/[(3/3.3 s)2(9.8 m/s2)] = 0.59.
P5-16 (a) Whether you assume constant speed or constant energy, the tension in the string must
be the greatest at the bottom of the circle, so that’s where the string will break.
(b) The net force on the stone at the bottom is TW=mv2/r. Then
v=prg[T /W 1] = q(2.9 ft)(32 ft/s2)[(9.2 lb)/(0.82 lb) 1] = 31 ft/s.
P5-17 (a) In order to keep the ball moving in a circle there must be a net centripetal force Fc
directed horizontally toward the rod. There are only three forces which act on the ball: the force
of gravity, W=mg = (1.34 kg)(9.81 m/s2) = 13.1 N; the tension in the top string T1= 35.0 N, and
the tension in the bottom string, T2.
The components of the force from the tension in the top string are
T1,x = (35.0 N) cos 30= 30.3 N and T1,y = (35.0 N) sin 30= 17.5 N.
The vertical components do balance, so
T1,y +T2,y =W,
or T2,y = (13.1 N) (17.5 N) = 4.4 N. From this we can find the tension in the bottom string,
T2=T2,y/sin(30) = 8.8 N.
(b) The net force on the object will be the sum of the two horizontal components,
Fc= (30.3 N) + (8.8 N) cos 30= 37.9 N.
(c) The speed will be found from
v=acr=pFcr/m,
=p(37.9 m)(1.70 m) sin 60/(1.34 kg) = 6.45 m/s.
73
P5-18 The net force on the cube is F=mv2/r. The speed is 2πrω. (Note that we are using ωin
a non-standard way!) Then F= 4π2mrω2. There are three forces to consider: the normal force of
the funnel on the cube N; the force of gravity on the cube W; and the frictional force on the cube
f. The acceleration of the cube in circular motion is toward the center of the circle; this means the
net force on the cube is horizontal, toward the center. We will arrange our coordinate system so
that ris horizontal and zis vertical. Then the components of the normal force are Nr=Nsin θ
and Nz=Ncos θ; the components of the frictional force are fr=fcos θand fz=fsin θ.
The direction of the friction depends on the speed of the cube; it will point up if ωis small and
down if ωis large.
(a) If ωis small, Newton’s second law is
XFr=Nrfr=Nsin θfcos θ= 4π2mrω2,
XFz=Nz+fzW=Ncos θ+fsin θmg = 0.
We can substitute f=µsN. Solving for N,
N(cos θ+µssin θ) = mg.
Combining,
4π2rω2=gsin θµscos θ
cos θ+µssin θ.
Rearrange,
ω=1
2πsg
r
sin θµscos θ
cos θ+µssin θ.
This is the minimum value.
(b) Now the frictional force will point the other way, so Newton’s second law is now
XFr=Nr+fr=Nsin θ+fcos θ= 4π2mrω2,
XFz=NzfzW=Ncos θfsin θmg = 0.
We’ve swapped + and - signs, so
ω=1
2πsg
r
sin θ+µscos θ
cos θµssin θ
is the maximum value.
P5-19 (a) The radial distance from the axis of rotation at a latitude Lis Rcos L. The speed
in the circle is then v= 2πR cos L/T . The net force on a hanging object is F=mv2/(Rcos L) =
4π2mR cos L/T 2. This net force is not directed toward the center of the earth, but is instead directed
toward the axis of rotation. It makes an angle Lwith the Earth’s vertical. The tension in the cable
must then have two components: one which is vertical (compared to the Earth) and the other which
is horizontal. If the cable makes an angle θwith the vertical, then T|| =Tsin θand T=Tcos θ.
Then T|| =F|| and WT=F. Written with a little more detail,
Tsin θ= 4π2mR cos Lsin L/T 2T θ,
and
Tcos θ= 4π2mR cos2L/T 2+mg T.
74
But 4π2Rcos2L/T 2g, so it can be ignored in the last equation compared to g, and Tmg.
Then from the first equation,
θ= 2π2Rsin 2L/(gT 2).
(b) This is a maximum when sin 2Lis a maximum, which happens when L= 45. Then
θ= 2π2(6.37×106m)/[(9.8 m/s2)(86400 s)2] = 1.7×103rad.
(c) The deflection at both the equator and the poles would be zero.
P5-20 a= (F0/m)et/T . Then v=Rt
0a dt = (F0T/m)et/T , and x=Rt
0v dt = (F0T2/m)et/T .
(a) When t=T v = (F0T/m)e1= 0.368(F0T/m).
(b) When t=T x = (F0T2/m)e1= 0.368(F0T2/m).
75
E6-1 (a) v1= (m2/m1)v2= (2650 kg/816 kg)(16.0 km/h) = 52.0 km/h.
(b) v1= (m2/m1)v2= (9080 kg/816 kg)(16.0 km/h) = 178 km/h.
E6-2 ~
pi= (2000 kg)(40 km/h)ˆ
j= 8.00×104kg ·km/hˆ
j.~
pf= (2000 kg)(50 km/h)ˆ
i= 1.00×105kg ·
km/hˆ
i. ∆~
p=~
pf~
pi= 1.00×105kg ·km/hˆ
i8.00×104kg ·km/hˆ
j. ∆p=p(∆px)2+ (∆py)2=
1.28×105kg ·km/h. The direction is 38.7south of east.
E6-3 The figure below shows the initial and final momentum vectors arranged to geometrically
show ~
pf~
pi= ∆~
p. We can use the cosine law to find the length of ∆~
p.
pf
-pi
p
θ
The angle α= 42+ 42,pi=mv = (4.88 kg)(31.4 m/s) = 153 kg·m/s. Then the magnitude of
~
pis
p=p(153 kg·m/s)2+ (153 kg·m/s)22(153 kg·m/s)2cos(84) = 205 kg·m/s,
directed up from the plate. By symmetry it must be perpendicular.
E6-4 The change in momentum is ∆p=mv =(2300 kg)(15 m/s) = 3.5×104kg ·m/s. The
average force is F= ∆p/t= (3.5×104kg ·m/s)/(0.54 s) = 6.5×104N.
E6-5 (a) The change in momentum is ∆p= (mv)mv; the average force is F= ∆p/t=
2mv/t.
(b) F=2(0.14 kg)(7.8 m/s)/(3.9×103s) = 560 N.
E6-6 (a) J= ∆p= (0.046 kg)(52.2 m/s) 0 = 2.4 N ·s.
(b) The impulse imparted to the club is opposite that imparted to the ball.
(c) F= ∆p/t= (2.4 N ·s)/(1.20×103s) = 2000 N.
E6-7 Choose the coordinate system so that the ball is only moving along the xaxis, with away
from the batter as positive. Then pfx=mvfx= (0.150 kg)(61.5 m/s) = 9.23 kg·m/s and pix=
mvix= (0.150 kg)(41.6 m/s) = 6.24 kg·m/s. The impulse is given by Jx=pfxpix= 15.47
kg·m/s. We can find the average force by application of Eq. 6-7:
Fav,x =Jx
t=(15.47 kg ·m/s)
(4.7×103s) = 3290 N.
76
E6-8 The magnitude of the impulse is J=F δt = (984 N)(0.0270 s) = 26.6 N ·s. Then pf=
pi+ ∆p, so
vf=(0.420 kg)(13.8 m/s) + (26.6 N ·s)
(0.420 kg) =49.5 m/s.
The ball moves backward!
E6-9 The change in momentum of the ball is ∆p= (mv)(mv) = 2mv = 2(0.058 kg)(32 m/s) =
3.7 kg ·m/s. The impulse is the area under a force - time graph; for the trapezoid in the figure this
area is J=Fmax(2 ms + 6 ms)/2 = (4 ms)Fmax. Then Fmax = (3.7 kg ·m/s)/(4 ms) = 930 N.
E6-10 The final speed of each object is given by vi=J/mi, where irefers to which object (as
opposed to “initial”). The object are going in different directions, so the relative speed will be the
sum. Then
vrel =v1+v2= (300 N ·s)[1/(1200 kg)+1/(1800 kg)] = 0.42 m/s.
E6-11 Use Simpson’s rule. Then the area is given by
Jx=1
3h(f0+ 4f1+ 2f2+ 4f3+... + 4f13 + f14) ,
=1
3(0.2 ms) (200 + 4 ·800 + 2 ·1200... N)
which gives Jx= 4.28 kg·m/s.
Since the impulse is the change in momentum, and the ball started from rest, pfx=Jx+pix= 4.28
kg·m/s. The final velocity is then found from vx=px/m = 8.6 m/s.
E6-12 (a) The average speed during the the time the hand is in contact with the board is half of the
initial speed, or vav = 4.8 m/s. The time of contact is then t=y/vav = (0.028 m)/(4.8 m/s) = 5.8 ms.
(b) The impulse given to the board is the same as the magnitude in the change in momentum of
the hand, or J= (0.54 kg)(9.5 m/s) = 5.1 N ·s. Then Fav = (5.1 N ·s)/(5.8 ms) = 880 N.
E6-13 p=J=Ft= (3000 N)(65.0 s) = 1.95×105N·s. The direction of the thrust relative to
the velocity doesn’t matter in this exercise.
E6-14 (a) p=mv = (2.14×103kg)(483 m/s) = 1.03 kg ·m/s.
(b) The impulse imparted to the wall in one second is ten times the above momentum, or
J= 10.3 kg ·m/s. The average force is then Fav = (10.3 kg ·m/s)/(1.0 s) = 10.3 N.
(c) The average force for each individual particle is Fav = (1.03 kg ·m/s)/(1.25×103s) = 830 N.
E6-15 A transverse direction means at right angles, so the thrusters have imparted a momentum
sufficient to direct the spacecraft 100+3400 = 3500 km to the side of the original path. The spacecraft
is half-way through the six-month journey, so it has three months to move the 3500 km to the side.
This corresponds to a transverse speed of v= (3500×103m)/(90×86400 s) = 0.45 m/s.The required
time for the rocket to fire is ∆t= (5400 kg)(0.45 m/s)/(1200 N) = 2.0 s.
E6-16 Total initial momentum is zero, so
vm=ms
mm
vs=msg
mmgvs=(0.158 lb)
(195 lb) (12.7 ft/s) = 1.0×102ft/s.
77
E6-17 Conservation of momentum:
pf,m+pf,c=pi,m+pi,c,
mmvf,m+mcvf,c=mmvi,m+mcvi,c,
vf,cvi,c=mmvi,mmmvf,m
mc
,
vc=(75.2 kg)(2.33 m/s) (75.2 kg)(0)
(38.6 kg) ,
= 4.54 m/s.
The answer is positive; the cart speed increases.
E6-18 Conservation of momentum:
pf,m+pf,c=pi,m+pi,c,
mm(vf,cvrel) + mcvf,c= (mm+mc)vi,c,
(mm+mc)vf,cmmvrel = (mm+mc)vi,c,
vc=mmvrel/(mm+mc),
=wvrel/(w+W).
E6-19 Conservation of momentum. Let mrefer to motor and crefer to command module:
pf,m+pf,c=pi,m+pi,c,
mm(vf,cvrel) + mcvf,c= (mm+mc)vi,c,
(mm+mc)vf,cmmvrel = (mm+mc)vi,c,
vf,c=mmvrel + (mm+mc)vi,c
(mm+mc),
=4mc(125 km/h) + (4mc+mc)(3860 km/h)
(4mc+mc)= 3960 km/h.
E6-20 Conservation of momentum. The block on the left is 1, the other is 2.
m1v1,f+m2v2,f=m1v1,i+m2v2,i,
v1,f=v1,i+m2
m1
(v2,iv2,f),
= (5.5 m/s) + (2.4 kg)
(1.6 kg) [(2.5 m/s) (4.9 m/s)],
= 1.9 m/s.
E6-21 Conservation of momentum. The block on the left is 1, the other is 2.
m1v1,f+m2v2,f=m1v1,i+m2v2,i,
v1,f=v1,i+m2
m1
(v2,iv2,f),
= (5.5 m/s) + (2.4 kg)
(1.6 kg) [(2.5 m/s) (4.9 m/s)],
=5.6 m/s.
78
E6-22 Assume a completely inelastic collision. Call the Earth 1 and the meteorite 2. Then
m1v1,f+m2v2,f=m1v1,i+m2v2,i,
v1,f=m2v2,i
m1+m2
,
=(5×1010kg)(7200 m/s)
(5.98×1024kg) + (5×1010kg) = 7×1011m/s.
That’s 2 mm/y!
E6-23 Conservation of momentum is used to solve the problem:
Pf=Pi,
pf,bl +pf,bu =pi,bl +pi,bu,
mblvf,bl +mbuvf,bu =mblvi,bl +mbuvi,bu,
(715 g)vf,bl + (5.18 g)(428 m/s) = (715 g)(0) + (5.18 g)(672 m/s),
which has solution vf,bl = 1.77 m/s.
E6-24 The ycomponent of the initial momentum is zero; therefore the magnitudes of the ycom-
ponents of the two particles must be equal after the collision. Then
mαvαsin θα=mOvOsin θO,
vα=mOvOsin θO
mαvαsin θα
,
=(16 u)(1.20×105m/s) sin(51)
(4.00 u) sin(64)= 4.15×105m/s.
E6-25 The total momentum is
~
p= (2.0 kg)[(15 m/s)ˆ
i+ (30 m/s)ˆ
j] + (3.0 kg)[(10 m/s)ˆ
i+ (5 m/s)ˆ
j],
= 75 kg ·m/sˆ
j.
The final velocity of Bis
~
vBf=1
mB
(~
pmA~
vAf),
=1
(3.0 kg) {(75 kg ·m/sˆ
j)(2.0 kg)[(6.0 m/s)ˆ
i+ (30 m/s)ˆ
j]},
= (4.0 m/s)ˆ
i+ (5.0 m/s)ˆ
j.
E6-26 Assume electron travels in +xdirection while neutrino travels in +ydirection. Conservation
of momentum requires that
~
p=(1.2×1022kg ·m/s)ˆ
i(6.4×1023kg ·m/s)ˆ
j
be the momentum of the nucleus after the decay. This has a magnitude of p= 1.4×1022kg ·m/s
and be directed 152from the electron.
79
E6-27 What we know:
~
p1,i= (1.50×105kg)(6.20 m/s)ˆ
i= 9.30×105kg ·m/sˆ
i,
~
p2,i= (2.78×105kg)(4.30 m/s)ˆ
j= 1.20×106kg ·m/sˆ
j,
~
p2,f= (2.78×105kg)(5.10 m/s)[sin(18)ˆ
i+ cos(18)ˆ
j],
= 4.38×105kg ·m/sˆ
i+ 1.35×106kg ·m/sˆ
j.
Conservation of momentum then requires
~
p1,f= (9.30×105kg ·m/sˆ
i)(4.38×105kg ·m/sˆ
i)
+(1.20×106kg ·m/sˆ
j)(1.35×106kg ·m/sˆ
j),
= 4.92×105kg ·m/sˆ
i1.50×105kg ·m/sˆ
i.
This corresponds to a velocity of
~
v1,f= 3.28 m/sˆ
i1.00 m/sˆ
j,
which has a magnitude of 3.43 m/s directed 17to the right.
E6-28 vf=2.1 m/s.
E6-29 We want to solve Eq. 6-24 for m2given that v1,f= 0 and v1,i=v2,i. Making these
substitutions
(0) = m1m2
m1+m2
v1,i+2m2
m1+m2
(v1,i),
0=(m1m2)v1,i(2m2)v1,i,
3m2=m1
so m2= 100 g.
E6-30 (a) Rearrange Eq. 6-27:
m2=m1
v1i v1f
v1i +v1f
= (0.342 kg) (1.24 m/s) (0.636 m/s)
(1.24 m/s) + (0.636 m/s) = 0.110 kg.
(b) v2f = 2(0.342 kg)(1.24 m/s)/(0.342 kg + 0.110 kg) = 1.88 m/s.
E6-31 Rearrange Eq. 6-27:
m2=m1
v1i v1f
v1i +v1f
= (2.0 kg) v1i v1i/4
v1i +v1i/4= 1.2 kg.
E6-32 I’ll multiply all momentum equations by g, then I can use weight directly without converting
to mass.
(a) vf= [(31.8 T)(5.20 ft/s) + (24.2 T)(2.90 ft/s)]/(31.8 T + 24.2 T) = 4.21 ft/s.
(b) Evaluate:
v1f =31.8 T 24.2 T
31.8 T + 24.2 T (5.20 ft/s) + 2(24.2 T)
31.8 T + 24.2 T (2.90 ft/s) = 3.21 ft/s.
v2f =31.8 T 24.2 T
31.8 T + 24.2 T (2.90 ft/s) + 2(31.8 T)
31.8 T + 24.2 T (5.20 ft/s) = 5.51 ft/s.
80
E6-33 Let the initial momentum of the first object be ~
p1,i=m~
v1,i, that of the second object be
~
p2,i=m~
v2,i, and that of the combined final object be ~
pf= 2m~
vf. Then
~
p1,i+~
p2,i=~
pf,
implies that we can find a triangle with sides of length p1,i,p2,i, and pf. These lengths are
p1,i=mvi,
p2,i=mvi,
pf= 2mvf= 2mvi/2 = mvi,
so this is an equilateral triangle. This means the angle between the initial velocities is 120.
E6-34 We need to change to the center of mass system. Since both particles have the same
mass, the conservation of momentum problem is effectively the same as a (vector) conservation of
velocity problem. Since one of the particles is originally at rest, the center of mass moves with
speed vcm =v1i/2. In the figure below the center of mass velocities are primed; the transformation
velocity is vt.
v’
v’
1f
1i
t
v
v
v
v’
v’
v
1f
2i
2f
t
2f
Note that since vt=v01i =v02i =v01f =v02f the entire problem can be inscribed in a rhombus.
The diagonals of the rhombus are the directions of v1f and v2f ; note that the diagonals of a rhombus
are necessarily at right angles!
(a) The target proton moves off at 90to the direction the incident proton moves after the
collision, or 26away from the incident protons original direction.
(b) The ycomponents of the final momenta must be equal, so v2f sin(26) = v1f sin(64), or v2f =
v1f tan(64). The xcomponents must add to the original momentum, so (514 m/s) = v2f cos(26) +
v1f cos(64), or
v1f = (514 m/s)/{tan(64) cos(26) + cos(64)}= 225 m/s,
and
v2f = (225 m/s) tan(64) = 461 m/s.
E6-35 vcm ={(3.16 kg)(15.6 m/s)+(2.84 kg)(12.2 m/s)}/{(3.16 kg)+(2.84 kg)}= 2.44 m/s, pos-
itive means to the left.
81
P6-1 The force is the change in momentum over change in time; the momentum is the mass time
velocity, so
F=p
t=mv
t= ∆vm
t= 2uµ,
since µis the mass per unit time.
P6-2 (a) The initial momentum is ~
pi= (1420 kg)(5.28 m/s)ˆ
j= 7500 kg ·m/sˆ
j. After making the
right hand turn the final momentum is ~
pf= 7500 kg ·m/sˆ
i. The impulse is ~
J= 7500 kg ·m/sˆ
i
7500 kg ·m/sˆ
j, which has magnitude J= 10600 kg ·m/s.
(b) During the collision the impulse is ~
J= 07500 kg·m/sˆ
i. The magnitude is J= 7500 kg·m/s.
(c) The average force is F=J/t = (10600 kg ·m/s)/(4.60 s) = 2300 N.
(d) The average force is F=J/t = (7500 kg ·m/s)/(0.350 s) = 21400 N.
P6-3 (a) Only the component of the momentum which is perpendicular to the wall changes. Then
~
J= ∆~
p=2(0.325 kg)(6.22 m/s) sin(33)ˆ
j=2.20 kg ·m/sˆ
j.
(b) ~
F=~
J/t =(2.20 kg ·m/sˆ
j)/(0.0104 s) = 212 N.
P6-4 The change in momentum of one bullet is ∆p= 2mv = 2(0.0030 kg)(500 m/s) = 3.0 kg ·m/s.
The average force is the total impulse in one minute divided by one minute, or
Fav = 100(3.0 kg ·m/s)/(60 s) = 5.0 N.
P6-5 (a) The volume of a hailstone is V= 4πr3/3 = 4π(0.5 cm)3/3 = 0.524 cm3.The mass of a
hailstone is m=ρV = (9.2×104kg/cm3)(0.524 cm3) = 4.8×104kg.
(b) The change in momentum of one hailstone when it hits the ground is
p= (4.8×104kg)(25 m/s) = 1.2×102kg ·m/s.
The hailstones fall at 25 m/s, which means that in one second the hailstones in a column 25 m high
hit the ground. Over an area of 10 m×20 m then there would be (25 m)(10 m)(20 m) = 500 m3worth
of hailstones, or 6.00×105hailstones per second striking the surface. Then
Fav = 6.00×105(1.2×102kg ·m/s)/(1 s) = 7200 N.
P6-6 Assume the links are not connected once the top link is released. Consider the link that
starts habove the table; it falls a distance hin a time t=p2h/g and hits the table with a speed
v=gt =2hg. When the link hits the table hof the chain is already on the table, and Lhis yet
to come. The linear mass density of the chain is M/L, so when this link strikes the table the mass is
hitting the table at a rate dm/dt = (M/L)v= (M/L)2hg. The average force required to stop the
falling link is then v dm/dt = (M/L)2hg = 2(M/L)hg. But the weight of the chain that is already
on the table is (M/L)hg, so the net force on the table is the sum of these two terms, or F= 3W.
P6-7 The weight of the marbles in the box after a time tis mgRt because Rt is the number of
marbles in the box.
The marbles fall a distance hfrom rest; the time required to fall this distance is t=p2h/g,
the speed of the marbles when they strike the box is v=gt =2gh. The momentum each marble
imparts on the box is then m2gh. If the marbles strike at a rate Rthen the force required to stop
them is Rm2gh.
82
The reading on the scale is then
W=mR(p2gh +gt).
This will give a numerical result of
(4.60×103kg)(115 s1)p2(9.81 m/s2)(9.62 m) + (9.81 m/s2)(6.50 s)= 41.0 N.
P6-8 (a) v= (108 kg)(9.74 m/s)/(108 kg + 1930 kg) = 0.516 m/s.
(b) Label the person as object 1 and the car as object 2. Then m1v1+m2v2= (108 kg)(9.74 m/s)
and v1=v2+ 0.520 m/s. Combining,
v2= [1050 kg ·m/s(0.520 m/s)(108 kg)]/(108 kg + 1930 kg) = 0.488 m/s.
P6-9 (a) It takes a time t1=p2h/g to fall h= 6.5 ft. An object will be moving at a speed
v1=gt1=2hg after falling this distance. If there is an inelastic collision with the pile then the
two will move together with a speed of v2=Mv1/(M+m) after the collision.
If the pile then stops within d= 1.5 inches, then the time of stopping is given by t2=d/(v2/2) =
2d/v2.
For inelastic collisions this corresponds to an average force of
Fav =(M+m)v2
t2
=(M+m)v2
2
2d=M2v2
1
2(M+m)d=(gM)2
g(M+m)
h
d.
Note that we multiply through by gto get weights. The numerical result is Fav = 130 t.
(b) For an elastic collision v2= 2M v1/(M+m); the time of stopping is still expressed by
t2= 2d/v2, but we now know Fav instead of d. Then
Fav =mv2
t2
=mv2
2
2d=4Mmv2
1
(M+m)d=2(gM)(gm)
g(M+m)
h
d.
or
d=2(gM)(gm)
g(M+m)
h
Fav
,
which has a numerical result of d= 0.51 inches.
But wait! The weight, which just had an elastic collision, “bounced” off of the pile, and then hit
it again. This drives the pile deeper into the earth. The weight hits the pile a second time with a
speed of v3= (Mm)/(M+m)v1; the pile will (in this second elastic collision) then have a speed of
v4= 2M(M+m)v3= [(Mm)/(M+m)]v2. In other words, we have an infinite series of distances
traveled by the pile, and if α= [(Mm)/(M+m)] = 0.71, the depth driven by the pile is
df=d(1 + α2+α4+α6···) = d
1α2,
or d= 1.03.
P6-10 The cat jumps off of sled A; conservation of momentum requires that MvA,1+m(vA,1+vc) =
0, or
vA,1=mvc/(m+M) = (3.63 kg)(3.05 m/s)/(22.7 kg + 3.63 kg) = 0.420 m/s.
The cat lands on sled B; conservation of momentum requires vB,1=m(vA,1+vc)/(m+M). The
cat jumps off of sled B; conservation of momentum is now
MvB,2+m(vB,2vc) = m(vA,1+vc),
83
or
vB,2= 2mvc/(m+M) = (3.63 kg)[(0.420 m/s) + 2(3.05 m/s)]/(22.7 kg + 3.63 kg) = 0.783 m/s.
The cat then lands on cart A; conservation of momentum requires that (M+m)vA,2=MvB,2, or
vA,2=(22.7 kg)(0.783 m/s)/(22.7 kg + 3.63 kg) = 0.675 m/s.
P6-11 We align the coordinate system so that west is +xand south is +y. The each car
contributes the following to the initial momentum
A: (2720 lb/g)(38.5 mi/h)ˆ
i= 1.05×105lb ·mi/h/gˆ
i,
B: (3640 lb/g)(58.0 mi/h)ˆ
j= 2.11×105lb ·mi/h/g ˆ
j.
These become the components of the final momentum. The direction is then
θ= arctan 2.11×105lb ·mi/h/g
1.05×105lb ·mi/h/g = 63.5,
south of west. The magnitude is the square root of the sum of the squares,
2.36×105lb ·mi/h/g,
and we divide this by the mass (6360 lb/g) to get the final speed after the collision: 37.1 mi/h.
P6-12 (a) Ball Amust carry off a momentum of ~
p=mBvˆ
imBv/2ˆ
j, which would be in a direction
θ= arctan(0.5/1) = 27from the original direction of B, or 117from the final direction.
(b) No.
P6-13 (a) We assume all balls have a mass m. The collision imparts a “sideways” momentum to
the cue ball of m(3.50 m/s) sin(65) = m(3.17 m/s). The other ball must have an equal, but opposite
“sideways” momentum, so m(3.17 m/s) = m(6.75 m/s) sin θ, or θ=28.0.
(b) The final “forward” momentum is
m(3.50 m/s) cos(65) + m(6.75 m/s) cos(28) = m(7.44 m/s),
so the initial speed of the cue ball would have been 7.44 m/s.
P6-14 Assuming Mm, Eq. 6-25 becomes
v2f = 2v1i v1i = 2(13 km/s) (12 km/s) = 38 km/s.
P6-15 (a) We get
v2,f=2(220 g)
(220 g) + (46.0 g) (45.0 m/s) = 74.4 m/s.
(b) Doubling the mass of the clubhead we get
v2,f=2(440 g)
(440 g) + (46.0 g) (45.0 m/s) = 81.5 m/s.
(c) Tripling the mass of the clubhead we get
v2,f=2(660 g)
(660 g) + (46.0 g) (45.0 m/s) = 84.1 m/s.
Although the heavier club helps some, the maximum speed to get out of the ball will be less than
twice the speed of the club.
84
P6-16 There will always be at least two collisions. The balls are a,b, and cfrom left to right.
After the first collision between aand bone has
vb,1=v0and va,1= 0.
After the first collision between band cone has
vc,1= 2mv0/(m+M) and vb,2= (mM)v0/(m+M).
(a) If mMthen ball bcontinue to move to the right (or stops) and there are no more collisions.
(b) If m < M then ball bbounces back and strikes ball awhich was at rest. Then
va,2= (mM)v0/(m+M) and vb,3= 0.
P6-17 All three balls are identical in mass and radii? Then balls 2 and 3 will move off at 30to
the initial direction of the first ball. By symmetry we expect balls 2 and 3 to have the same speed.
The problem now is to define an elastic three body collision. It is no longer the case that the
balls bounce off with the same speed in the center of mass. One can’t even treat the problem as two
separate collisions, one right after the other. No amount of momentum conservation laws will help
solve the problem; we need some additional physics, but at this point in the text we don’t have it.
P6-18 The original speed is v0in the lab frame. Let αbe the angle of deflection in the cm frame
and ~
v0
1be the initial velocity in the cm frame. Then the velocity after the collision in the cm frame
is v0
1cos αˆ
i+v0
1sin αˆ
jand the velocity in the lab frame is (v0
1cos α+v)ˆ
i+v0
1sin αˆ
j, where vis the
speed of the cm frame. The deflection angle in the lab frame is
θ= arctan[(v0
1sin α)/(v0
1cos α+v)],
but v=m1v0/(m1+m2) and v0
1=v0vso v0
1=m2v0/(m1+m2) and
θ= arctan[(m2sin α)/(m2cos α+m1)].
(c) θis a maximum when (cos α+m1/m2)/sin αis a minimum, which happens when cos α=
m1/m2if m1m2. Then [(m2sin α)/(m2cos α+m1)] can have any value between −∞ and ,
so θcan be between 0 and π.
(a) If m1> m2then (cos α+m1/m2)/sin αis a minimum when cos α=m2/m1, then
[(m2sin α)/(m2cos α+m1)] = m2/qm2
1m2
2.
If tan θ=m2/pm2
1m2
2then m1is like a hypotenuse and m2the opposite side. Then
cos θ=qm2
1m2
2/m1=p1(m2/m1)2.
(b) We need to change to the center of mass system. Since both particles have the same mass,
the conservation of momentum problem is effectively the same as a (vector) conservation of velocity
problem. Since one of the particles is originally at rest, the center of mass moves with speed
vcm =v1i/2. In the figure below the center of mass velocities are primed; the transformation
velocity is vt.
85
v’
v’
1f
1i
t
v
v
v
v’
v’
v
1f
2i
2f
t
2f
Note that since vt=v01i =v02i =v01f =v02f the entire problem can be inscribed in a rhombus.
The diagonals of the rhombus are the directions of v1f and v2f ; note that the diagonals of a rhombus
are necessarily at right angles!
P6-19 (a) The speed of the bullet after leaving the first block but before entering the second can
be determined by momentum conservation.
Pf=Pi,
pf,bl +pf,bu =pi,bl +pi,bu,
mblvf,bl +mbuvf,bu =mblvi,bl +mbuvi,bu,
(1.78kg)(1.48 m/s)+(3.54×103kg)(1.48 m/s) = (1.78kg)(0)+(3.54×103kg)vi,bu,
which has solution vi,bl = 746 m/s.
(b) We do the same steps again, except applied to the first block,
Pf=Pi,
pf,bl +pf,bu =pi,bl +pi,bu,
mblvf,bl +mbuvf,bu =mblvi,bl +mbuvi,bu,
(1.22kg)(0.63 m/s)+(3.54×103kg)(746 m/s) = (1.22kg)(0)+(3.54×103kg)vi,bu,
which has solution vi,bl = 963 m/s.
P6-20 The acceleration of the block down the ramp is a1=gsin(22). The ramp has a length of
d=h/ sin(22), so it takes a time t1=p2d/a1=p2h/g/ sin(22) to reach the bottom. The speed
when it reaches the bottom is v1=a1t1=2gh. Notice that it is independent of the angle!
The collision is inelastic, so the two stick together and move with an initial speed of v2=
m1v1/(m1+m2). They slide a distance xbefore stopping; the average speed while decelerating is
vav =v2/2, so the stopping time is t2= 2x/v2and the deceleration is a2=v2/t2=v2
2/(2x). If the
retarding force is f= (m1+m2)a2, then f=µk(m1+m2)g. Glue it all together and
µk=m2
1
(m1+m2)2
h
x=(2.0 kg)2
(2.0 kg + 3.5 kg)2
(0.65 m)
(0.57 m) = 0.15.
86
P6-21 (a) For an object with initial speed vand deceleration awhich travels a distance xbefore
stopping, the time tto stop is t=v/a, the average speed while stopping is v/2, and d=at2/2.
Combining, v=2ax. The deceleration in this case is given by a=µkg.
Then just after the collision
vA=p2(0.130)(9.81 m/s2)(8.20 m) = 4.57 m/s,
while
vB=p2(0.130)(9.81 m/s2)(6.10 m) = 3.94 m/s,
(b) v0= [(1100 kg)(4.57 m/s) + (1400kg)(3.94 m/s)]/(1400 kg) = 7.53 m/s.
87
E7-1 xcm = (7.36×1022kg)(3.82×108m)/(7.36×1022kg + 5.98×1034kg) = 4.64×106m. This is less
than the radius of the Earth.
E7-2 If the particles are lapart then
x1=m1l(m1+m2)
is the distance from particle 1 to the center of mass and
x2=m2l(m1+m2)
is the distance from particle 2 to the center of mass. Divide the top equation by the bottom and
x1/x2=m1/m2.
E7-3 The center of mass velocity is given by Eq. 7-1,
~
vcm =m1~
v1+m2~
v2
m1+m2
,
=(2210 kg)(105 km/h) + (2080 kg)(43.5 km/h)
(2210 kg) + (2080 kg) = 75.2 km/h.
E7-4 They will meet at the center of mass, so
xcm = (65 kg)(9.7 m)/(65 kg + 42 kg) = 5.9 m.
E7-5 (a) No external forces, center of mass doesn’t move.
(b) The collide at the center of mass,
xcm = (4.29 kg)(1.64 m)/(4.29 kg + 1.43 kg) = 1.23 m.
E7-6 The range of the center of mass is
R=v2
0sin 2θ /g = (466 m/s)2sin(2 ×57.4)/(9.81 m/s2) = 2.01×104m.
Half lands directly underneath the highest point, or 1.00×104m. The other piece must land at x,
such that
2.01×104m = (1.00×104m + x)/2;
then x= 3.02×104m.
E7-7 The center of mass of the boat + dog doesn’t move because there are no external forces on
the system. Define the coordinate system so that distances are measured from the shore, so toward
the shore is in the negative xdirection. The change in position of the center of mass is given by
xcm =mdxd+mbxb
md+mb
= 0,
Both ∆xdand ∆xbare measured with respect to the shore; we are given ∆xdb =8.50 ft, the
displacement of the dog with respect to the boat. But
xd= ∆xdb + ∆xb.
88
Since we want to find out about the dog, we’ll substitute for the boat’s displacement,
0 = mdxd+mb(∆xdxdb)
md+mb
.
Rearrange and solve for ∆xd. Use W=mg and multiply the top and bottom of the expression by
g. Then
xd=mbxdb
md+mb
g
g=(46.4 lb)(8.50 ft)
(10.8 lb) + (46.4 lb) =6.90 ft.
The dog is now 21.46.9 = 14.5 feet from shore.
E7-8 Richard has too much time on his hands.
The center of mass of the system is xcm away from the center of the boat. Switching seats is
effectively the same thing as rotating the canoe through 180, so the center of mass of the system
has moved through a distance of 2xcm = 0.412 m. Then xcm = 0.206 m. Then
xcm = (Ml ml)/(M+m+mc) = 0.206 m,
where l= 1.47 m, M= 78.4 kg, mc= 31.6 kg, and mis Judy’s mass. Rearrange,
m=Ml (M+mc)xcm
l+xcm
=(78.4 kg)(1.47 m) (78.4 kg + 31.6 kg)(0.206 m)
(1.47 m) + (0.206 m) = 55.2 kg.
E7-9 It takes the man t= (18.2 m)/(2.08 m/s) = 8.75 s to walk to the front of the boat. During
this time the center of mass of the system has moved forward x= (4.16 m/s)(8.75 s) = 36.4 m. But
in walking forward to the front of the boat the man shifted the center of mass by a distance of
(84.4 kg)(18.2 m)/(84.4 kg + 425 kg) = 3.02 m, so the boat only traveled 36.4 m 3.02 m = 33.4 m.
E7-10 Do each coordinate separately.
xcm =(3 kg)(0) + (8 kg)(1 m) + (4 kg)(2 m)
(3 kg) + (8 kg) + (4 kg) = 1.07 m
and
ycm =(3 kg)(0) + (8 kg)(2 m) + (4 kg)(1 m)
(3 kg) + (8 kg) + (4 kg) = 1.33 m
E7-11 The center of mass of the three hydrogen atoms will be at the center of the pyramid
base. The problem is then reduced to finding the center of mass of the nitrogen atom and the three
hydrogen atom triangle. This molecular center of mass must lie on the dotted line in Fig. 7-27.
The location of the plane of the hydrogen atoms can be found from Pythagoras theorem
yh=p(10.14×1011m)2(9.40×1011m)2= 3.8×1011m.
This distance can be used to find the center of mass of the molecule. From Eq. 7-2,
ycm =mnyn+mhyh
mn+mh
=(13.9mh)(0) + (3mh)(3.8×1011m)
(13.9mh) + (3mh)= 6.75×1012m.
E7-12 The velocity components of the center of mass at t= 1.42 s are vcm,x = 7.3 m/s and
vcm,y = (10.0 m/s) (9.81 m/s)(1.42 s) = 3.93 m/s. Then the velocity components of the “other”
piece are
v2,x = [(9.6 kg)(7.3 m/s) (6.5 kg)(11.4 m/s)]/(3.1 kg) = 1.30 m/s.
and
v2,y = [(9.6 kg)(3.9 m/s) (6.5 kg)(4.6 m/s)]/(3.1 kg) = 2.4 m/s.
89
E7-13 The center of mass should lie on the perpendicular bisector of the rod of mass 3M. We
can view the system as having two parts: the heavy rod of mass 3Mand the two light rods each of
mass M. The two light rods have a center of mass at the center of the square.
Both of these center of masses are located along the vertical line of symmetry for the object.
The center of mass of the heavy bar is at yh,cm = 0, while the combined center of mass of the two
light bars is at yl,cm =L/2, where down is positive. The center of mass of the system is then at
ycm =2Myl,cm + 3Myh,cm
2M+ 3M=2(L/2)
5=L/5.
E7-14 The two slabs have the same volume and have mass mi=ρiV. The center of mass is
located at
xcm =m1lm2l
m1+m2
=lρ1ρ2
ρ1+ρ2
= (5.5 cm)(7.85 g/cm3)(2.70 g/cm3)
(7.85 g/cm3) + (2.70 g/cm3)= 2.68 cm
from the boundary inside the iron; it is centered in the yand zdirections.
E7-15 Treat the four sides of the box as one thing of mass 4mwith a mass located l/2 above the
base. Then the center of mass is
zcm = (l/2)(4m)/(4m+m) = 2l/5 = 2(0.4 m)/5 = 0.16 m,
xcm =ycm = 0.2 m.
E7-16 One piece moves off with momentum m(31.4 m/s)ˆ
i, another moves off with momentum
2m(31.4 m/s)ˆ
j. The third piece must then have momentum m(31.4 m/s)ˆ
i2m(31.4 m/s)ˆ
jand
velocity (1/3)(31.4 m/s)ˆ
i2/3(31.4 m/s)ˆ
j=10.5 m/sˆ
i20.9 m/sˆ
j. The magnitude of v3is
23.4 m/s and direction 63.3away from the lighter piece.
E7-17 It will take an impulse of (84.7 kg)(3.87 m/s) = 328 kg ·m/s to stop the animal. This would
come from firing nbullets where n= (328 kg ·m/s)/[(0.0126 kg)(975 m/s) = 27.
E7-18 Conservation of momentum for firing one cannon ball of mass mwith muzzle speed vc
forward out of a cannon on a trolley of original total mass Mmoving forward with original speed
v0is
Mv0= (Mm)v1+m(vc+v1) = Mv1+mvc,
where v1is the speed of the trolley after the cannonball is fired. Then to stop the trolley we require
ncannonballs be fired so that
n= (Mv0)/(mvc) = [(3500 kg)(45 m/s)]/[(65 kg)(625 m/s)] = 3.88,
so n= 4.
E7-19 Label the velocities of the various containers as ~
vkwhere kis an integer between one and
twelve. The mass of each container is m. The subscript “g” refers to the goo; the subscript krefers
to the kth container.
The total momentum before the collision is given by
~
P=X
k
m~
vk,i+mg~
vg,i= 12m~
vcont.,cm +mg~
vg,i.
90
We are told, however, that the initial velocity of the center of mass of the containers is at rest, so
the initial momentum simplifies to ~
P=mg~
vg,i, and has a magnitude of 4000 kg·m/s.
(a) Then
vcm =P
12m+mg
=(4000 kg·m/s)
12(100.0 kg) + (50 kg) = 3.2 m/s.
(b) It doesn’t matter if the cord breaks, we’ll get the same answer for the motion of the center
of mass.
E7-20 (a) F= (3270 m/s)(480 kg/s) = 1.57×106N.
(b) m= 2.55×105kg (480 kg/s)(250 s) = 1.35×105kg.
(c) Eq. 7-32:
vf= (3270 m/s) ln(1.35×105kg/2.55×105kg) = 2080 m/s.
E7-21 Use Eq. 7-32. The initial velocity of the rocket is 0. The mass ratio can then be found
from a minor rearrangement; Mi
Mf
=e|vf/vrel|
The “flipping” of the left hand side of this expression is possible because the exhaust velocity is
negative with respect to the rocket. For part (a) Mi/M f=e= 2.72. For part (b) Mi/Mf=e2=
7.39.
E7-22 Eq. 7-32 rearranged:
Mf
Mi
=e−|v/vrel |=e(22.6m/s)/(1230m/s) = 0.982.
The fraction of the initial mass which is discarded is 0.0182.
E7-23 The loaded rocket has a weight of (1.11×105kg)(9.81 m/s2) = 1.09×106N; the thrust must be
at least this large to get the rocket off the ground. Then v(1.09×106N)/(820 kg/s) = 1.33×103m/s
is the minimum exhaust speed.
E7-24 The acceleration down the incline is (9.8 m/s2) sin(26)=4.3 m/s2. It will take t=
p2(93 m)/(4.3 m/s2) = 6.6 s. The sand doesn’t affect the problem, so long as it only “leaks” out.
E7-25 We’ll use Eq. 7-4 to solve this problem, but since we are given weights instead of mass
we’ll multiply the top and bottom by glike we did in Exercise 7-7. Then
~
vcm =m1~
v1+m2~
v2
m1+m2
g
g=W1~
v1+W2~
v2
W1+W2
.
Now for the numbers
vcm =(9.75 T)(1.36 m/s) + (0.50 T)(0)
(9.75 T) + (0.50 T) = 1.29 m/s.
P7-1 (a) The balloon moves down so that the center of mass is stationary;
0 = Mvb+mvm=Mvb+m(v+vb),
or vb=mv/(m+M).
(b) When the man stops so does the balloon.
91
P7-2 (a) The center of mass is midway between them.
(b) Measure from the heavier mass.
xcm = (0.0560 m)(0.816 kg)/(1.700 kg) = 0.0269 m,
which is 1.12 mm closer to the heavier mass than in part (a).
(c) Think Atwood’s machine. The acceleration of the two masses is
a= 2∆m g/(m1+m2) = 2(0.034 kg)g/(1.700 kg) = 0.0400g,
the heavier going down while the lighter moves up. The acceleration of the center of mass is
acm = (am1am2)/(m1+m2) = (0.0400g)2(0.034 kg)g/(1.700 kg) = 0.00160g.
P7-3 This is a glorified Atwood’s machine problem. The total mass on the right side is the mass
per unit length times the length, mr=λx; similarly the mass on the left is given by ml=λ(Lx).
Then
a=m2m1
m2+m1
g=λx λ(Lx)
λx +λ(Lx)g=2xL
Lg
which solves the problem. The acceleration is in the direction of the side of length xif x > L/2.
P7-4 (a) Assume the car is massless. Then moving the cannonballs is moving the center of mass,
unless the cannonballs don’t move but instead the car does. How far? L.
(b) Once the cannonballs stop moving so does the rail car.
P7-5 By symmetry, the center of mass of the empty storage tank should be in the very center,
along the axis at a height yt,cm =H/2. We can pretend that the entire mass of the tank, mt=M,
is located at this point.
The center of mass of the gasoline is also, by symmetry, located along the axis at half the height of
the gasoline, yg,cm =x/2. The mass, if the tank were filled to a height H, is m; assuming a uniform
density for the gasoline, the mass present when the level of gas reaches a height xis mg=mx/H.
(a) The center of mass of the entire system is at the center of the cylinder when the tank is full
and when the tank is empty. When the tank is half full the center of mass is below the center. So as
the tank changes from full to empty the center of mass drops, reaches some lowest point, and then
rises back to the center of the tank.
(b) The center of mass of the entire system is found from
ycm =mgyg,cm +mtyt,cm
mg+mt
=(mx/H)(x/2) + (M)(H/2)
(mx/H)+(M)=mx2+MH2
2mx + 2MH .
Take the derivative:
dycm
dx =mmx2+ 2xMH MH2
(mx +MH)2
Set this equal to zero to find the minimum; this means we want the numerator to vanish, or mx2+
2xMH MH2= 0. Then
x=M+M2+mM
mH.
92
P7-6 The center of mass will be located along symmetry axis. Call this the xaxis. Then
xcm =1
MZxdm,
=4
πR2ZR
0ZR2x2
0
x dy dx,
=4
πR2ZR
0
xpR2x2dx,
=4
πR2R3/3 = 4R
3π.
P7-7 (a) The components of the shell velocity with respect to the cannon are
v0
x= (556 m/s) cos(39.0) = 432 m/s and v0
y= (556 m/s) sin(39.0) = 350 m/s.
The vertical component with respect to the ground is the same, vy=v0
y, but the horizontal compo-
nent is found from conservation of momentum:
M(vxv0
x) + m(vx) = 0,
so vx= (1400 kg)(432 m/s)(70.0 kg + 1400 kg) = 411 m/s. The resulting speed is v= 540 m/s.
(b) The direction is θ= arctan(350/411) = 40.4.
P7-8 v= (2870 kg)(252 m/s)/(2870 kg + 917 kg) = 191 m/s.
P7-9 It takes (1.5 m/s)(20 kg) = 30 N to accelerate the luggage to the speed of the belt. The
people when taking the luggage off will (on average) also need to exert a 30 N force to remove it;
this force (because of friction) will be exerted on the belt. So the belt requires 60 N of additional
force.
P7-10 (a) The thrust must be at least equal to the weight, so
dm/dt = (5860 kg)(9.81 m/s2)/(1170 m/s) = 49.1 kg/s.
(b) The net force on the rocket will need to be F= (5860 kg)(18.3 m/s2) = 107000 N. Add this
to the weight to find the thrust, so
dm/dt = [107000 N + (5860 kg)(9.81 m/s2)]/(1170 m/s) = 141 kg/s
P7-11 Consider Eq. 7-31. We want the barges to continue at constant speed, so the left hand
side of that equation vanishes. Then
X~
Fext =~
vrel
dM
dt .
We are told that the frictional force is independent of the weight, since the speed doesn’t change the
frictional force should be constant and equal in magnitude to the force exerted by the engine before
the shoveling happens. Then P~
Fext is equal to the additional force required from the engines. We’ll
call it ~
P.
The relative speed of the coal to the faster moving cart has magnitude: 21.29.65 = 11.6
km/h= 3.22 m/s. The mass flux is 15.4 kg/s, so P= (3.22 m/s)(15.4kg/s) = 49.6 N. The faster
moving cart will need to increase the engine force by 49.6 N. The slower cart won’t need to do
anything, because the coal left the slower barge with a relative speed of zero according to our
approximation.
93
P7-12 (a) Nothing is ejected from the string, so vrel = 0. Then Eq. 7-31 reduces to m dv/dt =Fext.
(b) Since Fext is from the weight of the hanging string, and the fraction that is hanging is y/L,
Fext =mgy/L. The equation of motion is then d2y/dt2=gy/L.
(c) Take first derivative:
dy
dt =y0
2(pg/L)eg/Lt eg/Lt,
and then second derivative,
d2y
dt2=y0
2(pg/L)2eg/Lt +eg/Lt.
Substitute into equation of motion. It works! Note that when t= 0 we have y=y0.
94
E8-1 An n-dimensional object can be oriented by stating the position of ndifferent carefully
chosen points Piinside the body. Since each point has ncoordinates, one might think there are n2
coordinates required to completely specify the position of the body. But if the body is rigid then
the distances between the points are fixed. There is a distance dij for every pair of points Piand
Pj. For each distance dij we need one fewer coordinate to specify the position of the body. There
are n(n1)/2 ways to connect nobjects in pairs, so n2n(n1)/2 = n(n+ 1)/2 is the number
of coordinates required.
E8-2 (1 rev/min)(2πrad/rev)/(60 s/min) = 0.105 rad/s.
E8-3 (a) ω=a+ 3bt24ct3.
(b) α= 6bt 12t2.
E8-4 (a) The radius is r= (2.3×104ly)(3.0×108m/s) = 6.9×1012m·y/s. The time to make one
revolution is t= (2π6.9×1012m·y/s)/(250×103m/s) = 1.7×108y.
(b) The Sun has made 4.5×109y/1.7×108y = 26 revolutions.
E8-5 (a) Integrate.
ωz=ω0+Zt
04at33bt2dt =ω0+at4bt3
(b) Integrate, again.
θ=Zt
0
ωzdt =Zt
0ω0+at4bt3dt =ω0t+1
5at51
4bt4
E8-6 (a) (1 rev/min)(2πrad/rev)/(60 s/min) = 0.105 rad/s.
(b) (1 rev/h)(2πrad/rev)/(3600 s/h) = 1.75×103rad/s.
(c) (1/12 rev/h)(2πrad/rev)/(3600 s/h) = 1.45×103rad/s.
E8-7 85 mi/h = 125 ft/s. The ball takes t= (60 ft)/(125 ft/s) = 0.48 s to reach the plate. It
makes (30 rev/s)(0.48 s) = 14 revolutions in that time.
E8-8 It takes t=p2(10 m)/(9.81 m/s2)=1.43 s to fall 10 m. The average angular velocity is
then ω= (2.5)(2πrad)/(1.43 s) = 11 rad/s.
E8-9 (a) Since there are eight spokes, this means the wheel can make no more than 1/8 of a
revolution while the arrow traverses the plane of the wheel. The wheel rotates at 2.5 rev/s; it
makes one revolution every 1/2.5=0.4 s; so the arrow must pass through the wheel in less than
0.4/8 = 0.05 s.
The arrow is 0.24 m long, and it must move at least one arrow length in 0.05 s. The corresponding
minimum speed is (0.24 m)/(0.05 s) = 4.8 m/s.
(b) It does not matter where you aim, because the wheel is rigid. It is the angle through which
the spokes have turned, not the distance, which matters here.
95
E8-10 We look for the times when the Sun, the Earth, and the other planet are collinear in some
specified order.
Since the outer planets revolve around the Sun more slowly than Earth, after one year the Earth
has returned to the original position, but the outer planet has completed less than one revolution.
The Earth will then “catch up” with the outer planet before the planet has completed a revolution.
If θEis the angle through which Earth moved and θPis the angle through which the planet moved,
then θE=θP+ 2π, since the Earth completed one more revolution than the planet.
If ωPis the angular velocity of the planet, then the angle through which it moves during the
time TS(the time for the planet to line up with the Earth). Then
θE=θP+ 2π,
ωETS=ωPTS+ 2π,
ωE=ωP+ 2π/TS
The angular velocity of a planet is ω= 2π/T , where Tis the period of revolution. Substituting this
into the last equation above yields
1/TE= 1/TP+ 1/TS.
E8-11 We look for the times when the Sun, the Earth, and the other planet are collinear in some
specified order.
Since the inner planets revolve around the Sun more quickly than Earth, after one year the Earth
has returned to the original position, but the inner planet has completed more than one revolution.
The inner planet must then have “caught-up” with the Earth before the Earth has completed a
revolution. If θEis the angle through which Earth moved and θPis the angle through which the
planet moved, then θP=θE+ 2π, since the inner planet completed one more revolution than the
Earth.
96
If ωPis the angular velocity of the planet, then the angle through which it moves during the
time TS(the time for the planet to line up with the Earth). Then
θP=θE+ 2π,
ωPTS=ωETS+ 2π,
ωP=ωE+ 2π/TS
The angular velocity of a planet is ω= 2π/T , where Tis the period of revolution. Substituting this
into the last equation above yields
1/TP= 1/TE+ 1/TS.
E8-12 (a) α= (78 rev/min)/(0.533 min) = 150 rev/min2.
(b) Average angular speed while slowing down is 39 rev/min, so (39 rev/min)(0.533 min) =
21 rev.
E8-13 (a) α= (2880 rev/min 1170 rev/min)/(0.210 min) = 8140 rev/min2.
(b) Average angular speed while accelerating is 2030 rev/min, so (2030 rev/min)(0.210 min) =
425 rev.
E8-14 Find area under curve.
1
2(5 min + 2.5 min)(3000 rev/min) = 1.13×104rev.
E8-15 (a) ω0z= 25.2 rad/s; ωz= 0; t= 19.7 s; and αzand φare unknown. From Eq. 8-6,
ωz=ω0z+αzt,
(0) = (25.2 rad/s) + αz(19.7 s),
αz=1.28 rad/s2
(b) We use Eq. 8-7 to find the angle through which the wheel rotates.
φ=φ0+ω0zt+1
2αzt2= (0)+(25.2 rad/s)(19.7 s)+ 1
2(1.28 rad/s2)(19.7 s)2= 248 rad.
97
(c) φ= 248 rad 1rev
2πrad = 39.5 rev.
E8-16 (a) α= (225 rev/min 315 rev/min)/(1.00 min) = 90.0 rev/min2.
(b) t= (0 225 rev/min)/(90.0 rev/min2) = 2.50 min.
(c) (90.0 rev/min2)(2.50 min)2/2 + (225 rev/min)(2.50 min) = 281 rev.
E8-17 (a) The average angular speed was (90 rev)/(15 s) = 6.0 rev/s. The angular speed at the
beginning of the interval was then 2(6.0 rev/s) (10 rev/s) = 2.0 rev/s.
(b) The angular acceleration was (10 rev/s2.0 rev/s)/(15 s) = 0.533 rev/s2. The time required
to get the wheel to 2.0 rev/s was t= (2.0 rev/s)/(0.533 rev/s2) = 3.8 s.
E8-18 (a) The wheel will rotate through an angle φwhere
φ= (563 cm)/(8.14 cm/2) = 138 rad.
(b) t=q2(138 rad)/(1.47 rad/s2) = 13.7 s.
E8-19 (a) We are given φ= 42.3 rev= 266 rad, ω0z= 1.44 rad/s, and ωz= 0. Assuming a
uniform deceleration, the average angular velocity during the interval is
ωav,z =1
2(ω0z+ωz) = 0.72 rad/s.
Then the time taken for deceleration is given by φ=ωav,zt, so t= 369 s.
(b) The angular acceleration can be found from Eq. 8-6,
ωz=ω0z+αzt,
(0) = (1.44 rad/s) + αz(369 s),
αz=3.9×103rad/s2.
(c) We’ll solve Eq. 8-7 for t,
φ=φ0+ω0zt+1
2αzt2,
(133 rad) = (0) + (1.44 rad/s)t+1
2(3.9×103rad/s2)t2,
0 = 133 + (1.44 s1)t(1.95 ×103s2)t2.
Solving this quadratic expression yields two answers: t= 108 s and t= 630 s.
E8-20 The angular acceleration is α= (4.96 rad/s)/(2.33 s) = 2.13 rad/s2.The angle through
which the wheel turned while accelerating is φ= (2.13 rad/s2)(23.0 s)2/2 = 563 rad.The angular
speed at this time is ω= (2.13 rad/s2)(23.0 s) = 49.0 rad/s.The wheel spins through an additional
angle of (49.0 rad/s)(46 s 23 s) = 1130 rad, for a total angle of 1690 rad.
E8-21 ω= (14.6 m/s)/(110 m) = 0.133 rad/s.
E8-22 The linear acceleration is (25 m/s12 m/s)/(6.2 s) = 2.1 m/s2. The angular acceleration is
α= (2.1 m/s2)/(0.75 m/2) = 5.6 rad/s.
98
E8-23 (a) The angular speed is given by vT=ωr. So ω=vT/r = (28,700 km/hr)/(3220
km) = 8.91 rad/hr. That’s the same thing as 2.48×103rad/s.
(b) aR=ω2r= (8.91 rad/h)2(3220 km) = 256000 km/h2, or
aR= 256000 km/h2(1/3600 h/s)2(1000 m/km) = 19.8 m/s2.
(c) If the speed is constant then the tangential acceleration is zero, regardless of the shape of the
trajectory!
E8-24 The bar needs to make
(1.50 cm)(12.0 turns/cm) = 18 turns.
This will happen is (18 rev)/(237 rev/min) = 4.56 s.
E8-25 (a) The angular speed is ω= (2πrad)/(86400 s) = 7.27×105rad/s.
(b) The distance from the polar axis is r= (6.37×106m) cos(40) = 4.88×106m. The linear speed
is then v= (7.27×105rad/s)(4.88×106m) = 355 m/s.
(c) The angular speed is the same as part (a). The distance from the polar axis is r= (6.37×
106m) cos(0) = 6.37×106m. The linear speed is then v= (7.27×105rad/s)(6.37×106m) = 463 m/s.
E8-26 (a) aT= (14.2 rad/s2)(0.0283 m) = 0.402 m/s2.
(b) Full speed is ω= 289 rad/s. aR= (289 rad/s)2(0.0283 m) = 2360 m/s2.
(c) It takes
t= (289 rad/s)/(14.2 rad/s2) = 20.4 s
to get up to full speed. Then x= (0.402 m/s2)(20.4 s)2/2 = 83.6 m is the distance through which a
point on the rim moves.
E8-27 (a) The pilot sees the propeller rotate, no more. So the tip of the propeller is moving with
a tangential velocity of vT=ωr = (2000 rev/min)(2πrad/rev)(1.5 m) = 18900 m/min. This is the
same thing as 315 m/s.
(b) The observer on the ground sees this tangential motion and sees the forward motion of
the plane. These two velocity components are perpendicular, so the magnitude of the sum is
p(315 m/s)2+ (133 m/s)2= 342 m/s.
E8-28 aT=aRwhen rα =2=r(αt)2, or t=q1/(0.236 rad/s2) = 2.06 s.
E8-29 (a) aR=rω2=rα2t2.
(b) aT=rα.
(c) Since aR=aTtan(57.0), t=ptan(57.0). Then
φ=1
2αt2=1
2tan(57.0) = 0.77 rad = 44.1.
E8-30 (a) The tangential speed of the edge of the wheel relative axle is v= 27 m/s. ω=
(27 m/s)/(0.38 m) = 71 rad/s.
(b) The average angular speed while slowing is 71 rad/s/2, the time required to stop is then
t= (30 ×2πrad)/(71 rad/s/2) = 5.3 s. The angular acceleration is then α= (71 rad/s)/(5.3 s) =
13 rad/s.
(c) The car moves forward (27 m/s/2)(5.3 s) = 72 m.
99
E8-31 Yes, the speed would be wrong. The angular velocity of the small wheel would be ω=vt/rs,
but the reported velocity would be v=ωrl=vtrl/rs. This would be in error by a fraction
v
vt
=(72 cm)
(62 cm) 1 = 0.16.
E8-32 (a) Square both equations and then add them:
x2+y2= (Rcos ωt)2+ (Rsin ωt)2=R2,
which is the equation for a circle of radius R.
(b) vx=sin ωt =ωy;vy=cos ωt =ωx. Square and add, v=ωR. The direction is
tangent to the circle.
(b) ax=2cos ωt =ω2x;ay=2sin ωt =ω2y. Square and add, a=ω2R. The
direction is toward the center.
E8-33 (a) The object is “slowing down”, so ~α = (2.66 rad/s2)ˆ
k. We know the direction
because it is rotating about the zaxis and we are given the direction of ~ω. Then from Eq. 8-19,
~
v=~ω ×~
R= (14.3 rad/s)ˆ
k×[(1.83 m)ˆ
j+ (1.26 m)ˆ
k]. But only the cross term ˆ
k׈
jsurvives, so
~
v= (26.2 m/s)ˆ
i.
(b) We find the acceleration from Eq. 8-21,
~
a=~α ×~
R+~ω ×~
v,
= (2.66 rad/s2)ˆ
k×[(1.83 m)ˆ
j+ (1.26 m)ˆ
k] + (14.3 rad/s)ˆ
k×(26.2 m/s)ˆ
i,
= (4.87 m/s2)ˆ
i+ (375 m/s2)ˆ
j.
E8-34 (a) ~
F=2m~ω ×~
v=2v cos θ, where θis the latitude. Then
F= 2(12 kg)(2πrad/86400 s)(35 m/s) cos(45) = 0.043 N,
and is directed west.
(b) Reversing the velocity will reverse the direction, so east.
(c) No. The Coriolis force pushes it to the west on the way up and gives it a westerly velocity;
on the way down the Coriolis force slows down the westerly motion, but does not push it back east.
The object lands to the west of the starting point.
P8-1 (a) ω= (4.0 rad/s) (6.0 rad/s2)t+ (3.0 rad/s)t2. Then ω(2.0 s) = 4.0 rad/s and ω(4.0 s) =
28.0 rad/s.
(b) αav = (28.0 rad/s 4.0 rad/s)/(4.0 s 2.0 s) = 12 rad/s2.
(c) α=(6.0 rad/s2) + (6.0 rad/s)t. Then α(2.0 s) = 6.0 rad/s2and α(4.0 s) = 18.0 rad/s2.
P8-2 If the wheel really does move counterclockwise at 4.0 rev/min, then it turns through
(4.0 rev/min)/[(60 s/min)(24 frames/s)] = 2.78×103rev/frame.
This means that a spoke has moved 2.78×103rev.There are 16 spokes each located 1/16 of a
revolution around the wheel. If instead of moving counterclockwise the wheel was instead moving
clockwise so that a different spoke had moved 1/16 rev 2.78 ×103rev = 0.0597 rev, then the
same effect would be present. The wheel then would be turning clockwise with a speed of ω=
(0.0597 rev)(60 s/min)(24 frames/s) = 86 rev/min.
100
P8-3 (a) In the diagram below the Earth is shown at two locations a day apart. The Earth rotates
clockwise in this figure.
Note that the Earth rotates through 2πrad in order to be correctly oriented for a complete
sidereal day, but because the Earth has moved in the orbit it needs to go farther through an angle
θin order to complete a solar day. By the time the Earth has gone all of the way around the sun
the total angle θwill be 2πrad, which means that there was one more sidereal day than solar day.
(b) There are (365.25 d)(24.000 h/d) = 8.7660×103hours in a year with 265.25 solar days. But
there are 366.25 sidereal days, so each one has a length of 8.7660×103/366.25 = 23.934 hours, or 23
hours and 56 minutes and 4 seconds.
P8-4 (a) The period is time per complete rotation, so ω= 2π/T .
(b) α= ∆ω/t, so
α=2π
T0+ ∆T2π
T0/(∆t),
=2π
tT
T0(T0+ ∆T),
2π
tT
T2
0
,
=2π
(3.16×107s) (1.26×105s)
(0.033 s)2=2.30×109rad/s2.
(c) t= (2π/0.033 s)/(2.30×109rad/s2) = 8.28×1010s, or 2600 years.
(d) 2π/T0= 2π/T αt, or
T0=1/(0.033 s) (2.3×109rad/s2)(3.0×1010 s)/(2π)1= 0.024 s.
P8-5 The final angular velocity during the acceleration phase is ωz=αzt= (3.0 rad/s)(4.0
s) = 12.0 rad/s. Since both the acceleration and deceleration phases are uniform with endpoints
ωz= 0, the average angular velocity for both phases is the same, and given by half of the maximum:
ωav,z = 6.0 rad/s.
101
The angle through which the wheel turns is then
φ=ωav,zt= (6.0 rad/s)(4.1 s) = 24.6 rad.
The time is the total for both phases.
(a) The first student sees the wheel rotate through the smallest angle less than one revolution;
this student would have no idea that the disk had rotated more than once. Since the disk moved
through 3.92 revolutions, the first student will either assume the disk moved forward through 0.92
revolutions or backward through 0.08 revolutions.
(b) According to whom? We’ve already answered from the perspective of the second student.
P8-6 ω= (0.652 rad/s2)tand α= (0.652 rad/s2).
(a) ω= (0.652 rad/s2)(5.60 s) = 3.65 rad/s
(b) vT=ωr = (3.65 rad/s)(10.4 m) = 38 m/s.
(c) aT=αr = (0.652 rad/s2)(10.4 m) = 6.78 m/s2.
(d) aR=ω2r= (3.65 rad/s)2(10.4 m) = 139 m/s2.
P8-7 (a) ω= (2πrad)/(3.16×107s) = 1.99×107rad/s.
(b) vT=ωR = (1.99×107rad/s)(1.50×1011m) = 2.99×104m/s.
(c) aR=ω2R= (1.99×107rad/s)2(1.50×1011m) = 5.94×103m/s2.
P8-8 (a) α= (156 rev/min)/(2.2×60 min) = 1.18 rev/min2.
(b) The average angular speed while slowing down is 78 rev/min, so the wheel turns through
(78 rev/min)(2.2×60 min) = 10300 revolutions.
(c) aT= (2πrad/rev)(1.18 rev/min2)(0.524 m) = 3.89 m/min2. That’s the same as 1.08×
103m/s2.
(d) aR= (2πrad/rev)(72.5 rev/min)2(0.524 m) = 1.73 ×104m/min2. That’s the same as
4.81m/s2. This is so much larger than the aTterm that the magnitude of the total linear ac-
celeration is simply 4.81m/s2.
P8-9 (a) There are 500 teeth (and 500 spaces between these teeth); so disk rotates 2π/500 rad
between the outgoing light pulse and the incoming light pulse. The light traveled 1000 m, so the
elapsed time is t= (1000 m)/(3×108m/s) = 3.33×106s.
Then the angular speed of the disk is ωz=φ/t = 1.26×102rad)/(3.33×106s) = 3800 rad/s.
(b) The linear speed of a point on the edge of the would be
vT=ωR = (3800 rad/s)(0.05 m) = 190 m/s.
P8-10 The linear acceleration of the belt is a=αArA. The angular acceleration of Cis αC=
a/rC=αA(rA/rC). The time required for Cto get up to speed is
t=(2πrad/rev)(100 rev/min)(1/60 min/s)
(1.60 rad/s2)(10.0/25.0) = 16.4 s.
P8-11 (a) The final angular speed is ωo= (130 cm/s)/(5.80 cm) = 22.4 rad/s.
(b) The recording area is π(Ro2Ri2), the recorded track has a length land width w, so
l=π[(5.80 cm)2(2.50 cm2)]
(1.60×104cm) = 5.38×105cm.
(c) Playing time is t= (5.38×105cm)/(130 cm/s) = 4140 s, or 69 minutes.
102
P8-12 The angular position is given by φ= arctan(vt/b). The derivative (Maple!) is
ω=vb
b2+v2t2,
and is directed up. Take the derivative again,
α=2bv3t
(b2+v2t2)2,
but is directed down.
P8-13 (a) Let the rocket sled move along the line x=b. The observer is at the origin and
sees the rocket move with a constant angular speed, so the angle made with the xaxis increases
according to θ=ωt. The observer, rocket, and starting point form a right triangle; the position y
of the rocket is the opposite side of this triangle, so
tan θ=y/b implies y=b/ tan ωt.
We want to take the derivative of this with respect to time and get
v(t) = ωb/ cos2(ωt).
(b) The speed becomes infinite (which is clearly unphysical) when t=π/2ω.
103
E9-1 (a) First, ~
F= (5.0 N)ˆ
i.
~τ = [yFzzFy]ˆ
i+ [zFxxFz]ˆ
j+ [xFyyFx]ˆ
k,
= [y(0) (0)(0)]ˆ
i+ [(0)Fxx(0)]ˆ
j+ [x(0) yFx]ˆ
k,
= [yFx]ˆ
k=(3.0 m)(5.0 N)ˆ
k=(15.0 N ·m)ˆ
k.
(b) Now ~
F= (5.0 N)ˆ
j. Ignoring all zero terms,
~τ = [xFy]ˆ
k= (2.0 m)(5.0 N)ˆ
k= (10 N ·m)ˆ
k.
(c) Finally, ~
F= (5.0 N)ˆ
i.
~τ = [yFx]ˆ
k=(3.0 m)(5.0 N)ˆ
k= (15.0 N ·m)ˆ
k.
E9-2 (a) Everything is in the plane of the page, so the net torque will either be directed normal
to the page. Let out be positive, then the net torque is τ=r1F1sin θ1r2F2sin θ2.
(b) τ= (1.30 m)(4.20 N) sin(75.0)(2.15 m)(4.90 N) sin(58.0) = 3.66N ·m.
E9-4 Everything is in the plane of the page, so the net torque will either be directed normal to
the page. Let out be positive, then the net torque is
τ= (8.0 m)(10 N) sin(45)(4.0 m)(16 N) sin(90) + (3.0 m)(19 N) sin(20) = 12N ·m.
E9-5 Since ~
rand ~
slie in the xy plane, then ~
t=~
r×~
smust be perpendicular to that plane, and
can only point along the zaxis.
The angle between ~
rand ~
sis 32085= 235. So |~
t|=rs|sin θ|= (4.5)(7.3)|sin(235)|= 27.
Now for the direction of ~
t. The smaller rotation to bring ~
rinto ~
sis through a counterclockwise
rotation; the right hand rule would then show that the cross product points along the positive z
direction.
E9-6 ~
a= (3.20)[cos(63.0)ˆ
j+ sin(63.0)ˆ
k] and ~
b= (1.40)[cos(48.0)ˆ
i+ sin(48.0)ˆ
k]. Then
~
a×~
b= (3.20) cos(63.0)(1.40) sin(48.0)ˆ
i
+(3.20) sin(63.0)(1.40) cos(48.0)ˆ
j
(3.20) cos(63.0)(1.40) cos(48.0)ˆ
k
= 1.51ˆ
i+ 2.67ˆ
j1.36ˆ
k.
E9-7 ~
b×~
ahas magnitude ab sin φand points in the negative zdirection. It is then perpendicular
to ~
a, so ~
chas magnitude a2bsin φ. The direction of ~
cis perpendicular to ~
abut lies in the plane
containing vectors ~
aand ~
b. Then it makes an angle π/2φwith ~
b.
E9-8 (a) In unit vector notation,
~
c= [(3)(3) (2)(1)]ˆ
i+ [(1)(4) (2)(3)]ˆ
j+ [(2)(2) (3)(4)]ˆ
k,
= 11ˆ
i+ 10ˆ
j+ 8ˆ
k.
(b) Evaluate arcsin[|~
a×b|/(ab)], finding magnitudes with the Pythagoras relationship:
φ= arcsin (16.8)/[(3.74)(5.39)] = 56.
104
E9-9 This exercise is a three dimensional generalization of Ex. 9-1, except nothing is zero.
~τ = [yFzzFy]ˆ
i+ [zFxxFz]ˆ
j+ [xFyyFx]ˆ
k,
= [(2.0 m)(4.3 N) (1.6 m)(2.4 N)]ˆ
i+ [(1.6 m)(3.5 N) (1.5 m)(4.3 N)]ˆ
j
+[(1.5 m)(2.4 N) (2.0 m)(3.5 N)]ˆ
k,
= [4.8 N·m]ˆ
i+ [0.85 N·m]ˆ
j+ [3.4 N·m]ˆ
k.
E9-10 (a) ~
F= (2.6 N)ˆ
i, then ~τ = (0.85 m)(2.6 N)ˆ
j(0.36 m)(2.6 N)ˆ
k= 2.2 N ·mˆ
j+ 0.94 N ·mˆ
k.
(b) ~
F= (2.6 N)ˆ
k, then ~τ = (0.36 m)(2.6 N)ˆ
i(0.54 m)(2.6 N)ˆ
j= 0.93 N ·mˆ
i+ 1.4 N ·mˆ
j.
E9-11 (a) The rotational inertia about an axis through the origin is
I=mr2= (0.025 kg)(0.74 m)2= 1.4×102kg ·m2.
(b) α= (0.74 m)(22 N) sin(50)/(1.4×102kg ·m2) = 890 rad/s.
E9-12 (a) I0= (0.052 kg)(0.27 m)2+ (0.035 kg)(0.45 m)2+ (0.024 kg)(0.65 m)2= 2.1×102kg ·m2.
(b) The center of mass is located at
xcm =(0.052 kg)(0.27 m) + (0.035 kg)(0.45 m) + (0.024 kg)(0.65 m)
(0.052 kg) + (0.035 kg) + (0.024 kg) = 0.41 m.
Applying the parallel axis theorem yields Icm = 2.1×102kg ·m2(0.11 kg)(0.41 m)2= 2.5×103kg ·
m2.
E9-13 (a) Rotational inertia is additive so long as we consider the inertia about the same axis.
We can use Eq. 9-10:
I=Xmnr2
n= (0.075 kg)(0.42 m)2+ (0.030 kg)(0.65 m)2= 0.026 kg·m2.
(b) No change.
E9-14 ~τ = [(0.42 m)(2.5 N) (0.65 m)(3.6 N)]ˆ
k=1.29 N ·mˆ
k. Using the result from E9-13,
~α = (1.29 N ·mˆ
k)/(0.026 kg·m2) = 50 rad/s2ˆ
k.That’s clockwise if viewed from above.
E9-15 (a) F=2r= (110 kg)(33.5 rad/s)2(3.90 m) = 4.81×105N.
(b) The angular acceleration is α= (33.5 rad/s)/(6.70 s) = 5.00 rad/s2. The rotational inertia
about the axis of rotation is I= (110 kg)(7.80 m)2/3=2.23×103kg ·m2. τ =Iα = (2.23×103kg ·
m2)(5.00 rad/s2) = 1.12×104N·m.
E9-16 We can add the inertias for the three rods together,
I= 3 1
3ML2= (240 kg)(5.20 m)2= 6.49×103kg ·m2.
E9-17 The diagonal distance from the axis through the center of mass and the axis through the
edge is h=p(a/2)2+ (b/2)2, so
I=Icm +Mh2=1
12Ma2+b2+M(a/2)2+ (b/2)2=1
12 +1
4Ma2+b2.
Simplifying, I=1
3Ma2+b2.
105
E9-18 I=Icm +Mh2= (0.56 kg)(1.0 m)2/12 + (0.56)(0.30 m)2= 9.7×102kg ·m2.
E9-19 For particle one I1=mr2=mL2; for particle two I2=mr2=m(2L)2= 4mL2. The
rotational inertia of the rod is Irod =1
3(2M)(2L)2=8
3ML2. Add the three inertias:
I=5m+8
3ML2.
E9-20 (a) I=MR2/2 = M(R/2)2.
(b) Let Ibe the rotational inertia. Assuming that kis the radius of a hoop with an equivalent
rotational inertia, then I=Mk2, or k=pI/M.
E9-21 Note the mistakes in the equation in the part (b) of the exercise text.
(a) mn=M/N.
(b) Each piece has a thickness t=L/N, the distance from the end to the nth piece is xn=
(n1/2)t= (n1/2)L/N. The axis of rotation is the center, so the distance from the center is
rn=xnL/2 = nL/N (1 + 1/2N)L.
(c) The rotational inertia is
I=
N
X
n=1
mnr2
n,
=ML2
N3
N
X
n=1
(n1/2N)2,
=ML2
N3
N
X
n=1 n2(2N+ 1)n+ (N+ 1/2)2,
=ML2
N3N(N+ 1)(2N+ 1)
6(2N+ 1)N(N+ 1)
2+ (N+ 1/2)2N,
ML2
N32N3
62N3
2+N3,
=ML2/3.
E9-22 F= (46 N)(2.6 cm)/(13 cm) = 9.2 N.
E9-23 Tower topples when center of gravity is no longer above base. Assuming center of gravity
is located at the very center of the tower, then when the tower leans 7.0 m then the tower falls. This
is 2.5 m farther than the present.
(b) θ= arcsin(7.0 m/55 m) = 7.3.
E9-24 If the torque from the force is sufficient to lift edge the cube then the cube will tip. The
net torque about the edge which stays in contact with the ground will be τ=F d mgd/2 if Fis
sufficiently large. Then Fmg/2 is the minimum force which will cause the cube to tip.
The minimum force to get the cube to slide is Fµsmg = (0.46)mg. The cube will slide first.
106
E9-25 The ladder slips if the force of static friction required to keep the ladder up exceeds µsN.
Equations 9-31 give us the normal force in terms of the masses of the ladder and the firefighter,
N= (m+M)g, and is independent of the location of the firefighter on the ladder. Also from Eq.
9-31 is the relationship between the force from the wall and the force of friction; the condition at
which slipping occurs is Fwµs(m+M)g.
Now go straight to Eq. 9-32. The a/2 in the second term is the location of the firefighter, who in
the example was halfway between the base of the ladder and the top of the ladder. In the exercise
we don’t know where the firefighter is, so we’ll replace a/2 with x. Then
Fwh+Mgx +mga
3= 0
is an expression for rotational equilibrium. Substitute in the condition of Fwwhen slipping just
starts, and we get
(µs(m+M)g)h+Mgx +mga
3= 0.
Solve this for x,
x=µsm
M+ 1hma
3M= (0.54) 45 kg
72 kg + 1(9.3 m) (45 kg)(7.6 m)
3(72 kg) = 6.6 m
This is the horizontal distance; the fraction of the total length along the ladder is then given by
x/a = (6.6 m)/(7.6 m) = 0.87. The firefighter can climb (0.87)(12 m) = 10.4 m up the ladder.
E9-26 (a) The net torque about the rear axle is (1360 kg)(9.8 m/s2)(3.05 m1.78 m)Ff(3.05 m) =
0, which has solution Ff= 5.55×103N. Each of the front tires support half of this, or 2.77×103N.
(b) The net torque about the front axle is (1360 kg)(9.8 m/s2)(1.78 m) Ff(3.05 m) = 0, which
has solution Ff= 7.78×103N. Each of the front tires support half of this, or 3.89×103N.
E9-27 The net torque on the bridge about the end closest to the person is
(160 lb)L/4 + (600 lb)L/2FfL= 0,
which has a solution for the supporting force on the far end of Ff= 340 lb.
The net force on the bridge is (160 lb)L/4 + (600 lb)L/2(340 lb) Fc= 0, so the force on the
close end of the bridge is Fc= 420 lb.
E9-28 The net torque on the board about the left end is
Fr(1.55 m) (142 N)(2.24 m) (582 N)(4.48 m) = 0,
which has a solution for the supporting force for the right pedestal of Fr= 1890 N. The force on the
board from the pedestal is up, so the force on the pedestal from the board is down (compression).
The net force on the board is Fl+(1890 N)(142 N)(582 N) = 0, so the force from the pedestal
on the left is Fl=1170 N. The negative sign means up, so the pedestal is under tension.
E9-29 We can assume that both the force ~
Fand the force of gravity ~
Wact on the center of the
wheel. Then the wheel will just start to lift when
~
W×~
r+~
F×~
r= 0,
or
Wsin θ=Fcos θ,
107
where θis the angle between the vertical (pointing down) and the line between the center of the
wheel and the point of contact with the step. The use of the sine on the left is a straightforward
application of Eq. 9-2. Why the cosine on the right? Because
sin(90θ) = cos θ.
Then F=Wtan θ. We can express the angle θin terms of trig functions, h, and r.rcos θis the
vertical distance from the center of the wheel to the top of the step, or rh. Then
cos θ= 1 h
rand sin θ=s11h
r2
.
Finally by combining the above we get
F=Wq2h
rh2
r2
1h
r
=W2hr h2
rh.
E9-30 (a) Assume that each of the two support points for the square sign experience the same
tension, equal to half of the weight of the sign. The net torque on the rod about an axis through
the hinge is
(52.3 kg/2)(9.81 m/s2)(0.95 m) + (52.3 kg/2)(9.81 m/s2)(2.88 m) (2.88 m)Tsin θ= 0,
where Tis the tension in the cable and θis the angle between the cable and the rod. The angle can
be found from θ= arctan(4.12 m/2.88 m) = 55.0, so T= 416 N.
(b) There are two components to the tension, one which is vertical, (416 N) sin(55.0) = 341 N,
and another which is horizontal, (416 N) cos(55.0) = 239 N. The horizontal force exerted by the wall
must then be 239 N. The net vertical force on the rod is F+ (341 N) (52.3 kg/2)(9.81 m/s2) = 0,
which has solution F= 172 N as the vertical upward force of the wall on the rod.
E9-31 (a) The net torque on the rod about an axis through the hinge is
τ=W(L/2) cos(54.0)T L sin(153.0) = 0.
or T= (52.7 lb/2)(sin 54.0/sin 153.0) = 47.0 lb.
(b) The vertical upward force of the wire on the rod is Ty=Tcos(27.0). The vertical upward
force of the wall on the rod is Py=WTcos(27.0), where Wis the weight of the rod. Then
Py= (52.7 lb) (47.0 lb) cos(27.0) = 10.8 lb
The horizontal force from the wall is balanced by the horizontal force from the wire. Then Px=
(47.0 lb) sin(27.0) = 21.3 lb.
E9-32 If the ladder is not slipping then the torque about an axis through the point of contact with
the ground is
τ= (W L/2) cos θNh/ sin θ= 0,
where Nis the normal force of the edge on the ladder. Then N=W L cos θsin θ /(2h).
Nhas two components; one which is vertically up, Ny=Ncos θ, and another which is horizontal,
Nx=Nsin θ. The horizontal force must be offset by the static friction.
The normal force on the ladder from the ground is given by
Ng=WNcos θ=W[1 Lcos2θsin θ /(2h)].
108
The force of static friction can be as large as f=µsNg, so
µs=W L cos θsin2θ /(2h)
W[1 Lcos2θsin θ /(2h)] =Lcos θsin2θ
2hLcos2θsin θ.
Put in the numbers and θ= 68.0. Then µs= 0.407.
E9-33 Let out be positive. The net torque about the axis is then
τ= (0.118 m)(5.88 N) (0.118 m)(4.13 m) (0.0493 m)(2.12 N) = 0.102 N ·m.
The rotational inertia of the disk is I= (1.92 kg)(0.118 m)2/2 = 1.34 ×102kg ·m2. Then α=
(0.102 N ·m)/(1.34×102kg ·m2) = 7.61 rad/s2.
E9-34 (a) I=τ= (960 N ·m)/(6.23 rad/s2) = 154 kg ·m2.
(b) m= (3/2)I/r2= (1.5)(154 kg ·m2)/(1.88 m)2= 65.4 kg.
E9-35 (a) The angular acceleration is
α=ω
t=6.20 rad/s
0.22 s = 28.2 rad/s2
(b) From Eq. 9-11, τ=Iα = (12.0 kg·m2)(28.2 rad/s2) = 338 N·m.
E9-36 The angular acceleration is α= 2(π/2 rad)/(30 s)2= 3.5×103rad/s2. The required force
is then
F=τ/r =Iα/r = (8.7×104kg ·m2)(3.5×103rad/s2)/(2.4 m) = 127 N.
Don’t let the door slam...
E9-37 The torque is τ=rF , the angular acceleration is α=τ/I =rF/I. The angular velocity is
ω=Zt
0
α dt =rAt2
2I+rBt3
3I,
so when t= 3.60 s,
ω=(9.88×102m)(0.496 N/s)(3.60 s)2
2(1.14×103kg ·m2)+(9.88×102m)(0.305 N/s2)(3.60 s)3
3(1.14×103kg ·m2)= 690 rad/s.
E9-38 (a) α= 2θ/t2.
(b) a=αR = 2θR/t2.
(c) T1and T2are not equal. Instead, (T1T2)R=Iα. For the hanging block Mg T1=Ma.
Then
T1=Mg 2M/t2,
and
T2=Mg 2M/t22(I/R)θ/t2.
109
E9-39 Apply a kinematic equation from chapter 2 to find the acceleration:
y=v0yt+1
2ayt2,
ay=2y
t2=2(0.765 m)
(5.11 s)2= 0.0586 m/s2
Closely follow the approach in Sample Problem 9-10. For the heavier block, m1= 0.512 kg, and
Newton’s second law gives
m1gT1=m1ay,
where ayis positive and down. For the lighter block, m2= 0.463 kg, and Newton’s second law gives
T2m2g=m2ay,
where ayis positive and up. We do know that T1> T2; the net force on the pulley creates a torque
which results in the pulley rotating toward the heavier mass. That net force is T1T2; so the
rotational form of Newton’s second law gives
(T1T2)R=Iαz=IaT/R,
where R= 0.049 m is the radius of the pulley and aTis the tangential acceleration. But this
acceleration is equal to ay, because everything— both blocks and the pulley— are moving together.
We then have three equations and three unknowns. We’ll add the first two together,
m1gT1+T2m2g=m1ay+m2ay,
T1T2= (gay)m1(g+ay)m2,
and then combine this with the third equation by substituting for T1T2,
(gay)m1(g+ay)m2=Iay/R2,
g
ay1m1g
ay
+ 1m2R2=I.
Now for the numbers:
(9.81 m/s2)
(0.0586 m/s2)1(0.512 kg) (9.81 m/s2)
(0.0586 m/s2)+ 1(0.463 kg) = 7.23 kg,
(7.23 kg)(0.049 m)2= 0.0174 kg·m2.
E9-40 The wheel turns with an initial angular speed of ω0= 88.0 rad/s. The average speed while
decelerating is ωav =ω0/2. The wheel stops turning in a time t=φ/ωav = 2φ/ω0. The deceleration
is then α=ω0/t =ω2
0/(2φ).
The rotational inertia is I=MR2/2, so the torque required to stop the disk is τ=Iα =
MR2ω2
0/(4φ).The force of friction on the disk is f=µN, so τ=Rf. Then
µ=M2
0
4Nφ =(1.40 kg)(0.23 m)(88.0 rad/s)2
4(130 N)(17.6 rad) = 0.272.
E9-41 (a) The automobile has an initial speed of v0= 21.8 m/s. The angular speed is then
ω0= (21.8 m/s)/(0.385 m) = 56.6 rad/s.
(b) The average speed while decelerating is ωav =ω0/2. The wheel stops turning in a time
t=φ/ωav = 2φ/ω0. The deceleration is then
α=ω0/t =ω2
0/(2φ) = (56.6 rad/s)2/[2(180 rad)] = 8.90 rad/s.
(c) The automobile traveled x=φr = (180 rad)(0.385 m) = 69.3 m.
110
E9-42 (a) The angular acceleration is derived in Sample Problem 9-13,
α=g
R0
1
1 + I/(MR2
0)=(981 cm/s2)
(0.320 cm)
1
1 + (0.950 kg ·cm2)/[(0.120 kg)(0.320 cm)2]= 39.1 rad/s2.
The acceleration is a=αR0= (39.1 rad/s2)(0.320 cm) = 12.5 cm/s2.
(b) Starting from rest, t=p2x/a =q2(134 cm)/(12.5 cm/s2) = 4.63 s.
(c) ω=αt = (39.1 rad/s2)(4.63 s) = 181 rad/s. This is the same as 28.8 rev/s.
(d) The yo-yo accelerates toward the ground according to y=at2+v0t, where down is positive.
The time required to move to the end of the string is found from
t=v0+pv2
0+ 4ay
2a=(1.30 m/s) + p(1.30 m/s)2+ 4(0.125 m/s2)(1.34 m)
2(0.125 m/s2)= 0.945 s
The initial rotational speed was ω0= (1.30 m/s)/(3.2×103m) = 406 rad/s. Then
ω=ω0+αt = (406 rad/s) + (39.1 rad/s2)(0.945 s) = 443 rad/s,
which is the same as 70.5 rev/s.
E9-43 (a) Assuming a perfect hinge at B, the only two vertical forces on the tire will be the
normal force from the belt and the force of gravity. Then N=W=mg, or N= (15.0 kg)(9.8
m/s2) = 147 N.
While the tire skids we have kinetic friction, so f=µkN= (0.600)(147 N) = 88.2 N. The force
of gravity and and the pull from the holding rod AB both act at the axis of rotation, so can’t
contribute to the net torque. The normal force acts at a point which is parallel to the displacement
from the axis of rotation, so it doesn’t contribute to the torque either (because the cross product
would vanish); so the only contribution to the torque is from the frictional force.
The frictional force is perpendicular to the radial vector, so the magnitude of the torque is just
τ=rf = (0.300 m)(88.2 N) = 26.5 N·m. This means the angular acceleration will be α=τ/I =
(26.5 N·m)/(0.750 kg·m2) = 35.3 rad/s2.
When ωR =vT= 12.0 m/s the tire is no longer slipping. We solve for ωand get ω= 40 rad/s.
Now we solve ω=ω0+αt for the time. The wheel started from rest, so t= 1.13 s.
(b) The length of the skid is x=vt = (12.0 m/s)(1.13 s) = 13.6 m long.
P9-1 The problem of sliding down the ramp has been solved (see Sample Problem 5-8); the
critical angle θsis given by tan θs=µs.
The problem of tipping is actually not that much harder: an object tips when the center of
gravity is no longer over the base. The important angle for tipping is shown in the figure below; we
can find that by trigonometry to be
tan θt=O
A=(0.56 m)
(0.56 m) + (0.28 m) = 0.67,
so θt= 34.
111
W
Nf
θ
(a) If µs= 0.60 then θs= 31and the crate slides.
(b) If µs= 0.70 then θs= 35and the crate tips before sliding; it tips at 34.
P9-2 (a) The total force up on the chain needs to be equal to the total force down; the force
down is W. Assuming the tension at the end points is Tthen Tsin θis the upward component, so
T=W/(2 sin θ).
(b) There is a horizontal component to the tension Tcos θat the wall; this must be the tension
at the horizontal point at the bottom of the cable. Then Tbottom =W/(2 tan θ).
P9-3 (a) The rope exerts a force on the sphere which has horizontal Tsin θand vertical Tcos θ
components, where θ= arctan(r/L). The weight of the sphere is balanced by the upward force from
the rope, so Tcos θ=W. But cos θ=L/r2+L2, so T=Wp1 + r2/L2.
(b) The wall pushes outward against the sphere equal to the inward push on the sphere from the
rope, or P=Tsin θ=Wtan θ=W r/L.
P9-4 Treat the problem as having two forces: the man at one end lifting with force F=W/3
and the two men acting together a distance xaway from the first man and lifting with a force
2F= 2W/3. Then the torque about an axis through the end of the beam where the first man is
lifting is τ= 2xW/3W L/2, where Lis the length of the beam. This expression equal zero when
x= 3L/4.
P9-5 (a) We can solve this problem with Eq. 9-32 after a few modifications. We’ll assume the
center of mass of the ladder is at the center, then the third term of Eq. 9-32 is mga/2. The cleaner
didn’t climb half-way, he climbed 3.10/5.12 = 60.5% of the way, so the second term of Eq. 9-32
becomes Mga(0.605). h,L, and aare related by L2=a2+h2, so h=p(5.12 m)2(2.45 m)2= 4.5
m. Then, putting the correction into Eq. 9-32,
Fw=1
hhMga(0.605) + mga
2i,
=1
(4.5 m) h(74.6 kg)(9.81 m/s2)(2.45 m)(0.605) ,
+ (10.3 kg)(9.81 m/s2)(2.45 m)/2i,
= 269 N
(b) The vertical component of the force of the ground on the ground is the sum of the weight of
the window cleaner and the weight of the ladder, or 833 N.
112
The horizontal component is equal in magnitude to the force of the ladder on the window. Then
the net force of the ground on the ladder has magnitude
p(269 N)2+ (833 N)2= 875 N
and direction
θ= arctan(833/269) = 72above the horizontal.
P9-6 (a) There are no upward forces other than the normal force on the bottom ball, so the force
exerted on the bottom ball by the container is 2W.
(c) The bottom ball must exert a force on the top ball which has a vertical component equal to the
weight of the top ball. Then W=Nsin θor the force of contact between the balls is N=W/ sin θ.
(b) The force of contact between the balls has a horizontal component P=Ncos θ=W/ tan θ,
this must also be the force of the walls on the balls.
P9-7 (a) There are three forces on the ball: weight ~
W, the normal force from the lower plane ~
N1,
and the normal force from the upper plane ~
N2. The force from the lower plane has components
N1,x =N1sin θ1and N1,y =N1cos θ1. The force from the upper plane has components N2,x =
N2sin θ2and N2,y =N2cos θ2. Then N1sin θ1=N2sin θ2and N1cos θ1=W+N2cos θ2.
Solving for N2by dividing one expression by the other,
cos θ1
sin θ1
=W
N2sin θ2
+cos θ2
sin θ2
,
or
N2=W
sin θ2cos θ1
sin θ1cos θ2
sin θ21
,
=W
sin θ2
cos θ1sin θ2cos θ2sin θ1
sin θ1sin θ2
,
=Wsin θ1
sin(θ2θ1).
Then solve for N1,
N1=Wsin θ2
sin(θ2θ1).
(b) Friction changes everything.
P9-8 (a) The net torque about a line through Ais
τ=W x T L sin θ= 0,
so T=W x/(Lsin θ).
(b) The horizontal force on the pin is equal in magnitude to the horizontal component of the
tension: Tcos θ=W x/(Ltan θ). The vertical component balances the weight: WW x/L.
(c) x= (520 N)(2.75 m) sin(32.0)/(315 N) = 2.41 m.
P9-9 (a) As long as the center of gravity of an object (even if combined) is above the base, then
the object will not tip.
Stack the bricks from the top down. The center of gravity of the top brick is L/2 from the edge
of the top brick. This top brick can be offset nor more than L/2 from the one beneath. The center
of gravity of the top two bricks is located at
xcm = [(L/2) + (L)]/2 = 3L/4.
113
These top two bricks can be offset no more than L/4 from the brick beneath. The center of gravity
of the top three bricks is located at
xcm = [(L/2) + 2(L)]/3 = 5L/6.
These top three bricks can be offset no more than L/6 from the brick beneath. The total offset is
then L/2 + L/4 + L/6 = 11L/12.
(b) Actually, we never need to know the location of the center of gravity; we now realize that
each brick is located with an offset L/(2n) with the brick beneath, where nis the number of the
brick counting from the top. The series is then of the form
(L/2)[(1/1) + (1/2) + (1/3) + (1/4) + ···],
a series (harmonic, for those of you who care) which does not converge.
(c) The center of gravity would be half way between the ends of the two extreme bricks. This
would be at NL/n; the pile will topple when this value exceeds L, or when N=n.
P9-10 (a) For a planar object which lies in the xyplane, Ix=Rx2dm and Iy=Ry2dm. Then
Ix+Iy=R(x2+y2)dm =Rr2dm. But this is the rotational inertia about the zaxis, sent ris the
distance from the zaxis.
(b) Since the rotational inertia about one diameter (Ix) should be the same as the rotational
inertia about any other (Iy) then Ix=Iyand Ix=Iz/2 = MR2/4.
P9-11 Problem 9-10 says that Ix+Iy=Izfor any thin, flat object which lies only in the
xyplane.It doesn’t matter in which direction the xand yaxes are chosen, so long as they are
perpendicular. We can then orient our square as in either of the pictures below:
y
x
y
x
By symmetry Ix=Iyin either picture. Consequently, Ix=Iy=Iz/2 for either picture. It is
the same square, so Izis the same for both picture. Then Ixis also the same for both orientations.
P9-12 Let M0be the mass of the plate before the holes are cut out. Then M1=M0(a/L)2is the
mass of the part cut out of each hole and M=M09M1is the mass of the plate. The rotational
inertia (about an axis perpendicular to the plane through the center of the square) for the large
uncut square is M0L2/6 and for each smaller cut out is M1a2/6.
From the large uncut square’s inertia we need to remove M1a2/6 for the center cut-out, M1a2/6+
M1(L/3)2for each of the four edge cut-outs, and M1a2/6 + M1(2L/3)2for each of the corner
sections.
114
Then
I=M0L2
69M1a2
64M1L2
942M1L2
9,
=M0L2
63M0a4
2L24M0a2
3.
P9-13 (a) From Eq. 9-15, I=Rr2dm about some axis of rotation when ris measured from that
axis. If we consider the xaxis as our axis of rotation, then r=py2+z2, since the distance to the
xaxis depends only on the yand zcoordinates. We have similar equations for the yand zaxes, so
Ix=Zy2+z2dm,
Iy=Zx2+z2dm,
Iz=Zx2+y2dm.
These three equations can be added together to give
Ix+Iy+Iz= 2 Zx2+y2+z2dm,
so if we now define rto be measured from the origin (which is not the definition used above), then
we end up with the answer in the text.
(b) The right hand side of the equation is integrated over the entire body, regardless of how
the axes are defined. So the integral should be the same, no matter how the coordinate system is
rotated.
P9-14 (a) Since the shell is spherically symmetric Ix=Iy=Iz, so Ix= (2/3) Rr2dm =
(2R2/3) Rdm = 2MR2/3.
(b) Since the solid ball is spherically symmetric Ix=Iy=Iz, so
Ix=2
3Zr23Mr2dr
R3=2
5MR2.
P9-15 (a) A simple ratio will suffice:
dm
2πr dr =M
πR2or dm =2Mr
R2dr.
(b) dI =r2dm = (2Mr3/R2)dr.
(c) I=RR
0(2Mr3/R2)dr =MR2/2.
P9-16 (a) Another simple ratio will suffice:
dm
πr2dz =M
(4/3)πR3or dm =3M(R2z2)
4R3dz.
(b) dI =r2dm/2 = [3M(R2z2)2/8R3]dz.
115
(c) There are a few steps to do here:
I=ZR
R
3M(R2z2)2
8R3dz,
=3M
4R3ZR
0
(R42R2z2+z4)dz,
=3M
4R3(R52R5/3 + R5/5) = 2
5MR2.
P9-17 The rotational acceleration will be given by αz=Pτ /I.
The torque about the pivot comes from the force of gravity on each block. This forces will both
originally be at right angles to the pivot arm, so the net torque will be Pτ=mgL2mgL1, where
clockwise as seen on the page is positive.
The rotational inertia about the pivot is given by I=Pmnr2
n=m(L2
2+L2
1). So we can now
find the rotational acceleration,
α=Pτ
I=mgL2mgL1
m(L2
2+L2
1)=gL2L1
L2
2+L2
1
= 8.66 rad/s2.
The linear acceleration is the tangential acceleration, aT=αR. For the left block, aT= 1.73 m/s2;
for the right block aT= 6.93 m/s2.
P9-18 (a) The force of friction on the hub is µkMg. The torque is τ=µkM ga. The angular
acceleration has magnitude α=τ/I =µkga/k2. The time it takes to stop the wheel will be
t=ω0=ω0k2/(µkga).
(b) The average rotational speed while slowing is ω0/2. The angle through which it turns while
slowing is ω0t/2 radians, or ω0t/(4π) = ω2
0k2/(4πµkga)
P9-19 (a) Consider a differential ring of the disk. The torque on the ring because of friction is
=r dF =rµkMg
πR22πr dr =2µkM gr2
R2dr.
The net torque is then
τ=Z=ZR
0
2µkMgr2
R2dr =2
3µkMgR.
(b) The rotational acceleration has magnitude α=τ/I =4
3µkg/R. Then it will take a time
t=ω0=30
4µkg
to stop.
P9-20 We need only show that the two objects have the same acceleration.
Consider first the hoop. There is a force W|| =Wsin θ=mg sin θpulling it down the ramp
and a frictional force fpulling it up the ramp. The frictional force is just large enough to cause
a torque that will allow the hoop to roll without slipping. This means a=αR; consequently,
fR =αI =aI/R. In this case I=mR2.
The acceleration down the plane is
ma =mg sin θf=mg sin θmaI/R2=mg sin θma.
116
Then a=gsin θ /2. The mass and radius are irrelevant!
For a block sliding with friction there are also two forces: W|| =Wsin θ=mg sin θand f=
µkmg cos θ. Then the acceleration down the plane will be given by
a=gsin θµkgcos θ,
which will be equal to that of the hoop if
µk=sin θsin θ/2
cos θ=1
2tan θ.
P9-21 This problem is equivalent to Sample Problem 9-11, except that we have a sphere instead
of a cylinder. We’ll have the same two equations for Newton’s second law,
Mg sin θf=Macm and NMg cos θ= 0.
Newton’s second law for rotation will look like
fR =Icmα.
The conditions for accelerating without slipping are acm =αR, rearrange the rotational equation to
get
f=Icmα
R=Icm(acm)
R2,
and then
Mg sin θIcm(acm)
R2=Macm,
and solve for acm. For fun, let’s write the rotational inertia as I=βM R2, where β= 2/5 for the
sphere. Then, upon some mild rearranging, we get
acm =gsin θ
1 + β
For the sphere, acm = 5/7gsin θ.
(a) If acm = 0.133g, then sin θ= 7/5(0.133) = 0.186, and θ= 10.7.
(b) A frictionless block has no rotational properties; in this case β= 0! Then acm =gsin θ=
0.186g.
P9-22 (a) There are three forces on the cylinder: gravity Wand the tension from each cable T.
The downward acceleration of the cylinder is then given by ma =W2T.
The ropes unwind according to α=a/R, but α=τ/I and I=mR2/2. Then
a=τR/I = (2T R)R/(mR2/2) = 4T/m.
Combining the above, 4T=W2T, or T=W/6.
(b) a= 4(mg/6)/m = 2g/3.
P9-23 The force of friction required to keep the cylinder rolling is given by
f=1
3Mg sin θ;
the normal force is given to be N=Mg cos θ; so the coefficient of static friction is given by
µsf
N=1
3tan θ.
117
P9-24 a=F/M, since Fis the net force on the disk. The torque about the center of mass is F R,
so the disk has an angular acceleration of
α=F R
I=F R
MR2/2=2F
M R .
P9-25 This problem is equivalent to Sample Problem 9-11, except that we have an unknown rolling
object. We’ll have the same two equations for Newton’s second law,
Mg sin θf=Macm and NMg cos θ= 0.
Newton’s second law for rotation will look like
fR =Icmα.
The conditions for accelerating without slipping are acm =αR, rearrange the rotational equation to
get
f=Icmα
R=Icm(acm)
R2,
and then
Mg sin θIcm(acm)
R2=Macm,
and solve for acm. Write the rotational inertia as I=βMR2, where β= 2/5 for a sphere, β= 1/2
for a cylinder, and β= 1 for a hoop. Then, upon some mild rearranging, we get
acm =gsin θ
1 + β
Note that ais largest when βis smallest; consequently the cylinder wins. Neither Mnor Rentered
into the final equation.
118
E10-1 l=rp =mvr = (13.7×103kg)(380 m/s)(0.12 m) = 0.62 kg ·m2/s.
E10-2 (a) ~
l=m~
r×~
v, or
~
l=m(yvzzvy)ˆ
i+m(zvxxvz)ˆ
j+m(xvyyvx)ˆ
k.
(b) If ~
vand ~
rexist only in the xy plane then z=vz= 0, so only the uk term survives.
E10-3 If the angular momentum ~
lis constant in time, then d
~
l/dt = 0. Trying this on Eq. 10-1,
d
~
l
dt =d
dt (~
r×~
p),
=d
dt (~
r×m~
v),
=md~
r
dt ×~
v+m~
r×d~
v
dt ,
=m~
v×~
v+m~
r×~
a.
Now the cross product of a vector with itself is zero, so the first term vanishes. But in the exercise
we are told the particle has constant velocity, so ~
a= 0, and consequently the second term vanishes.
Hence, ~
lis constant for a single particle if ~
vis constant.
E10-4 (a) L=Pli;li=rimivi. Putting the numbers in for each planet and then summing (I
won’t bore you with the arithmetic details) yields L= 3.15×1043kg ·m2/s.
(b) Jupiter has l= 1.94×1043kg ·m2/s,which is 61.6% of the total.
E10-5 l=mvr =m(2πr/T )r= 2π(84.3 kg)(6.37×106m)2/(86400 s) = 2.49×1011kg ·m2/s.
E10-6 (a) Substitute and expand:
~
L=X(~
rcm +~
r0
i)×(mi~
vcm +~
p0
i),
=X(mi~
rcm ×~
vcm +~
rcm ×~
p0
i+mi~
r0
i×~
vcm +~
r0
i×~
p0
i),
=M~
rcm ×~
vcm +~
rcm ×(X~
p0
i)+(Xmi~
r0
i)×~
vcm +X~
r0
i×~
p0
i.
(b) But P~
p0
i= 0 and Pmi~
r0
i= 0, because these two quantities are in the center of momentum
and center of mass. Then
~
L=M~
rcm ×~
vcm +X~
r0
i×~
p0
i=~
L0+M~
rcm ×~
vcm.
E10-7 (a) Substitute and expand:
~
p0
i=mi
d~
r0
i
dt =mi
d~
ri
dt mi
d~
rcm
dt =~
pimi~
vcm.
(b) Substitute and expand:
d~
L0
dt =Xd~
r0
i
dt ×~
p0
i+X~
r0
i×d~
p0
i
dt =X~
r0
i×d~
p0
i
dt .
The first term vanished because ~
v0
iis parallel to ~
p0
i.
119
(c) Substitute and expand:
d~
L0
dt =X~
r0
i×d(~
pimi~
vcm)
dt ,
=X~
r0
i×(mi~
aimi~
acm),
=X~
r0
i×mi~
ai+Xmi~
r0
i×~
acm)
The second term vanishes because of the definition of the center of mass. Then
d~
L0
dt =X~
r0
i×~
Fi,
where ~
Fiis the net force on the ith particle. The force ~
Fimay include both internal and external
components. If there is an internal component, say between the ith and jth particles, then the
torques from these two third law components will cancel out. Consequently,
d~
L0
dt =X~τi=~τext.
E10-8 (a) Integrate.
Z~τ dt =Zd~
L
dt dt =Zd~
L= ∆~
L.
(b) If Iis fixed, ∆L=Iω. Not only that,
Zτdt =ZF r dt =rZF dt =rF avt,
where we use the definition of average that depends on time.
E10-9 (a) ~τt= ∆
~
l.The disk starts from rest, so ∆
~
l=~
l~
l0=~
l. We need only concern
ourselves with the magnitudes, so
l= ∆l=τt= (15.8 N·m)(0.033 s) = 0.521 kg·m2/s.
(b) ω=l/I = (0.521 kg·m2/s)/(1.22×103kg·m2) = 427 rad/s.
E10-10 (a) Let v0be the initial speed; the average speed while slowing to a stop is v0/2; the time
required to stop is t= 2x/v0; the acceleration is a=v0/t =v2
0/(2x). Then
a=(43.3 m/s)2/[2(225 m/s)] = 4.17 m/s2.
(b) α=a/r = (4.17 m/s2)/(0.247 m) = 16.9 rad/s2.
(c) τ=Iα = (0.155 kg ·m2)(16.9 rad/s2) = 2.62 N ·m.
E10-11 Let ~
ri=~
z+~
r0
i.From the figure, ~
p1=~
p2and ~
r0
1=~
r0
2. Then
~
L=~
l1+~
l2=~
r1×~
p1+~
r2×~
p2,
= (~
r1~
r2)×~
p1,
= (~
r0
1~
r0
2)×~
p1,
= 2~
r0
1×~
p1.
Since ~
r0
1and ~
p1both lie in the xy plane then ~
Lmust be along the zaxis.
120
E10-12 Expand:
~
L=X~
li=X~
ri×~
pi,
=Xmi~
ri×~
vi=Xmi~
ri×(~ω ×~
ri)
=Xmi[(~
ri·~
ri)~ω (~
ri·~ω)~
r)i],
=Xmi[r2
i~ω (z2
iω)ˆ
k(zixiω)ˆ
i(ziyiω)ˆ
j],
but if the body is symmetric about the zaxis then the last two terms vanish, leaving
~
L=Xmi[r2
i~ω (z2
iω)ˆ
k] = Xmi(x2
i+y2
i)~ω =I~ω.
E10-13 An impulse of 12.8 N·s will change the linear momentum by 12.8 N·s; the stick starts
from rest, so the final momentum must be 12.8 N·s. Since p=mv, we then can find v=p/m = (12.8
N·s)/(4.42 kg) = 2.90 m/s.
Impulse is a vector, given by R~
Fdt. We can take the cross product of the impulse with the
displacement vector ~
r(measured from the axis of rotation to the point where the force is applied)
and get
~
r×Z~
Fdt Z~
r×~
Fdt,
The two sides of the above expression are only equal if ~
rhas a constant magnitude and direction. This
won’t be true, but if the force is of sufficiently short duration then it hopefully won’t change much.
The right hand side is an integral over a torque, and will equal the change in angular momentum of
the stick.
The exercise states that the force is perpendicular to the stick, then |~
r×~
F|=rF , and the “torque
impulse” is then (0.464 m)(12.8 N·s) = 5.94 kg·m/s. This “torque impulse” is equal to the change
in the angular momentum, but the stick started from rest, so the final angular momentum of the
stick is 5.94 kg·m/s.
But how fast is it rotating? We can use Fig. 9-15 to find the rotational inertia about the center
of the stick: I=1
12 ML2=1
12 (4.42 kg)(1.23 m)2= 0.557 kg·m2. The angular velocity of the stick is
ω=l/I = (5.94 kg·m/s)/(0.557 kg·m2) = 10.7 rad/s.
E10-14 The point of rotation is the point of contact with the plane; the torque about that point
is τ=rmg sin θ. The angular momentum is Iω, so τ=Iα. In this case I=mr2/2 + mr2, the
second term from the parallel axis theorem. Then
a=rα = /I =mr2gsin θ/(3mr2/2) = 2
3gsin θ.
E10-15 From Exercise 8 we can immediately write
I1(ω1ω0)/r1=I2(ω20)/r2,
but we also have r1ω1=r2ω2. Then
ω2=r1r2I1ω0
r2
1I2r2
2I1
.
121
E10-16 (a) ∆ω= (1/T11/T2)/(1/T1) = (T2T1)/T2=T/T , which in this case is
(6.0×103s)(8.64×104s) = 6.9×108.
(b) Assuming conservation of angular momentum, ∆I/I =ω. Then the fractional change
would be 6.9×108.
E10-17 The rotational inertia of a solid sphere is I=2
5MR2; so as the sun collapses
~
Li=~
Lf,
Ii~ωi=If~ωf,
2
5MRi2~ωi=2
5MRf2~ωf,
Ri2~ωi=Rf2~ωf.
The angular frequency is inversely proportional to the period of rotation, so
Tf=Ti
Rf2
Ri2= (3.6×104min) (6.37×106m)
(6.96×108m) 2
= 3.0 min.
E10-18 The final angular velocity of the train with respect to the tracks is ωtt =Rv. The
conservation of angular momentum implies
0 = MR2ω+mR2(ωtt +ω),
or
ω=mv
(m+M)R.
E10-19 This is much like a center of mass problem.
0 = Ipφp+Im(φmp +φp),
or
φmp =(Ip+Im)φp
Im≈ − (12.6 kg ·m2)(25)
(2.47×103kg ·m2)= 1.28×105.
That’s 354 rotations!
E10-20 ωf= (Ii/If)ωi= [(6.13 kg ·m2)/(1.97 kg ·m2)](1.22 rev/s) = 3.80 rev/s.
E10-21 We have two disks which are originally not in contact which then come into contact;
there are no external torques. We can write
~
l1,i+~
l2,i=~
l1,f+~
l2,f,
I1~ω1,i+I2~ω2,i=I1~ω1,f+I2~ω2,f.
The final angular velocities of the two disks will be equal, so the above equation can be simplified
and rearranged to yield
ωf=I1
I1+I2
ω1,i=(1.27 kg ·m2)
(1.27 kg ·m2) + (4.85 kg ·m2)(824 rev/min) = 171 rev/min
E10-22 l=lcos θ=mvr cos θ=mvh.
122
E10-23 (a) ωf= (I1/I2)ωi,I1= (3.66 kg)(0.363 m)2= 0.482 kg ·m2. Then
ωf= [(0.482 kg ·m2)/(2.88 kg ·m2)](57.7 rad/s) = 9.66 rad/s,
with the same rotational sense as the original wheel.
(b) Same answer, since friction is an internal force internal here.
E10-24 (a) Assume the merry-go-round is a disk. Then conservation of angular momentum yields
(1
2mmR2+mgR2)ω+ (mrR2)(v/R) = 0,
or
ω=(1.13 kg)(7.82 m/s)/(3.72 m)
(827 kg)/2 + (50.6 kg) =5.12×103rad/s.
(b) v=ωR = (5.12×103rad/s)(3.72 m) = 1.90×102m/s.
E10-25 Conservation of angular momentum:
(mmk2+mgR2)ω=mgR2(v/R),
so
ω=(44.3 kg)(2.92 m/s)/(1.22 m)
(176 kg)(0.916 m)2+ (44.3 kg)(1.22 m)2= 0.496 rad/s.
E10-26 Use Eq. 10-22:
ωP=Mgr
Iω =(0.492 kg)(9.81 m/s2)(3.88×102m)
(5.12×104kg ·m2)(2π28.6 rad/s) = 2.04 rad/s = 0.324 rev/s.
E10-27 The relevant precession expression is Eq. 10-22.
The rotational inertia will be a sum of the contributions from both the disk and the axle, but
the radius of the axle is probably very small compared to the disk, probably as small as 0.5 cm.
Since Iis proportional to the radius squared, we expect contributions from the axle to be less than
(1/100)2of the value for the disk. For the disk only we use
I=1
2MR2=1
2(1.14 kg)(0.487 m)2= 0.135 kg ·m2.
Now for ω,
ω= 975 rev/min 2πrad
1 rev 1 min
60 s = 102 rad/s.
Then L=Iω = 13.8 kg·m2/s.
Back to Eq. 10-22,
ωp=Mgr
L=(1.27 kg)(9.81 m/s2)(0.0610 m)
13.8 kg ·m2/s= 0.0551 rad/s.
The time for one precession is
t=1rev
ωp
=2πrad
(0.0551 rad/s) = 114 s.
123
P10-1 Positive zis out of the page.
(a) ~
l=rmv sin θˆ
k= (2.91 m)(2.13 kg)(4.18 m) sin(147)ˆ
k= 14.1 kg ·m2/sˆ
k.
(b) ~τ =rF sin θˆ
k= (2.91 m)(1.88 N) sin(26)ˆ
k= 2.40 N ·mˆ
k.
P10-2 Regardless of where the origin is located one can orient the coordinate system so that the
two paths lie in the xy plane and are both parallel to the yaxis. The one of the particles travels
along the path x=vt,y=a,z=b; the momentum of this particle is ~
p1=mvˆ
i. The other
particle will then travel along a path x=cvt,y=a+d,z=b; the momentum of this particle is
~
p2=mvˆ
i. The angular momentum of the first particle is
~
l1=mvbˆ
jmvaˆ
k,
while that of the second is ~
l2=mvbˆ
j+mv(a+d)ˆ
k,
so the total is ~
l1+~
l2=mvdˆ
k.
P10-3 Assume that the cue stick strikes the ball horizontally with a force of constant magnitude
Ffor a time ∆t. Then the magnitude of the change in linear momentum of the ball is given by
Ft= ∆p=p, since the initial momentum is zero.
If the force is applied a distance xabove the center of the ball, then the magnitude of the torque
about a horizontal axis through the center of the ball is τ=xF . The change in angular momentum
of the ball is given by τt= ∆l=l, since initially the ball is not rotating.
For the ball to roll without slipping we need v=ωR. We can start with this:
v=ωR,
p
m=lR
I,
Ft
m=τtR
I,
F
m=xF R
I.
Then x=I/mR is the condition for rolling without sliding from the start. For a solid sphere,
I=2
5mR2, so x=2
5R.
P10-4 The change in momentum of the block is M(v2v1), this is equal to the magnitude the
impulse delivered to the cylinder. According to E10-8 we can write M(v2v1)R=Iωf. But in the
end the box isn’t slipping, so ωf=v2/R. Then
Mv2Mv1= (I/R2)v2,
or
v2=v1/(1 + I/M R2).
P10-5 Assume that the cue stick strikes the ball horizontally with a force of constant magnitude
Ffor a time ∆t. Then the magnitude of the change in linear momentum of the ball is given by
Ft= ∆p=p, since the initial momentum is zero. Consequently, Ft=mv0.
If the force is applied a distance habove the center of the ball, then the magnitude of the torque
about a horizontal axis through the center of the ball is τ=hF . The change in angular momentum
of the ball is given by τt= ∆l=l0,since initially the ball is not rotating. Consequently, the initial
angular momentum of the ball is l0=hmv0=Iω0.
124
The ball originally slips while moving, but eventually it rolls. When it has begun to roll without
slipping we have v=. Applying the results from E10-8,
m(vv0)R+I(ωω0) = 0,
or
m(vv0)R+2
5mR2v
Rhmv0= 0,
then, if v= 9v0/7,
h=9
71R+2
5R9
7=4
5R.
P10-6 (a) Refer to the previous answer. We now want v=ω= 0, so
m(vv0)R+2
5mR2v
Rhmv0= 0,
becomes
v0Rhv0= 0,
or h=R. That’ll scratch the felt.
(b) Assuming only a horizontal force then
v=(h+R)v0
R(1 + 2/5),
which can only be negative if h < R, which means hitting below the ball. Can’t happen. If instead
we allow for a downward component, then we can increase the “reverse English” as much as we want
without increasing the initial forward velocity, and as such it would be possible to get the ball to
move backwards.
P10-7 We assume the bowling ball is solid, so the rotational inertia will be I= (2/5)MR2(see
Figure 9-15).
The normal force on the bowling ball will be N=Mg, where Mis the mass of the bowling ball.
The kinetic friction on the bowling ball is Ff=µkN=µkMg. The magnitude of the net torque on
the bowling ball while skidding is then τ=µkMgR.
Originally the angular momentum of the ball is zero; the final angular momentum will have
magnitude l=Iω =Iv/R, where vis the final translational speed of the ball.
(a) The time requires for the ball to stop skidding is the time required to change the angular
momentum to l, so
t=l
τ=(2/5)MR2v/R
µkMgR =2v
5µkg.
Since we don’t know v, we can’t solve this for ∆t. But the same time through which the angular
momentum of the ball is increasing the linear momentum of the ball is decreasing, so we also have
t=p
Ff
=Mv Mv0
µkMg =v0v
µkg.
Combining,
t=v0v
µkg,
=v05µkgt/2
µkg,
125
2µkgt= 2v05µkgt,
t=2v0
7µkg,
=2(8.50 m/s)
7(0.210)(9.81 m/s2)= 1.18 s.
(d) Use the expression for angular momentum and torque,
v= 5µkgt/2 = 5(0.210)(9.81 m/s2)(1.18 s)/2 = 6.08 m/s.
(b) The acceleration of the ball is F/M =µg. The distance traveled is then given by
x=1
2at2+v0t,
=1
2(0.210)(9.81 m/s2)(1.18 s)2+ (8.50 m/s)(1.18 s) = 8.6 m,
(c) The angular acceleration is τ/I = 5µkg/(2R). Then
θ=1
2αt2+ω0t,
=5(0.210)(9.81 m/s2)
4(0.11 m) (1.18 s)2= 32.6 rad = 5.19 revolutions.
P10-8 (a) l=Iω0= (1/2)MR2ω0.
(b) The initial speed is v0=0. The chip decelerates in a time t=v0/g, and during this time
the chip travels with an average speed of v0/2 through a distance of
y=vavt=v0
2
v0
g=R2ω2
2g.
(c) Loosing the chip won’t change the angular velocity of the wheel.
P10-9 Since L=Iω = 2πI/T and Lis constant, then IT. But IR2, so R2Tand
T
T=2RR
R2=2∆R
R.
Then
T= (86400 s) 2(30 m)
(6.37×106m) 0.8 s.
P10-10 Originally the rotational inertia was
Ii=2
5MR2=8π
15 ρ0R5.
The average density can be found from Appendix C. Now the rotational inertia is
If=8π
15 (ρ1ρ2)R5
1+8π
15 ρ2R5,
where ρ1is the density of the core, R1is the radius of the core, and ρ2is the density of the mantle.
Since the angular momentum is constant we have ∆T /T = ∆I/I. Then
T
T=ρ1ρ2
ρ0
R5
1
R5+ρ2
ρ01 = 10.34.50
5.52
35705
63705+4.50
5.52 1 = 0.127,
so the day is getting longer.
126
P10-11 The cockroach initially has an angular speed of ωc,i=v/r. The rotational inertia of
the cockroach about the axis of the turntable is Ic=mR2. Then conservation of angular momentum
gives
lc,i+ls,i=lc,f+ls,f,
Icωc,i+Isωs,i=Icωc,f+Isωs,f,
mR2v/r +Iω = (mR2+I)ωf,
ωf=Iω mvR
I+mR2.
P10-12 (a) The skaters move in a circle of radius R= (2.92 m)/2=1.46 m centered midway
between the skaters. The angular velocity of the system will be ωi=v/R = (1.38 m/s)/(1.46 m) =
0.945 rad/s.
(b) Moving closer will decrease the rotational inertia, so
ωf=2MRi2
2MRf2ωi=(1.46 m)2
(0.470 m)2(0.945 rad/s) = 9.12 rad/s.
127
E11-1 (a) Apply Eq. 11-2, W=F s cos φ= (190 N)(3.3 m) cos(22) = 580 J.
(b) The force of gravity is perpendicular to the displacement of the crate, so there is no work
done by the force of gravity.
(c) The normal force is perpendicular to the displacement of the crate, so there is no work done
by the normal force.
E11-2 (a) The force required is F=ma = (106 kg)(1.97 m/s2) = 209 N. The object moves with
an average velocity vav =v0/2 in a time t=v0/a through a distance x=vavt=v2
0/(2a). So
x= (51.3 m/s)2/[2(1.97 m/s2)] = 668 m.
The work done is W=F x = (209 N)(668 m) = 1.40×105J.
(b) The force required is
F=ma = (106 kg)(4.82 m/s2) = 511 N.
x= (51.3 m/s)2/[2(4.82 m/s2)] = 273 m.The work done is W=F x = (511 N)(273 m) = 1.40×105J.
E11-3 (a) W=F x = (120 N)(3.6 m) = 430 J.
(b) W=F x cos θ=mgx cos θ= (25 kg)(9.8 m/s2)(3.6 m) cos(117) = 400 N.
(c) W=F x cos θ, but θ= 90, so W= 0.
E11-4 The worker pushes with a force ~
P; this force has components Px=Pcos θand Py=Psin θ,
where θ=32.0. The normal force of the ground on the crate is N=mg Py, so the force of
friction is f=µkN=µk(mg Py). The crate moves at constant speed, so Px=f. Then
Pcos θ=µk(mg Psin θ),
P=µkmg
cos θ+µksin θ.
The work done on the crate is
W=~
P·~
x=P x cos θ=µkxmg
1 + µktan θ,
=(0.21)(31.3 ft)(58.7 lb)
1 + (0.21) tan(32.0)= 444 ft ·lb.
E11-5 The components of the weight are W|| =mg sin θand W=mg cos θ. The push ~
Phas
components P|| =Pcos θand P=Psin θ.
The normal force on the trunk is N=W+Pso the force of friction is f=µk(mg cos θ+
Psin θ). The push required to move the trunk up at constant speed is then found by noting that
P|| =W|| +f.
Then
P=mg(tan θ+µk)
1µktan θ.
(a) The work done by the applied force is
W=P x cos θ=(52.3 kg)(9.81 m/s2)[sin(28.0) + (0.19) cos(28.0)](5.95 m)
1(0.19) tan(28.0)= 2160 J.
(b) The work done by the force of gravity is
W=mgx cos(θ+ 90) = (52.3 kg)(9.81 m/s2)(5.95 m) cos(118) = 1430 J.
128
E11-6 θ= arcsin(0.902 m/1.62 m) = 33.8.
The components of the weight are W|| =mg sin θand W=mg cos θ.
The normal force on the ice is N=Wso the force of friction is f=µkmg cos θ. The push
required to allow the ice to slide down at constant speed is then found by noting that P=W|| f.
Then P=mg(sin θµkcos θ).
(a) P= (47.2 kg)(9.81 m/s2)[sin(33.8)(0.110) cos(33.8)] = 215 N.
(b) The work done by the applied force is W=P x = (215 N)(1.62 m) = 348 J.
(c) The work done by the force of gravity is
W=mgx cos(90θ) = (47.2 kg)(9.81 m/s2)(1.62 m) cos(56.2) = 417 J.
E11-7 Equation 11-5 describes how to find the dot product of two vectors from components,
~
a·~
b=axbx+ayby+azbz,
= (3)(2) + (3)(1) + (3)(3) = 18.
Equation 11-3 can be used to find the angle between the vectors,
a=p(3)2+ (3)2+ (3)2= 5.19,
b=p(2)2+ (1)2+ (3)2= 3.74.
Now use Eq. 11-3,
cos φ=~
a·~
b
ab =(18)
(5.19)(3.74) = 0.927,
and then φ= 22.0.
E11-8 ~
a·~
b= (12)(5.8) cos(55) = 40.
E11-9 ~
r·~
s= (4.5)(7.3) cos(32085) = 19.
E11-10 (a) Add the components individually:
~
r= (5 + 2 + 4)ˆ
i+ (4 2 + 3)ˆ
j+ (63 + 2)ˆ
k= 11ˆ
i+ 5ˆ
j7ˆ
k.
(b) θ= arccos(7/112+ 52+ 72) = 120.
(c) θ= arccos(~
a·~
b/ab), or
θ=(5)(2) + (4)(2) + (6)(3)
p(52+ 42+ 62)(22+ 22+ 32)= 124.
E11-11 There are two forces on the woman, the force of gravity directed down and the normal
force of the floor directed up. These will be effectively equal, so N=W=mg. Consequently, the
57 kg woman must exert a force of F= (57kg)(9.8m/s2) = 560N to propel herself up the stairs.
From the reference frame of the woman the stairs are moving down, and she is exerting a force
down, so the work done by the woman is given by
W=F s = (560 N)(4.5 m) = 2500 J,
this work is positive because the force is in the same direction as the displacement.
The average power supplied by the woman is given by Eq. 11-7,
P=W/t = (2500 J)/(3.5 s) = 710 W.
129
E11-12 P=W/t =mgy/t = (100 ×667 N)(152 m)/(55.0 s) = 1.84×105W.
E11-13 P=F v = (110 N)(0.22 m/s) = 24 W.
E11-14 F=P/v, but the units are awkward.
F=(4800 hp)
(77 knots)
1 knot
1.688 ft/s
1 ft/s
0.3048 m/s
745.7 W
1 hp = 9.0×104N.
E11-15 P=F v = (720 N)(26 m/s) = 19000 W; in horsepower, P= 19000 W(1/745.7 hp/W) =
25 hp.
E11-16 Change to metric units! Then P= 4920 W, and the flow rate is Q= 13.9 L/s. The
density of water is approximately 1.00 kg/L , so the mass flow rate is R= 13.9 kg/s.
y=P
gR =(4920 kg)
(9.81 m/s2)(13.9 kg/s) = 36.1 m,
which is the same as approximately 120 feet.
E11-17 (a) Start by converting kilowatt-hours to Joules:
1 kW ·h = (1000 W)(3600 s) = 3.6×106J.
The car gets 30 mi/gal, and one gallon of gas produces 140 MJ of energy. The gas required to
produce 3.6×106J is
3.6×106J1 gal
140×106J= 0.026 gal.
The distance traveled on this much gasoline is
0.026 gal 30 mi
1 gal = 0.78 mi.
(b) At 55 mi/h, it will take
0.78 mi 1 hr
55 mi= 0.014 h = 51 s.
The rate of energy expenditure is then (3.6×106J)/(51 s) = 71000 W.
E11-18 The linear speed is v= 2π(0.207 m)(2.53 rev/s) = 3.29 m/s. The frictional force is f=
µkN= (0.32)(180 N) = 57.6 N. The power developed is P=F v = (57.6 N)(3.29 m) = 190 W.
E11-19 The net force required will be (1380 kg 1220 kg)(9.81 m/s2) = 1570 N. The work is
W=F y, the power output is P=W/t = (1570 N)(54.5 m)/(43.0 s) = 1990 W, or P= 2.67 hp.
E11-20 (a) The momentum change of the ejected material in one second is
p= (70.2 kg)(497 m/s184 m/s) + (2.92 kg)(497 m/s) = 2.34×104kg ·m/s.
The thrust is then F= ∆p/t= 2.34×104N.
(b) The power is P=F v = (2.34×104N)(184 m/s) = 4.31×106W. That’s 5780 hp.
130
E11-21 The acceleration on the object as a function of position is given by
a=20 m/s2
8 m x,
The work done on the object is given by Eq. 11-14,
W=Z8
0
Fxdx =Z8
0
(10 kg) 20 m/s2
8 m x dx = 800 J.
E11-22 Work is area between the curve and the line F= 0. Then
W= (10 N)(2 s) + 1
2(10 N)(2 s) + 1
2(5 N)(2 s) = 25 J.
E11-23 (a) For a spring, F=kx, and ∆F=kx.
k=F
x=(240 N) (110 N)
(0.060 m) (0.040 m) = 6500 N/m.
With no force on the spring,
x=F
k=(0) (110 N)
(6500 N/m) =0.017 m.
This is the amount less than the 40 mm mark, so the position of the spring with no force on it is 23
mm.
(b) ∆x=10 mm compared to the 100 N picture, so
F=kx=(6500 N/m)(0.010 m) = 65 N.
The weight of the last object is 110 N 65 N = 45 N.
E11-24 (a) W=1
2k(xf2xi2) = 1
2(1500 N/m)(7.60×103m)2= 4.33×102J.
(b) W=1
2(1500 N/m)[(1.52×102m)2(7.60×103m)2= 1.30×101J.
E11-25 Start with Eq. 11-20, and let Fx= 0 while Fy=mg:
W=Zf
i
(Fxdx +Fydy) = mg Zf
i
dy =mgh.
E11-26 (a) F0=mv2
0/r0= (0.675 kg)(10.0 m/s)2/(0.500 m) = 135 N.
(b) Angular momentum is conserved, so v=v0(r0/r). The force then varies as F=mv2/r =
mv2
0r2
0/r3=F0(r0/r)3.The work done is
W=Z~
F·~
dr =(135 N)(0.500 m)3
2(0.300 m)2(0.500 m)2= 60.0 J.
E11-27 The kinetic energy of the electron is
4.2 eV 1.60 ×1019 J
1 eV = 6.7×1019 J.
Then
v=r2K
m=s2(6.7×1019J)
(9.1×1031kg) = 1.2×106m/s.
131
E11-28 (a) K=1
2(110 kg)(8.1 m/s)2= 3600 J.
(b) K=1
2(4.2×103kg)(950 m/s)2= 1900 J.
(c) m= 91,400 tons(907.2 kg/ton) = 8.29×107kg;
v= 32.0 knots(1.688 ft/s/knot)(0.3048 m/ft) = 16.5 m/s.
K=1
2(8.29×107kg)(16.5 m/s)2= 1.13×1010J.
E11-29 (b) ∆K=W=F x = (1.67×1027kg)(3.60×1015m/s2)(0.0350 m) = 2.10×1013J. That’s
2.10×1013J/(1.60×1019J/eV) = 1.31×106eV.
(a) Kf= 2.10×1013J + 1
2(1.67×1027kg)(2.40×107m/s2) = 6.91×1013J. Then
vf=p2K/m =p2(6.91×1013J)/(1.67×1027kg) = 2.88×107m/s.
E11-30 Work is negative if kinetic energy is decreasing. This happens only in region CD. The
work is zero in region BC. Otherwise it is positive.
E11-31 (a) Find the velocity of the particle by taking the time derivative of the position:
v=dx
dt = (3.0 m/s) (8.0 m/s2)t+ (3.0 m/s3)t2.
Find vat two times: t= 0 and t= 4 s.
v(0) = (3.0 m/s) (8.0 m/s2)(0) + (3.0 m/s3)(0)2= 3.0 m/s,
v(4) = (3.0 m/s) (8.0 m/s2)(4.0 s) + (3.0 m/s3)(4.0 s)2= 19.0 m/s
The initial kinetic energy is Ki=1
2(2.80 kg)(3.0 m/s)2= 13 J, while the final kinetic energy is
Kf=1
2(2.80 kg)(19.0 m/s)2= 505 J.
The work done by the force is given by Eq. 11-24,
W=KfKi= 505 J 13 J = 492 J.
(b) This question is asking for the instantaneous power when t= 3.0 s. P=F v, so first find a;
a=dv
dt =(8.0 m/s2) + (6.0 m/s3)t.
Then the power is given by P=mav, and when t= 3 s this gives
P=mav = (2.80 kg)(10 m/s2)(6 m/s) = 168 W.
E11-32 W= ∆K=Ki. Then
W=1
2(5.98×1024kg)(29.8×103m/s)2= 2.66×1033J.
E11-33 (a) K=1
2(1600 kg)(20 m/s)2= 3.2×105J.
(b) P=W/t = (3.2×105J)/(33 s) = 9.7×103W.
(c) P=F v =mav = (1600 kg)(20 m/s/33.0 s)(20 m/s) = 1.9×104W.
E11-34 (a) I= 1.40×104u·pm2(1.66×1027kg/mboxu) = 2.32×1047kg ·m2.
(b) K=1
2Iω2=1
2(2.32×1047kg ·m2)(4.30×1012 rad/s)2= 2.14×1022J.That’s 1.34 meV.
132
E11-35 The translational kinetic energy is Kt=1
2mv2, the rotational kinetic energy is Kr=
1
2Iω2=2
3Kt. Then
ω=r2m
3Iv=s2(5.30×1026kg)
3(1.94×1046kg ·m2)(500 m/s) = 6.75×1012 rad/s.
E11-36 Kr=1
2Iω2=1
4(512 kg)(0.976 m)2(624 rad/s)2= 4.75×107J.
(b) t=W/P = (4.75×107J)/(8130 W) = 5840 s, or 97.4 minutes.
E11-37 From Eq. 11-29, Ki=1
2Iωi2. The object is a hoop, so I=MR2. Then
Ki=1
2MR2ω2=1
2(31.4 kg)(1.21 m)2(29.6 rad/s)2= 2.01 ×104J.
Finally, the average power required to stop the wheel is
P=W
t=KfKi
t=(0) (2.01 ×104J)
(14.8 s) =1360 W.
E11-38 The wheels are connected by a belt, so rAωA=rBωB, or ωA= 3ωB.
(a) If lA=lBthen
IA
IB
=lAA
lBB
=ωB
ωA
=1
3.
(b) If instead KA=KBthen
IA
IB
=2KAA2
2KBB2=ωB2
ωA2=1
9.
E11-39 (a) ω= 2π/T , so
K=1
2Iω2=4π2
5
(5.98×1024kg)(6.37×106m)2
(86,400 s)2= 2.57×1029J
(b) t= (2.57×1029J)/(6.17×1012W) = 4.17×1016s, or 1.3 billion years.
E11-40 (a) The velocities relative to the center of mass are m1v1=m2v2; combine with v1+v2=
910.0 m/s and get
(290.0 kg)v1= (150.0 kg)(910 m/sv1),
or
v1= (150 kg)(910 m/s)/(290 kg + 150 kg) = 310 m/s
and v2= 600 m/s. The rocket case was sent back, so vc= 7600 m/s310 m/s = 7290 m/s. The
payload capsule was sent forward, so vp= 7600 m/s + 600 m/s = 8200 m/s.
(b) Before,
Ki=1
2(290 kg + 150 kg)(7600 m/s)2= 1.271×1010J.
After,
Kf=1
2(290 kg)(7290 m/s)2+1
2(150 kg)(8200 m/s)2= 1.275×1010J.
The “extra” energy came from the spring.
133
E11-41 Let the mass of the freight car be Mand the initial speed be vi. Let the mass of the
caboose be mand the final speed of the coupled cars be vf. The caboose is originally at rest, so the
expression of momentum conservation is
Mvi=Mvf+mvf= (M+m)vf
The decrease in kinetic energy is given by
KiKf=1
2Mvi21
2Mvf2+1
2mvf2,
=1
2Mvi2(M+m)vf2
What we really want is (KiKf)/Ki, so
KiKf
Ki
=Mvi2(M+m)vf2
Mvi2,
= 1 M+m
Mvf
vi2
,
= 1 M+m
MM
M+m2
,
where in the last line we substituted from the momentum conservation expression.
Then KiKf
Ki
= 1 M
M+m= 1 Mg
Mg +mg .
The left hand side is 27%. We want to solve this for mg, the weight of the caboose. Upon rearranging,
mg =Mg
10.27 Mg =(35.0 ton)
(0.73) (35.0 ton) = 12.9 ton.
E11-42 Since the body splits into two parts with equal mass then the velocity gained by one is
identical to the velocity “lost” by the other. The initial kinetic energy is
Ki=1
2(8.0 kg)(2.0 m/s)2= 16 J.
The final kinetic energy is 16 J greater than this, so
Kf= 32 J = 1
2(4.0 kg)(2.0 m/s + v)2+1
2(4.0 kg)(2.0 m/sv)2,
=1
2(8.0 kg)[(2.0 m/s)2+v2],
so 16.0 J = (4.0 kg)v2. Then v= 2.0 m/s; one chunk comes to a rest while the other moves off at a
speed of 4.0 m/s.
E11-43 The initial velocity of the neutron is v0ˆ
i, the final velocity is v1ˆ
j. By momentum conser-
vation the final momentum of the deuteron is mn(v0ˆ
iv1ˆ
j). Then mdv2=mnpv2
0+v2
1.
There is also conservation of kinetic energy:
1
2mnv2
0=1
2mnv2
1+1
2mdv2
2.
Rounding the numbers slightly we have md= 2mn, then 4v2
2=v2
0+v2
1is the momentum expression
and v2
0=v2
1+ 2v2
2is the energy expression. Combining,
2v2
0= 2v2
1+ (v2
0+v2
1),
or v2
1=v2
0/3. So the neutron is left with 1/3 of its original kinetic energy.
134
E11-44 (a) The third particle must have a momentum
~
p3=(16.7×1027kg)(6.22×106m/s)ˆ
i+ (8.35×1027kg)(7.85×106m/s)ˆ
j
= (1.04ˆ
i+ 0.655ˆ
j)×1019kg ·m/s.
(b) The kinetic energy can also be written as K=1
2mv2=1
2m(p/m)2=p2/2m. Then the
kinetic energy appearing in this process is
K=1
2(16.7×1027kg)(6.22×106m/s)2+1
2(8.35×1027kg)(7.85×106m/s)2
+1
2(11.7×1027kg) (1.23×1019kg ·m/s)2= 1.23×1012J.
This is the same as 7.66 MeV.
P11-1 Change your units! Then
F=W
s=(4.5 eV)(1.6×1019 J/eV)
(3.4×109m) = 2.1×1010 N.
P11-2 (a) If the acceleration is g/4 the the net force on the block is Mg/4, so the tension in
the cord must be T= 3Mg/4.
(a) The work done by the cord is W=~
F·~
s= (3Mg/4)(d) = (3/4)Mgd.
(b) The work done by gravity is W=~
F·~
s= (Mg)(d) = Mgd.
P11-3 (a) There are four cords which are attached to the bottom load L. Each supports a tension
F, so to lift the load 4F= (840 lb) + (20.0 lb), or F= 215 lb.
(b) The work done against gravity is W=~
F·~
s= (840 lb)(12.0 ft) = 10100 ft ·lb.
(c) To lift the load 12 feet each segment of the cord must be shortened by 12 ft; there are four
segments, so the end of the cord must be pulled through a distance of 48.0 ft.
(d) The work done by the applied force is W=~
F·~
s= (215 lb)(48.0 ft) = 10300 ft ·lb.
P11-4 The incline has a height hwhere h=W/mg = (680 J)/[(75 kg)(9.81 m/s2)]. The work
required to lift the block is the same regardless of the path, so the length of the incline lis l=
W/F = (680 J)/(320 N).The angle of the incline is then
θ= arcsin h
l= arcsin F
mg = arcsin (320 N)
(75 kg)(9.81 m/s2)= 25.8.
P11-5 (a) In 12 minutes the horse moves x= (5280 ft/mi)(6.20 mi/h)(0.200 h) = 6550 ft. The
work done by the horse in that time is W=~
F·~
s= (42.0 lb)(6550 ft) cos(27.0) = 2.45×105ft ·lb.
(b) The power output of the horse is
P=(2.45×105ft ·lb)
(720 s) = 340 ft ·lb/s 1 hp
550 ft ·lb/s = 0.618 hp.
P11-6 In this problem θ= arctan(28.2/39.4) = 35.5.
The weight of the block has components W|| =mg sin θand W=mg cos θ. The force of friction
on the block is f=µkN=µkmg cos θ. The tension in the rope must then be
T=mg(sin θ+µkcos θ)
in order to move the block up the ramp. The power supplied by the winch will be P=T v, so
P= (1380 kg)(9.81 m/s2)[sin(35.5) + (0.41) cos(35.5)](1.34 m/s) = 1.66×104W.
135
P11-7 If the power is constant then the force on the car is given by F=P/v. But the force is
related to the acceleration by F=ma and to the speed by F=mdv
dt for motion in one dimension.
Then
F=P
v,
mdv
dt =P
v,
mdx
dt
dv
dx =P
v,
mv dv
dx =P
v,
Zv
0
mv2dv =Zx
0
P dx,
1
3mv3=P x.
We can rearrange this final expression to get vas a function of x,v= (3xP/m)1/3.
P11-8 (a) If the drag is D=bv2, then the force required to move the plane forward at constant
speed is F=D=bv2, so the power required is P=F v =bv3.
(b) Pv3, so if the speed increases to 125% then Pincreases by a factor of 1.253= 1.953, or
increases by 95.3%.
P11-9 (a) P=mgh/t, but m/t is the persons per minute times the average mass, so
P= (100 people/min)(75.0kg)(9.81 m/s2)(8.20 m) = 1.01×104W.
(b) In 9.50 s the Escalator has moved (0.620 m/s)(9.50 s) = 5.89 m; so the Escalator has “lifted”
the man through a distance of (5.89 m)(8.20 m/13.3 m) = 3.63 m. The man did the rest himself.
The work done by the Escalator is then W= (83.5 kg)(9.81 m/s2)(3.63 m) = 2970 J.
(c) Yes, because the point of contact is moving in a direction with at least some non-zero com-
ponent of the force. The power is
P= (83.5 m/s2)(9.81 m/s2)(0.620 m/s)(8.20 m/13.3 m) = 313 W.
(d) If there is a force of contact between the man and the Escalator then the Escalator is doing
work on the man.
P11-10 (a) dP/dv =ab 3av2, so Pmax occurs when 3v2=b, or v=pb/3.
(b) F=P/v, so dF/dv =2v, which means Fis a maximum when v= 0.
(c) No; P= 0, but F=ab.
P11-11 (b) Integrate,
W=Z3x0
0
~
F·d~
s=F0
x0Z3x0
0
(xx0)dx =F0x09
23,
or W= 3F0x0/2.
136
P11-12 (a) Simpson’s rule gives
W=1
3[(10 N) + 4(2.4 N) + (0.8 N)] (1.0 m) = 6.8 J.
(b) W=RF ds =R(A/x2)dx =A/x, evaluating this between the limits of integration gives
W= (9 N ·m2)(1/1 m 1/3 m) = 6 J.
P11-13 The work required to stretch the spring from xito xfis given by
W=Zxf
xi
kx3dx =k
4xf4k
4xi4.
The problem gives
W0=k
4(l)4k
4(0)4=k
4l4.
We then want to find the work required to stretch from x=lto x= 2l, so
Wl2l=k
4(2l)4k
4(l)4,
= 16k
4l4k
4l4,
= 15k
4l4= 15W0.
P11-14 (a) The spring extension is δl =pl2
0+x2l0. The force from one spring has magnitude
kδl, but only the xcomponent contributes to the problem, so
F= 2kql2
0+x2l0x
pl2
0+x2
is the force required to move the point.
The work required is the integral, W=Rx
0F dx, which is
W=kx22kl0ql2
0+x2+ 2kl2
0
Note that it does reduce to the expected behavior for xl0.
(b) Binomial expansion of square root gives
ql2
0+x2=l01 + 1
2
x2
l2
01
8
x4
l4
0···,
so the first term in the above expansion cancels with the last term in W; the second term cancels
with the first term in W, leaving
W=1
4kx4
l2
0
.
P11-15 Number the springs clockwise from the top of the picture. Then the four forces on each
spring are
F1=k(l0px2+ (l0y)2),
F2=k(l0p(l0x)2+y2),
F3=k(l0px2+ (l0+y)2),
F4=k(l0p(l0+x)2+y2).
137
The directions are much harder to work out, but for small xand ywe can assume that
~
F1=k(l0px2+ (l0y)2)ˆ
j,
~
F2=k(l0p(l0x)2+y2)ˆ
i,
~
F3=k(l0px2+ (l0+y)2)ˆ
j,
~
F4=k(l0p(l0+x)2+y2)ˆ
i.
Then
W=Z~
F·d~
s=Z(F1+F3)dy +Z(F2+F4)dx,
Since xand yare small, expand the force(s) in a binomial expansion:
F1(x, y)F1(0,0) + F1
x x,y=0
x+F1
y x,y=0
y=ky;
there will be similar expression for the other four forces. Then
W=Z2ky dy +Z2kx dx =k(x2+y2) = kd2.
P11-16 (a) Ki=1
2(1100 kg)(12.8 m/s)2= 9.0×104J. Removing 51 kJ leaves 39 kJ behind, so
vf=p2Kf/m =p2(3.9×104J)/(1100 kg) = 8.4 m/s,
or 30 km/h.
(b) 39 kJ, as was found above.
P11-17 Let Mbe the mass of the man and mbe the mass of the boy. Let vMbe the original
speed of the man and vmbe the original speed of the boy. Then
1
2Mv2
M=1
21
2mv2
m
and 1
2M(vM+ 1.0 m/s)2=1
2mv2
m.
Combine these two expressions and solve for vM,
1
2Mv2
M=1
21
2M(vM+ 1.0 m/s)2,
v2
M=1
2(vM+ 1.0 m/s)2,
0 = v2
M+ (2.0 m/s)vM+ (1.0 m/s)2.
The last line can be solved as a quadratic, and vM= (1.0 m/s) ±(1.41 m/s). Now we use the very
first equation to find the speed of the boy,
1
2Mv2
M=1
21
2mv2
m,
v2
M=1
4v2
m,
2vM=vm.
138
P11-18 (a) The work done by gravity on the projectile as it is raised up to 140 m is W=
mgy =(0.550 kg)(9.81 m/s2)(140 m) = 755 J. Then the kinetic energy at the highest point is
1550 J 755 J = 795 J. Since the projectile must be moving horizontally at the highest point, the
horizontal velocity is vx=p2(795 J)/(0.550 kg) = 53.8 m/s.
(b) The magnitude of the velocity at launch is v=p2(1550 J)/(0.550 kg) = 75.1 m/s. Then
vy=p(75.1 m/s)2(53.8 m/s)2= 52.4 m/s.
(c) The kinetic energy at that point is 1
2(0.550 kg)[(65.0 m/s)2+ (53.8 m/s)2] = 1960 J. Since it
has extra kinetic energy, it must be below the launch point, and gravity has done 410 J of work on
it. That means it is y= (410 J)/[(0.550 kg)(9.81 m/s2)] = 76.0 m below the launch point.
P11-19 (a) K=1
2mv2=1
2(8.38 ×1011kg)(3.0×104m/s)2= 3.77 ×1020J.In terms of TNT,
K= 9.0×104megatons.
(b) The diameter will be 3
8.98×104= 45 km.
P11-20 (a) Wg=(0.263 kg)(9.81 m/s2)(0.118 m) = 0.304 J.
(b) Ws=1
2(252 N/m)(0.118 m)2=1.75 J.
(c) The kinetic energy just before hitting the block would be 1.75 J 0.304 J = 1.45 J. The speed
is then v=p2(1.45 J)/(0.263 kg) = 3.32 m/s.
(d) Doubling the speed quadruples the initial kinetic energy to 5.78 J. The compression will then
be given by
5.78 J = 1
2(252 N/m)y2(0.263 kg)(9.81 m/s2)y,
with solution y= 0.225 m.
P11-21 (a) We can solve this with a trick of integration.
W=Zx
0
F dx,
=Zx
0
max
dt
dt dx =maxZt
0
dx
dt dt
=maxZt
0
vxdt =maxZt
0
at dt,
=1
2ma2
xt2.
Basically, we changed the variable of integration from xto t, and then used the fact the the accel-
eration was constant so vx=v0x+axt. The object started at rest so v0x= 0, and we are given in
the problem that vf=atf. Combining,
W=1
2ma2
xt2=1
2mvf
tf2
t2.
(b) Instantaneous power will be the derivative of this, so
P=dW
dt =mvf
tf2
t.
139
P11-22 (a) α= (39.0 rev/s)(2πrad/rev)/(32.0 s) = 7.66 rad/s2.
(b) The total rotational inertia of the system about the axis of rotation is
I= (6.40 kg)(1.20 m)2/12 + 2(1.06 kg)(1.20 m/2)2= 1.53 kg ·m2.
The torque is then τ= (1.53 kg ·m2)(7.66 rad/s2) = 11.7 N ·m.
(c) K=1
2(1.53 kg ·m2)(245 rad/s)2= 4.59×104J.
(d) θ=ωavt= (39.0 rev/s/2)(32.0 s) = 624 rev.
(e) Only the loss in kinetic energy is independent of the behavior of the frictional torque.
P11-23 The wheel turn with angular speed ω=v/r, where ris the radius of the wheel and vthe
speed of the car. The total rotational kinetic energy in the four wheels is then
Kr = 41
2Iω2= 2 1
2(11.3 kg)r2hv
ri2= (11.3 kg)v2.
The translational kinetic energy is Kt=1
2(1040 kg)v2, so the fraction of the total which is due to
the rotation of the wheels is 11.3
520 + 11.3= 0.0213 or 2.13%.
P11-24 (a) Conservation of angular momentum: ωf= (6.13 kg ·m2/1.97 kg ·m2)(1.22 rev/s) =
3.80 rev/s.
(b) KrIω2l2/I, so
Kf/Ki=Ii/If= (6.13 kg ·m2)/(1.97 kg ·m2) = 3.11.
P11-25 We did the first part of the solution in Ex. 10-21. The initial kinetic energy is (again,
ignoring the shaft),
Ki=1
2I1~ω1,i2,
since the second wheel was originally at rest. The final kinetic energy is
Kf=1
2(I1+I2)~ωf2,
since the two wheels moved as one. Then
KiKf
Ki
=
1
2I1~ω1,i21
2(I1+I2)~ωf2
1
2I1~ω1,i2,
= 1 (I1+I2)~ωf2
I1~ω1,i2,
= 1 I1
I1+I2
,
where in the last line we substituted from the results of Ex. 10-21.
Using the numbers from Ex. 10-21,
KiKf
Ki
= 1 (1.27 kg·m2)
(1.27 kg·m2) + (4.85 kg·m2)= 79.2%.
140
P11-26 See the solution to P10-11.
Ki=I
2ω2+m
2v2
while
Kf=1
2(I+mR2)ωf2
according to P10-11,
ωf=Iω mvR
I+mR2.
Then
Kf=1
2
(Iω mvR)2
I+mR2.
Finally,
K=1
2
(Iω mvR)2(I+mR2)(Iω2+mv2)
I+mR2,
=1
2
ImR2ω2+ 2mvRIω +Imv2
I+mR2,
=Im
2
(+v)2
I+mR2.
P11-27 See the solution to P10-12.
(a) Ki=1
2Iiωi2, so
Ki=1
22(51.2 kg)(1.46 m)2(0.945 rad/s)2= 97.5 J.
(b) Kf=1
22(51.2 kg)(0.470 m)2(9.12 rad/s)2= 941 J.The energy comes from the work they
do while pulling themselves closer together.
P11-28 K=1
2mv2=1
2mp2=1
2m~
p·~
p. Then
Kf=1
2m(~
pi+ ∆~
p)·(~
pi+ ∆~
p),
=1
2mpi2+ 2~
pi·~
p+ (∆p)2,
K=1
2m2~
pi·~
p+ (∆p)2.
In all three cases ∆p= (3000 N)(65.0 s) = 1.95×105N·s and pi= (2500 kg)(300 m/s) = 7.50×105kg ·
m/s.
(a) If the thrust is backward (pushing rocket forward),
K=+2(7.50×105kg ·m/s)(1.95×105N·s) + (1.95×105N·s)2
2(2500 kg) = +6.61×107J.
(b) If the thrust is forward,
K=2(7.50×105kg ·m/s)(1.95×105N·s) + (1.95×105N·s)2
2(2500 kg) =5.09×107J.
(c) If the thrust is sideways the first term vanishes,
K=+(1.95×105N·s)2
2(2500 kg) = 7.61×106J.
141
P11-29 There’s nothing to integrate here! Start with the work-energy theorem
W=KfKi=1
2mvf21
2mvi2,
=1
2mvf2vi2,
=1
2m(vfvi) (vf+vi),
where in the last line we factored the difference of two squares. Continuing,
W=1
2(mvfmvi) (vf+vi),
=1
2(∆p) (vf+vi),
but ∆p=J, the impulse. That finishes this problem.
P11-30 Let Mbe the mass of the helicopter. It will take a force Mg to keep the helicopter
airborne. This force comes from pushing the air down at a rate ∆m/twith a speed of v; so
Mg =vm/t. The blades sweep out a cylinder of cross sectional area A, so ∆m/t=ρAv.
The force is then Mg =ρAv2; the speed that the air must be pushed down is v=pMg/ρA. The
minimum power is then
P=F v =MgsMg
ρA =s(1820 kg)3(9.81 m/s2)3
(1.23 kg/m3)π(4.88 m)2= 2.49×105W.
P11-31 (a) Inelastic collision, so vf=mvi/(m+M).
(b) K=1
2mv2=p2/2m, so
K
Ki
=1/m 1/(m+M)
1/m =M
m+M.
P11-32 Inelastic collision, so
vf=(1.88 kg)(10.3 m/s) + (4.92 kg)(3.27 m/s)
(1.88 kg) + (4.92 kg) = 5.21 m/s.
The loss in kinetic energy is
K=(1.88 kg)(10.3 m/s)2
2+(4.92 kg)(3.27 m/s)2
2(1.88 kg + 4.92 kg)(5.21 m/s)2
2= 33.7 J.
This change is because of work done on the spring, so
x=p2(33.7 J)/(1120 N/m) = 0.245 m
P11-33 ~
pf,B =~
pi,A +~
pi,B ~
pf,A, so
~
pf,B = [(2.0 kg)(15 m/s) + (3.0 kg)(10 m/s) (2.0 kg)(6.0 m/s)]ˆ
i
+[(2.0 kg)(30 m/s) + (3.0 kg)(5.0 m/s) (2.0 kg)(30 m/s)]ˆ
j,
= (12 kg ·m/s)ˆ
i+ (15 kg ·m/s)ˆ
j.
142
Then ~
vf,B = (4.0 m/s)ˆ
i+ (5.0 m/s)ˆ
j.Since K=m
2(v2
x+v2
y), the change in kinetic energy is
K=(2.0 kg)[(6.0 m/s)2+ (30 m/s)2(15 m/s)2(30 m/s)2]
2
+(3.0 kg)[(4.0 m/s)2+ (5 m/s)2(10 m/s)2(5.0 m/s)2]
2
=315 J.
P11-34 For the observer on the train the acceleration of the particle is a, the distance traveled is
xt=1
2at2, so the work done as measured by the train is Wt=maxt=1
2a2t2.The final speed of
the particle as measured by the train is vt=at, so the kinetic energy as measured by the train is
K=1
2mv2=1
2m(at)2.The particle started from rest, so ∆Kt=Wt.
For the observer on the ground the acceleration of the particle is a, the distance traveled is
xg=1
2at2+ut, so the work done as measured by the ground is Wg=maxg=1
2a2t2+maut.
The final speed of the particle as measured by the ground is vg=at +u, so the kinetic energy as
measured by the ground is
Kg=1
2mv2=1
2m(at +u)2=1
2a2t2+maut +1
2mu2.
But the initial kinetic energy as measured by the ground is 1
2mu2, so Wg= ∆Kg.
P11-35 (a) Ki=1
2m1v1,i2.
(b) After collision vf=m1v1,i/(m1+m2), so
Kf=1
2(m1+m2)m1v1,i
m1+m22
=1
2m1v1,i2m1
m1+m2.
(c) The fraction lost was
1m1
m1+m2
=m2
m1+m2
.
(d) Note that vcm =vf. The initial kinetic energy of the system is
Ki=1
2m1v01,i2+1
2m2v02,i2.
The final kinetic energy is zero (they stick together!), so the fraction lost is 1. The amount lost,
however, is the same.
P11-36 Only consider the first two collisions, the one between mand m0, and then the one between
m0and M.
Momentum conservation applied to the first collision means the speed of m0will be between
v0=mv0/(m+m0) (completely inelastic)and v0= 2mv0/(m+m0) (completely elastic). Momentum
conservation applied to the second collision means the speed of Mwill be between V=m0v0/(m0+M)
and V= 2m0v0/(m0+M). The largest kinetic energy for Mwill occur when it is moving the fastest,
so
v0=2mv0
m+m0and V=2m0v0
m0+M=4m0mv0
(m+m0)(m0+M).
We want to maximize Vas a function of m0, so take the derivative:
dV
dm0=4mv0(mM m02)
(m0+M)2(m+m0)2.
This vanishes when m0=mM .
143
E12-1 (a) Integrate.
U(x) = Zx
Gm1m2
x2dx =Gm1m2
x.
(b) W=U(x)U(x+d), so
W=Gm1m21
x1
x+d=Gm1m2
d
x(x+d).
E12-2 If d << x then x(x+d)x2, so
WGm1m2
x2d.
E12-3 Start with Eq. 12-6.
U(x)U(x0) = Zx
x0
Fx(x)dx,
=Zx
x0αxeβx2dx,
=α
2βeβx2
x
x0
.
Finishing the integration,
U(x) = U(x0) + α
2βeβx2
0eβx2.
If we choose x0=and U(x0) = 0 we would be left with
U(x) = α
2βeβx2.
E12-4 K=Uso ∆K=mgy. The power output is then
P= (58%)(1000 kg/m3)(73,800 m3)
(60 s) (9.81 m/s2)(96.3 m) = 6.74×108W.
E12-5 U=K, so 1
2kx2=1
2mv2. Then
k=mv2
x2=(2.38 kg)(10.0×103/s)
(1.47 m)2= 1.10×108N/m.
Wow.
E12-6 Ug+ ∆Us= 0, since K= 0 when the man jumps and when the man stops. Then
Us=mgy= (220 lb)(40.4 ft) = 8900 ft ·lb.
E12-7 Apply Eq. 12-15,
Kf+Uf=Ki+Ui,
1
2mvf2+mgyf=1
2mvi2+mgyi,
1
2vf2+g(r) = 1
2(0)2+g(0).
Rearranging,
vf=p2g(r) = p2(9.81 m/s2)(0.236 m) = 2.15 m/s.
144
E12-8 (a) K=1
2mv2=1
2(2.40 kg)(150 m/s)2= 2.70×104J.
(b) Assuming that the ground is zero, U=mgy = (2.40 kg)(9.81 m/s2)(125 m) = 2.94×103J.
(c) Kf=Ki+Uisince Uf= 0. Then
vf=s2(2.70×104J) + (2.94×103J)
(2.40 kg) = 158 m/s.
Only (a) and (b) depend on the mass.
E12-9 (a) Since ∆y= 0, then U= 0 and K= 0. Consequently, at B,v=v0.
(b) At C KC=KA+UAUC, or
1
2mvC2=1
2mv02+mgh mg h
2,
or
vB=rv02+ 2gh
2=pv02+gh.
(c) At D KD=KA+UAUD, or
1
2mvD2=1
2mv02+mgh mg(0),
or
vB=pv02+ 2gh.
E12-10 From the slope of the graph, k= (0.4 N)/(0.04 m) = 10 N/m.
(a) ∆K=U, so 1
2mvf2=1
2kxi2, or
vf=s(10 N/m)
(0.00380 kg) (0.0550 m) = 2.82 m/s.
(b) ∆K=U, so 1
2mvf2=1
2k(xi2xf2), or
vf=s(10 N/m)
(0.00380 kg) [(0.0550 m)2(0.0150 m)2] = 2.71 m/s.
E12-11 (a) The force constant of the spring is
k=F/x =mg/x = (7.94 kg)(9.81 m/s2)/(0.102 m) = 764 N/m.
(b) The potential energy stored in the spring is given by Eq. 12-8,
U=1
2kx2=1
2(764 N/m)(0.286 m + 0.102 m)2= 57.5 J.
(c) Conservation of energy,
Kf+Uf=Ki+Ui,
1
2mvf2+mgyf+1
2kxf2=1
2mvi2+mgyi+1
2kxi2,
1
2(0)2+mgh +1
2k(0)2=1
2(0)2+mg(0) + 1
2kxi2.
Rearranging,
h=k
2mg xi2=(764 N/m)
2(7.94 kg)(9.81 m/s2)(0.388 m)2= 0.738 m.
145
E12-12 The annual mass of water is m= (1000 kg/m3)(8 ×1012m2)(0.75 m). The change in
potential energy each year is then ∆U=mgy, where y=500 m. The power available is then
P=1
3(1000 kg/m3)(8×1012m2)(0.75 m)
(3.15×107m) (500 m) = 3.2×107W.
E12-13 (a) From kinematics, v=gt, so K=1
2mg2t2and U=U0K=mgh 1
2mg2t2.
(b) U=mgy so K=U0U=mg(hy).
E12-14 The potential energy is the same in both cases. Consequently, mgEyE=mgMyM, and
then
yM= (2.05 m 1.10 m)(9.81 m/s2)/(1.67 m/s2)+1.10 m = 6.68 m.
E12-15 The working is identical to Ex. 12-11,
Kf+Uf=Ki+Ui,
1
2mvf2+mgyf+1
2kxf2=1
2mvi2+mgyi+1
2kxi2,
1
2(0)2+mgh +1
2k(0)2=1
2(0)2+mg(0) + 1
2kxi2,
so
h=k
2mg xi2=(2080 N/m)
2(1.93 kg)(9.81 m/s2)(0.187 m)2= 1.92 m.
The distance up the incline is given by a trig relation,
d=h/ sin θ= (1.92 m)/sin(27) = 4.23 m.
E12-16 The vertical position of the pendulum is y=lcos θ, where θis measured from the
downward vertical and lis the length of the string. The total mechanical energy of the pendulum is
E=1
2mvb2
if we set U= 0 at the bottom of the path and vbis the speed at the bottom. In this case
U=mg(l+y).
(a) K=EU=1
2mvb2mgl(1 cos θ). Then
v=p(8.12 m/s)22(9.81 m/s2)(3.82 kg)(1 cos 58.0) = 5.54 m/s.
(b) U=EK, but at highest point K= 0. Then
θ= arccos 11
2
(8.12 m/s)2
(9.81 m/s2)(3.82 kg) = 83.1.
(c) E=1
2(1.33 kg)(8.12 m/s)2= 43.8 J.
E12-17 The equilibrium position is when F=ky =mg. Then ∆Ug=mgy and ∆Us=
1
2(ky)y=1
2mgy. So 2∆Us=Ug.
146
E12-18 Let the spring get compressed a distance x. If the object fell from a height h= 0.436 m,
then conservation of energy gives 1
2kx2=mg(x+h). Solving for x,
x=mg
k±rmg
k2+ 2mg
kh
only the positive answer is of interest, so
x=(2.14 kg)(9.81 m/s2)
(1860 N/m) ±s(2.14 kg)(9.81 m/s2)
(1860 N/m) 2
+ 2(2.14 kg)(9.81 m/s2)
(1860 N/m) (0.436 m) = 0.111 m.
E12-19 The horizontal distance traveled by the marble is R=vtf, where tfis the time of flight
and vis the speed of the marble when it leaves the gun. We find that speed using energy conservation
principles applied to the spring just before it is released and just after the marble leaves the gun.
Ki+Ui=Kf+Uf,
0 + 1
2kx2=1
2mv2+ 0.
Ki= 0 because the marble isn’t moving originally, and Uf= 0 because the spring is no longer
compressed. Substituting Rinto this,
1
2kx2=1
2mR
tf2
.
We have two values for the compression, x1and x2, and two ranges, R1and R2. We can put both
pairs into the above equation and get two expressions; if we divide one expression by the other we
get
x2
x12
=R2
R12
.
We can easily take the square root of both sides, then
x2
x1
=R2
R1
.
R1was Bobby’s try, and was equal to 2.20 0.27 = 1.93 m. x1= 1.1 cm was his compression. If
Rhoda wants to score, she wants R2= 2.2 m, then
x2=2.2 m
1.93 m1.1 cm = 1.25 cm.
E12-20 Conservation of energy— U1+K1=U2+K2— but U1=mgh,K1= 0, and U2= 0, so
K2=1
2mv2=mgh at the bottom of the swing.
The net force on Tarzan at the bottom of the swing is F=mv2/r, but this net force is equal to
the tension Tminus the weight W=mg. Then 2mgh/r =Tmg. Rearranging,
T= (180 lb) 2(8.5 ft)
(50 ft) + 1= 241 lb.
This isn’t enough to break the vine, but it is close.
147
E12-21 Let point 1 be the start position of the first mass, point 2 be the collision point, and point
3 be the highest point in the swing after the collision. Then U1=K2, or 1
2m1v12=m1gd, where v1
is the speed of m1just before it collides with m2. Then v1=2gd.
After the collision the speed of both objects is, by momentum conservation, v2=m1v1/(m1+m2).
Then, by energy conservation, U3=K0
2, or 1
2(m1+m2)v22= (m1+m2)gy, where yis the height
to which the combined masses rise.
Combining,
y=v22
2g=m12v12
2(m1+m2)2g=m1
m1+m22
d.
E12-22 K=U, so 1
2mv2+1
2Iω2=mgh,
where I=1
2MR2and ω=v/R. Combining,
1
2mv2+1
4Mv2=mgh,
so
v=r4mgh
2m+M=s4(0.0487 kg)(9.81 m/s2)(0.540 m)
2(0.0487 kg) + (0.396 kg) = 1.45 m/s.
E12-23 There are three contributions to the kinetic energy: rotational kinetic energy of the shell
(Ks), rotational kinetic energy of the pulley (Kp), and translational kinetic energy of the block
(Kb). The conservation of energy statement is then
Ks,i+Kp,i+Kb,i+Ui=Ks,f+Kp,f+Kb,f+Uf,
(0) + (0) + (0) + (0) = 1
2Isωs2+1
2Ipωp2+1
2mvb2+mgy.
Finally, y=hand
ωsR=ωpr=vb.
Combine all of this together, and our energy conservation statement will look like this:
0 = 1
22
3MR2vb
R2+1
2Ipvb
r2+1
2mvb2mgh
which can be fairly easily rearranged into
vb2=2mgh
2M/3 + IP/r2+m.
E12-24 The angular speed of the flywheel and the speed of the car are related by
k=ω
v=(1490 rad/s)
(24.0 m/s) = 62.1 rad/m.
The height of the slope is h= (1500 m) sin(5.00) = 131 m. The rotational inertia of the flywheel is
I=1
2
(194 N)
(9.81 m/s2)(0.54 m)2= 2.88 kg ·m2.
148
(a) Energy is conserved as the car moves down the slope: Ui=Kf, or
mgh =1
2mv2+1
2Iω2=1
2mv2+1
2Ik2v2,
or
v=r2mgh
m+Ik2=s2(822 kg)(9.81 m/s2)(131 m)
(822 kg) + (2.88 kg ·m2)(62.1 rad/m)2= 13.3 m/s,
or 47.9 m/s.
(b) The average speed down the slope is 13.3 m/s/2=6.65 m/s. The time to get to the bottom
is t= (1500 m)/(6.65 m/s) = 226 s. The angular acceleration of the disk is
α=ω
t=(13.3 m/s)(62.1 rad/m)
(226 s) = 3.65 rad/s2.
(c) P=τω =Iαω, so
P= (2.88 kg ·m2)(3.65 rad/s2)(13.3 m/s)(62.1 rad/m) = 8680 W.
E12-25 (a) For the solid sphere I=2
5mr2; if it rolls without slipping ω=v/r; conservation of
energy means Ki=Uf. Then
1
2mv2+1
22
5mr2v
r2=mgh.
or
h=(5.18 m/s)2
2(9.81 m/s2)+(5.18 m/s)2
5(9.81 m/s2)= 1.91 m.
The distance up the incline is (1.91 m)/sin(34.0) = 3.42 m.
(b) The sphere will travel a distance of 3.42 m with an average speed of 5.18 m/2, so t=
(3.42 m)/(2.59 m/s) = 1.32 s. But wait, it goes up then comes back down, so double this time to get
2.64 s.
(c) The total distance is 6.84 m, so the number of rotations is (6.84 m)/(0.0472 m)/(2π) = 23.1.
E12-26 Conservation of energy means 1
2mv2+1
2Iω2=mgh. But ω=v/r and we are told
h= 3v2/4g, so
1
2mv2+1
2Iv2
r2=mg 3v2
4g,
or
I= 2r23
4m1
2m=1
2mr2,
which could be a solid disk or cylinder.
E12-27 We assume the cannon ball is solid, so the rotational inertia will be I= (2/5)MR2
The normal force on the cannon ball will be N=Mg, where Mis the mass of the bowling ball.
The kinetic friction on the cannon ball is Ff=µkN=µkMg. The magnitude of the net torque on
the bowling ball while skidding is then τ=µkMgR.
Originally the angular momentum of the cannon ball is zero; the final angular momentum will
have magnitude l=Iω =Iv/R, where vis the final translational speed of the ball.
The time requires for the cannon ball to stop skidding is the time required to change the angular
momentum to l, so
t=l
τ=(2/5)MR2v/R
µkMgR =2v
5µkg.
149
Since we don’t know ∆t, we can’t solve this for v. But the same time through which the angular
momentum of the ball is increasing the linear momentum of the ball is decreasing, so we also have
t=p
Ff
=Mv Mv0
µkMg =v0v
µkg.
Combining,
2v
5µkg=v0v
µkg,
2v= 5(v0v),
v= 5v0/7
E12-28
0123456
0
1
2
3
4
0123456
-2
-1
0
1
2
F(x) (N)
K(x) (J)
x (m)x(m)
E12-29 (a) F=U/x=[(17 J) (3 J)]/[(4 m) (1 m)] = 4.7 N.
(b) The total energy is 1
2(2.0 kg)(2.0 m/s)2+ (7 J), or 3 J. The particle is constrained to
move between x= 1 m and x= 14 m.
(c) When x= 7 m K= (3 J) (17 J) = 14 J. The speed is v=p2(14 J)/(2.0 kg) = 3.7 m/s.
E12-30 Energy is conserved, so 1
2mv2
0=1
2mv2+mgy,
or
v=qv2
02gy,
which depends only on y.
E12-31 (a) We can find Fxand Fyfrom the appropriate derivatives of the potential,
Fx=U
x =kx,
Fy=U
y =ky.
The force at point (x, y) is then
~
F=Fxˆ
i+Fyˆ
j=kxˆ
ikyˆ
j.
(b) Since the force vector points directly toward the origin there is no angular component, and
Fθ= 0. Then Fr=kr where ris the distance from the origin.
(c) A spring which is attached to a point; the spring is free to rotate, perhaps?
150
E12-32 (a) By symmetry we expect Fx,Fy, and Fzto all have the same form.
Fx=U
x =kx
(x2+y2+z2)3/2,
with similar expressions for Fyand Fz. Then
~
F=k
(x2+y2+z2)3/2(xˆ
i+yˆ
j+zˆ
k).
(b) In spherical polar coordinates r2=x2+y2+z2. Then U=k/r and
Fr=U
r =k
r2.
E12-33 We’ll just do the paths, showing only non-zero terms.
Path 1: W=Rb
0(k2a)dy) = k2ab.
Path 2: W=Ra
0(k1b)dx) = k1ab.
Path 3: W= (cos φsin φ)Rd
0(k1k2)r dr =(k1+k2)ab/2.
These three are only equal if k1=k2.
P12-1 (a) We need to integrate an expression like
Zz
k
(z+l)2dz =k
z+l.
The second half is dealt with in a similar manner, yielding
U(z) = k
z+lk
zl.
(b) If zlthen we can expand the denominators, then
U(z) = k
z+lk
zl,
k
zkl
z2k
z+kl
z2,
=2kl
z2.
P12-2 The ball just reaches the top, so K2= 0. Then K1=U2U1=mgL, so v1=
p2(mgL)/m =2gL.
P12-3 Measure distances along the incline by x, where x= 0 is measured from the maximally
compressed spring. The vertical position of the mass is given by xsin θ. For the spring k=
(268 N)/(0.0233 m) = 1.15×104N/m. The total energy of the system is
1
2(1.15×104N/m)(0.0548 m)2= 17.3 J.
(a) The block needs to have moved a vertical distance xsin(32.0), where
17.3 J = (3.18 kg)(9.81 m/s2)xsin(32.0),
or x= 1.05 m.
(b) When the block hits the top of the spring the gravitational potential energy has changed by
U=(3.18 kg)(9.81 m/s2)(1.05 m 0.0548 m) sin(32.0) = 16.5,J;
hence the speed is v=p2(16.5 J)/(3.18 kg) = 3.22 m/s.
151
P12-4 The potential energy associated with the hanging part is
U=Z0
L/4
M
Lgy dy =Mg
2Ly2
0
L/4
=MgL
32 ,
so the work required is W=M gL/32.
P12-5 (a) Considering points Pand Qwe have
KP+UP=KQ+UQ,
(0) + mg(5R) = 1
2mv2+mg(R),
4mgR =1
2mv2,
p8gR =v.
There are two forces on the block, the normal force from the track,
N=mv2
R=m(8gR)
R= 8mg,
and the force of gravity W=mg. They are orthogonal so
Fnet =p(8mg)2+ (mg)2=65 mg
and the angle from the horizontal by
tan θ=mg
8mg =1
8,
or θ= 7.13below the horizontal.
(b) If the block barely makes it over the top of the track then the speed at the top of the loop
(point S, perhaps?) is just fast enough so that the centripetal force is equal in magnitude to the
weight,
mvS2/R =mg.
Assume the block was released from point T. The energy conservation problem is then
KT+UT=KS+US,
(0) + mgyT=1
2mv2
S+mgyS,
yT=1
2(R) + m(2R),
= 5R/2.
P12-6 The wedge slides to the left, the block to the right. Conservation of momentum requires
Mvw+mvb,x = 0. The block is constrained to move on the surface of the wedge, so
tan α=vb,y
vb,x vw
,
or
vb,y =vb,x tan α(1 + m/M).
152
Conservation of energy requires
1
2mvb2+1
2Mvw2=mgh.
Combining,
1
2mvb,x2+vb,y 2+1
2Mm
Mvb,x2=mgh,
tan2α(1 + m/M )2+1+ m
Mvb,x2= 2gh,
sin2α(M+m)2+M2cos2α+mM cos2αvb,x2= 2M2gh cos2α,
M2+mM +mM sin2α+m2sin2αvb,x2= 2M2gh cos2α,
(M+m)(M+msin2α)vb,x2= 2M2gh cos2α,
or
vb,x =Mcos αs2gh
(M+m)(M+msin2α).
Then
vw=mcos αs2gh
(M+m)(M+msin2α).
P12-7 U(x) = RFxdx =Ax2/2Bx3/3.
(a) U=(3.00 N/m)(2.26 m)2/2(5.00 N/m2)(2.26 m)3/3 = 26.9 J.
(b) There are two points to consider:
U1=(3.00 N/m)(4.91 m)2/2(5.00 N/m2)(4.91 m)3/3 = 233 J,
U2=(3.00 N/m)(1.77 m)2/2(5.00 N/m2)(1.77 m)3/3 = 13.9 J,
K1=1
2(1.18kg)(4.13 m/s)2= 10.1 J.
Then
v2=s2(10.1 J + 233 J 13.9 J)
(1.18 kg) = 19.7 m/s.
P12-8 Assume that U0=K0= 0. Then conservation of energy requires K=U; consequently,
v=p2g(y).
(a) v=p2(9.81 m/s2)(1.20 m) = 4.85 m/s.
(b) v=p2(9.81 m/s2)(1.20 m 0.45 m 0.45 m) = 2.43 m/s.
P12-9 Assume that U0=K0= 0. Then conservation of energy requires K=U; consequently,
v=p2g(y). If the ball barely swings around the top of the peg then the speed at the top of the
loop is just fast enough so that the centripetal force is equal in magnitude to the weight,
mv2/R =mg.
The energy conservation problem is then
mv2= 2mg(L2(Ld)) = 2mg(2dL)
mg(Ld)=2mg(2dL),
d= 3L/5.
153
P12-10 The speed at the top and the speed at the bottom are related by
1
2mvb2=1
2mvt2+ 2mgR.
The magnitude of the net force is F=mv2/R, the tension at the top is
Tt=mvt2/R mg,
while tension at the bottom is
Tb=mvb2/R +mg,
The difference is
T= 2mg +m(vb2vt2)/R = 2mg + 4mg = 6mg.
P12-11 Let the angle θbe measured from the horizontal to the point on the hemisphere where
the boy is located. There are then two components to the force of gravity— a component tangent
to the hemisphere, W|| =mg cos θ, and a component directed radially toward the center of the
hemisphere, W=mg sin θ.
While the boy is in contact with the hemisphere the motion is circular so
mv2/R =WN.
When the boy leaves the surface we have mv2/R =W, or mv2=mgR sin θ. Now for energy
conservation,
K+U=K0+U0,
1
2mv2+mgy =1
2m(0)2+mgR,
1
2gR sin θ+mgy =mgR,
1
2y+y=R,
y= 2R/3.
P12-12 (a) To be in contact at the top requires mvt2/R =mg. The speed at the bottom would
be given by energy conservation
1
2mvb2=1
2mvt2+ 2mgR,
so vb=5gR is the speed at the bottom that will allow the object to make it around the circle
without loosing contact.
(b) The particle will lose contact with the track if mv2/R mg sin θ. Energy conservation gives
1
2mv2
0=1
2mv2+mgR(1 + sin θ)
for points above the half-way point. Then the condition for “sticking” to the track is
1
Rv2
02g(1 + sin θ)gsin θ,
or, if v0= 0.775vm,
5(0.775)223 sin θ,
or θ= arcsin(1/3).
154
P12-13 The rotational inertia is
I=1
3ML2+ML2=4
3ML2.
Conservation of energy is 1
2Iω2= 3Mg(L/2),
so ω=p9g/(4L).
P12-14 The rotational speed of the sphere is ω=v/r; the rotational kinetic energy is Kr=
1
2Iω2=1
5mv2.
(a) For the marble to stay on the track mv2/R =mg at the top of the track. Then the marble
needs to be released from a point
mgh =1
2mv2+1
5mv2+ 2mgR,
or h=R/2 + R/5+2R= 2.7R.
(b) Energy conservation gives
6mgR =1
2mv2+1
5mv2+mgR,
or mv2/R = 50mg/7. This corresponds to the horizontal force acting on the marble.
P12-15 1
2mv2
0+mgy = 0, where yis the distance beneath the rim, or y=rcos θ0. Then
v0=p2gy =p2gr cos θ0.
P12-16 (a) For E1the atoms will eventually move apart completely.
(b) For E2the moving atom will bounce back and forth between a closest point and a farthest
point.
(c) U≈ −1.2×1019J.
(d) K=E1U2.2×1019J.
(e) Find the slope of the curve, so
F≈ −(1×1019J) (2×1019J)
(0.3×109m) (0.2×109m) =1×109N,
which would point toward the larger mass.
P12-17 The function needs to fall off at infinity in both directions; an exponential envelope
would work, but it will need to have an x2term to force the potential to zero on both sides. So we
propose something of the form
U(x) = P(x)eβx2
where P(x) is a polynomial in xand βis a positive constant.
We proposed the polynomial because we need a symmetric function which has two zeroes. A
quadratic of the form αx2U0would work, it has two zeroes, a minimum at x= 0, and is symmetric.
So our trial function is
U(x) = αx2U0eβx2.
155
This function should have three extrema. Take the derivative, and then we’ll set it equal to zero,
dU
dx = 2αxeβx22αx2U0βxeβx2.
Setting this equal to zero leaves two possibilities,
x= 0,
2α2αx2U0β= 0.
The first equation is trivial, the second is easily rearranged to give
x=±sα+βU0
βα
These are the points ±x1. We can, if we wanted, try to find αand βfrom the picture, but you
might notice we have one equation, U(x1) = U1and two unknowns. It really isn’t very illuminating
to take this problem much farther, but we could.
(b) The force is the derivative of the potential; this expression was found above.
(c) As long as the energy is less than the two peaks, then the motion would be oscillatory, trapped
in the well.
P12-18 (a) F=U/∂r, or
F=U0r0
r2+1
rer/r0.
(b) Evaluate the force at the four points:
F(r0) = 2(U0/r0)e1,
F(2r0) = (3/4)(U0/r0)e2,
F(4r0) = (5/16)(U0/r0)e4,
F(10r0) = (11/100)(U0/r0)e10.
The ratios are then
F(2r0)/F (r0) = (3/8)e1= 0.14,
F(4r0)/F (r0) = (5/32)e3= 7.8×103,
F(10r0)/F (r0) = (11/200)e9= 6.8×106.
156
E13-1 If the projectile had not experienced air drag it would have risen to a height y2, but
because of air drag 68 kJ of mechanical energy was dissipated so it only rose to a height y1. In
either case the initial velocity, and hence initial kinetic energy, was the same; and the velocity at
the highest point was zero. Then W= ∆U, so the potential energy would have been 68 kJ greater,
and
y= ∆U/mg = (68×103J)/(9.4 kg)(9.81 m/s2) = 740 m
is how much higher it would have gone without air friction.
E13-2 (a) The road incline is θ= arctan(0.08) = 4.57. The frictional forces are the same; the car
is now moving with a vertical upward speed of (15 m/s) sin(4.57) = 1.20 m/s. The additional power
required to drive up the hill is then ∆P=mgvy= (1700 kg)(9.81 m/s2)(1.20 m/s) = 20000 W. The
total power required is 36000 W.
(b) The car will “coast” if the power generated by rolling downhill is equal to 16000 W, or
vy= (16000 W)/[(1700 kg)(9.81 m/s2)] = 0.959 m/s,
down. Then the incline is
θ= arcsin(0.959 m/s/15 m/s) = 3.67.
This corresponds to a downward grade of tan(3.67) = 6.4 %.
E13-3 Apply energy conservation:
1
2mv2+mgy +1
2ky2= 0,
so
v=p2(9.81 m/s2)(0.084 m) (262 N/m)(0.084 m)2/(1.25 kg) = 0.41 m/s.
E13-4 The car climbs a vertical distance of (225 m) sin(10) = 39.1 m in coming to a stop. The
change in energy of the car is then
E=1
2
(16400 N)
(9.81 m/s2)(31.4 m/s)2+ (16400 N)(39.1 m) = 1.83×105J.
E13-5 (a) Applying conservation of energy to the points where the ball was dropped and where
it entered the oil,
1
2mvf2+mgyf=1
2mvi2+mgyi,
1
2vf2+g(0) = 1
2(0)2+gyi,
vf=p2gyi,
=p2(9.81 m/s2)(0.76 m) = 3.9 m/s.
(b) The change in internal energy of the ball + oil can be found by considering the points where
the ball was released and where the ball reached the bottom of the container.
E=Kf+UfKiUi,
=1
2mvf2+mgyf1
2m(0)2mgyi,
=1
2(12.2×103kg)(1.48m/s)2(12.2×103kg)(9.81m/s2)(0.55m 0.76m),
=0.143 J
157
E13-6 (a) Ui= (25.3 kg)(9.81 m/s2)(12.2 m) = 3030 J.
(b) Kf=1
2(25.3 kg)(5.56 m/s)2= 391 J.
(c) ∆Eint = 3030 J 391 J = 2640 J.
E13-7 (a) At atmospheric entry the kinetic energy is
K=1
2(7.9×104kg)(8.0×103m/s)2= 2.5×1012J.
The gravitational potential energy is
U= (7.9×104kg)(9.8 m/s2)(1.6×105m) = 1.2×1011J.
The total energy is 2.6×1012J.
(b) At touch down the kinetic energy is
K=1
2(7.9×104kg)(9.8×101m/s)2= 3.8×108J.
E13-8 E/t= (68 kg)(9.8 m/s2)(59 m/s) = 39000 J/s.
E13-9 Let mbe the mass of the water under consideration. Then the percentage of the potential
energy “lost” which appears as kinetic energy is
KfKi
UiUf
.
Then
KfKi
UiUf
=1
2mvf2vi2/(mgyimgyf),
=vf2vi2
2gy,
=(13 m/s)2(3.2 m/s)2
2(9.81 m/s2)(15 m),
= 54 %.
The rest of the energy would have been converted to sound and thermal energy.
E13-10 The change in energy is
E=1
2(524 kg)(62.6 m/s)2(524 kg)(9.81 m/s2)(292 m) = 4.74×105J.
E13-11 Uf=Ki(34.6 J). Then
h=1
2
(7.81 m/s)2
(9.81 m/s2)(34.6 J)
(4.26 kg)(9.81 m/s2)= 2.28 m;
which means the distance along the incline is (2.28 m)/sin(33.0) = 4.19 m.
158
E13-12 (a) Kf=UiUf, so
vf=p2(9.81 m/s2)[(862 m) (741 m)] = 48.7 m/s.
That’s a quick 175 km/h; but the speed at the bottom of the valley is 40% of the speed of sound!
(b) ∆E=UfUi, so
E= (54.4 kg)(9.81 m/s2)[(862 m) (741 m)] = 6.46×104J;
which means the internal energy of the snow and skis increased by 6.46×104J.
E13-13 The final potential energy is 15% less than the initial kinetic plus potential energy of the
ball, so
0.85(Ki+Ui) = Uf.
But Ui=Uf, so Ki= 0.15Uf/0.85,and then
vi=r0.15
0.852gh =p2(0.176)(9.81 m/s2)(12.4 m) = 6.54 m/s.
E13-14 Focus on the potential energy. After the nth bounce the ball will have a potential energy
at the top of the bounce of Un= 0.9Un1. Since Uh, one can write hn= (0.9)nh0. Solving for n,
n= log(hn/h0)/log(0.9) = log(3 ft/6 ft)/log(0.9) = 6.58,
which must be rounded up to 7.
E13-15 Let mbe the mass of the ball and Mbe the mass of the block.
The kinetic energy of the ball just before colliding with the block is given by K1=U0, so
v1=p2(9.81 m/s2)(0.687 m) = 3.67 m/s.
Momentum is conserved, so if v2and v3are velocities of the ball and block after the collision
then mv1=mv2+Mv3. Kinetic energy is not conserved, instead
1
21
2mv2
1=1
2mv2
2+1
2Mv2
3.
Combine the energy and momentum expressions to eliminate v3:
mv2
1= 2mv2
2+ 2Mm
M(v1v2)2,
Mv2
1= 2Mv2
2+ 2mv2
14mv1v2+ 2mv2
2,
which can be formed into a quadratic. The solution for v2is
v2=2m±p2(M2mM)
2(M+m)v1= (0.600 ±1.95) m/s.
The corresponding solutions for v3are then found from the momentum expression to be v3=
0.981 m/s and v3= 0.219. Since it is unlikely that the ball passed through the block we can toss
out the second set of answers.
E13-16 Ef=Kf+Uf= 3mgh, or
vf=p2(9.81 m/s2)2(0.18 m) = 2.66 m/s.
159
E13-17 We can find the kinetic energy of the center of mass of the woman when her feet leave
the ground by considering energy conservation and her highest point. Then
1
2mvi2+mgyi=1
2mvf2+mgyf,
1
2mvi=mgy,
= (55.0 kg)(9.81 m/s2)(1.20 m 0.90 m) = 162 J.
(a) During the jumping phase her potential energy changed by
U=mgy= (55.0 kg)(9.81 m/s2)(0.50 m) = 270 J
while she was moving up. Then
Fext =K+ ∆U
s=(162 J) + (270 J)
(0.5 m) = 864 N.
(b) Her fastest speed was when her feet left the ground,
v=2K
m=2(162 J)
(55.0 kg) = 2.42 m/s.
E13-18 (b) The ice skater needs to lose 1
2(116 kg)(3.24 m/s)2= 609 J of internal energy.
(a) The average force exerted on the rail is F= (609 J)/(0.340 m) = 1790 N.
E13-19 12.6 km/h is equal to 3.50 m/s; the initial kinetic energy of the car is
1
2(2340 kg)(3.50 m/s)2= 1.43×104J.
(a) The force exerted on the car is F= (1.43×104J)/(0.64 m) = 2.24×104N.
(b) The change increase in internal energy of the car is
Eint = (2.24×104N)(0.640 m 0.083 m) = 1.25×104J.
E13-20 Note that v2
n=v0
n22~
v0
n·~
vcm +vcm2.Then
K=X
n
1
2mnv0
n22mn~
v0
n·~
vcm +mnvcm2,
=X
n
1
2mnv0
n2 X
n
mn~
v0
n!·~
vcm + X
n
1
2mn!vcm2,
=Kint X
n
mn~
v0
n!·~
vcm +Kcm.
The middle term vanishes because of the definition of velocities relative to the center of mass.
160
E13-21 Momentum conservation requires mv0=mv+MV, where the sign indicates the direction.
We are assuming one dimensional collisions. Energy conservation requires
1
2mv2
0=1
2mv2+1
2MV 2+E.
Combining,
1
2mv2
0=1
2mv2+1
2Mm
Mv0m
Mv2+E,
Mv2
0=Mv2+m(v0v)2+ 2(M/m)E.
Arrange this as a quadratic in v,
(M+m)v2(2mv0)v+2(M/m)E+mv2
0Mv2
0= 0.
This will only have real solutions if the discriminant (b24ac) is greater than or equal to zero. Then
(2mv0)24 (M+m)2(M/m)E+mv2
0Mv2
0
is the condition for the minimum v0. Solving the equality condition,
4m2v2
0= 4(M+m)2(M/m)E+ (mM)v2
0,
or M2v2
0= 2(M+m)(M/m)E. One last rearrangement, and v0=p2(M+m)E/(mM).
P13-1 (a) The initial kinetic energy will equal the potential energy at the highest point plus the
amount of energy which is dissipated because of air drag.
mgh +fh =1
2mv2
0,
h=v2
0
2(g+f/m)=v2
0
2g(1 + f/w).
(b) The final kinetic energy when the stone lands will be equal to the initial kinetic energy minus
twice the energy dissipated on the way up, so
1
2mv2=1
2mv2
02fh,
=1
2mv2
02fv2
0
2g(1 + f/w),
=m
2f
g(1 + f/w)v2
0,
v2=12f
w+fv2
0,
v=wf
w+f1/2
v0.
P13-2 The object starts with U= (0.234 kg)(9.81 m/s2)(1.05 m) = 2.41 J. It will move back and
forth across the flat portion (2.41 J)/(0.688 J) = 3.50 times, which means it will come to a rest at
the center of the flat part while attempting one last right to left journey.
161
P13-3 (a) The work done on the block block because of friction is
(0.210)(2.41 kg)(9.81 m/s2)(1.83 m) = 9.09 J.
The energy dissipated because of friction is (9.09 J)/0.83 = 11.0 J.
The speed of the block just after the bullet comes to a rest is
v=p2K/m =p2(1.10 J)/(2.41 kg) = 3.02 m/s.
(b) The initial speed of the bullet is
v0=M+m
mv=(2.41 kg) + (0.00454 kg)
(0.00454 kg) (3.02 m/s) = 1.60×103m/s.
P13-4 The energy stored in the spring after compression is 1
2(193 N/m)(0.0416 m)2= 0.167 J.
Since 117 mJ was dissipated by friction, the kinetic energy of the block before colliding with the
spring was 0.284 J. The speed of the block was then
v=p2(0.284 J)/(1.34 kg) = 0.651 m/s.
P13-5 (a) Using Newton’s second law, F=ma, so for circular motion around the proton
mv2
r=F=ke2
r2.
The kinetic energy of the electron in an orbit is then
K=1
2mv2=1
2ke2
r.
The change in kinetic energy is
K=1
2ke21
r21
r1.
(b) The potential energy difference is
U=Zr2
r1
ke2
r2dr =ke21
r21
r1.
(c) The total energy change is
E= ∆K+ ∆U=1
2ke21
r21
r1.
P13-6 (a) The initial energy of the system is (4000 lb)(12ft) = 48,000 ft ·lb.The safety device
removes (1000 lb)(12ft) = 12,000 ft ·lb before the elevator hits the spring, so the elevator has a
kinetic energy of 36,000 ft ·lb when it hits the spring. The speed of the elevator when it hits the
spring is
v=s2(36,000 ft ·lb)(32.0 ft/s2)
(4000 lb) = 24.0 ft/s.
(b) Assuming the safety clamp remains in effect while the elevator is in contact with the spring
then the distance compressed will be found from
36,000 ft ·lb = 1
2(10,000 lb/ft)y2(4000 lb)y+ (1000 lb)y.
162
This is a quadratic expression in ywhich can be simplified to look like
5y23y36 = 0,
which has solutions y= (0.3±2.7) ft. Only y= 3.00 ft makes sense here.
(c) The distance through which the elevator will bounce back up is found from
33,000 ft = (4000 lb)y(1000 lb)y,
where yis measured from the most compressed point of the spring. Then y= 11 ft, or the elevator
bounces back up 8 feet.
(d) The elevator will bounce until it has traveled a total distance so that the safety device
dissipates all of the original energy, or 48 ft.
P13-7 The net force on the top block while it is being pulled is
11.0 N Ff= 11.0 N (0.35)(2.5 kg)(9.81 m/s2) = 2.42 N.
This means it is accelerating at (2.42 N)/(2.5 kg) = 0.968 m/s2. That acceleration will last a time
t=p2(0.30 m)/(0.968 m/s2)=0.787 s. The speed of the top block after the force stops pulling is
then (0.968 m/s2)(0.787 s) = 0.762 m/s. The force on the bottom block is Ff, so the acceleration of
the bottom block is
(0.35)(2.5 kg)(9.81 m/s2)/(10.0 kg) = 0.858 m/s2,
and the speed after the force stops pulling on the top block is (0.858 m/s2)(0.787 s) = 0.675 m/s.
(a) W=F s = (11.0 N)(0.30 m) = 3.3 J of energy were delivered to the system, but after the
force stops pulling only
1
2(2.5 kg)(0.762 m/s)2+1
2(10.0 kg)(0.675 m/s)2= 3.004 J
were present as kinetic energy. So 0.296 J is “missing” and would be now present as internal energy.
(b) The impulse received by the two block system is then J= (11.0 N)(0.787 s) = 8.66 N·s. This
impulse causes a change in momentum, so the speed of the two block system after the external
force stops pulling and both blocks move as one is (8.66 N·s)(12.5 kg) = 0.693 m/s. The final kinetic
energy is 1
2(12.5 kg)(0.693 m/s)2= 3.002 J;
this means that 0.002 J are dissipated.
P13-8 Hmm.
163
E14-1 FS/F E=MSrE2/M ErS2, since everything else cancels out in the expression. Then
FS
FE
=(1.99×1030kg)(3.84×108m)2
(5.98×1024)(1.50×1011m)2= 2.18
E14-2 Consider the force from the Sun and the force from the Earth. FS/F E=MSrE2/MErS2,
since everything else cancels out in the expression. We want the ratio to be one; we are also
constrained because rE+rS=Ris the distance from the Sun to the Earth. Then
ME(RrE)2=MSrE2,
RrE=rMS
ME
rE,
rE= (1.50×1011m)/ 1 + s(1.99×1030kg)
(5.98×1024)!= 2.6×108m.
E14-3 The masses of each object are m1= 20.0 kg and m2= 7.0 kg; the distance between the
centers of the two objects is 15 + 3 = 18 m.
The magnitude of the force from Newton’s law of gravitation is then
F=Gm1m2
r2=(6.67×1011N·m2/kg2)(20.0 kg)(7.0 kg)
(18 m)2= 2.9×1011N.
E14-4 (a) The magnitude of the force from Newton’s law of gravitation is
F=Gm1m2
r2=(6.67×1011N·m2/kg2)(12.7 kg)(9.85×103kg)
(0.108 m)2= 7.15×1010N.
(b) The torque is τ= 2(0.262 m)(7.15×1010N) = 3.75×1010N·m.
E14-5 The force of gravity on an object near the surface of the earth is given by
F=GMm
(re+y)2,
where Mis the mass of the Earth, mis the mass of the object, reis the radius of the Earth, and
yis the height above the surface of the Earth. Expand the expression since yre. We’ll use a
Taylor expansion, where F(re+y)F(re) + yF/∂re;
FGMm
r2
e2yGMm
r3
e
Since we are interested in the difference between the force at the top and the bottom, we really want
F= 2yGMm
r3
e
= 2 y
re
GMm
r2
e
= 2 y
re
W,
where in the last part we substituted for the weight, which is the same as the force of gravity,
W=GMm
r2
e
.
Finally,
F= 2(411 m)/(6.37×106m)(120 lb) = 0.015 lb.
164
E14-6 g1/r2, so g1/g2=r2
2/r2
1. Then
r2=p(9.81 m/s2)/(7.35 m/s2)(6.37×106m) = 7.36×106m.
That’s 990 kilometers above the surface of the Earth.
E14-7 (a) a=GM/r2, or
a=(6.67×1011N·m2/kg2)(1.99×1030kg)
(10.0×103m)2= 1.33×1012m/s2.
(b) v=2ax =p2(1.33×1012m/s2)(1.2 m/s) = 1.79×106m/s.
E14-8 (a) g0=GM/r2, or
g0=(6.67×1011N·m2/kg2)(7.36×1022kg)
(1.74×106m)2= 1.62 m/s2.
(b) Wm=We(gm/ge) so
Wm= (100 N)(1.62 m/s2/9.81 m/s2) = 16.5 N.
(c) Invert g=GM/r2;
r=pGM/g =q(6.67×1011N·m2/kg2)(5.98×1024kg)/(1.62 m/s2) = 1.57×107m.
That’s 2.46 Earth radii, or 1.46 Earth radii above the surface of the Earth.
E14-9 The object fell through y=10.0 m; the time required to fall would then be
t=p2y/g =p2(10.0 m)/(9.81 m/s2) = 1.43 s.
We are interested in the error, that means taking the total derivative of y=1
2gt2.and getting
δy =1
2δg t2gt δt.
δy = 0 so 1
2δg t =gδ t, which can be rearranged as
δt =δg t
2g
The percentage error in tneeds to be δt/t = 0.1 %/2=0.05 %. The absolute error is then δt =
(0.05 %)(1.43 s) = 0.7 ms.
E14-10 Treat mass which is inside a spherical shell as being located at the center of that shell.
Ignore any contributions from shells farther away from the center than the point in question.
(a) F=G(M1+M2)m/a2.
(b) F=G(M1)m/b2.
(c) F= 0.
165
E14-11 For a sphere of uniform density and radius R > r,
M(r)
4
3πr3=M
4
3πR3,
where Mis the total mass.
The force of gravity on the object of mass mis then
F=GMm
r2
r3
R3=GMmr
R3.
gis the free-fall acceleration of the object, and is the gravitational force divided by the mass, so
g=GMr
R3=GM
R2
r
R=GM
R2
RD
R.
Since Ris the distance from the center to the surface, and Dis the distance of the object beneath
the surface, then r=RDis the distance from the center to the object. The first fraction is the
free-fall acceleration on the surface, so
g=GM
R2
RD
R=gs
RD
R=gs1D
R
E14-12 The work required to move the object is GM Sm/r, where ris the gravitational radius.
But if this equals mc2we can write
mc2=GMSm/r,
r=GMS/c2.
For the sun, r= (6.67×1011N·m2/kg2)(1.99×1030kg)/(3.00×108m/s)2= 1.47×103m. That’s
2.1×106RS.
E14-13 The distance from the center is
r= (80000)(3.00×108m/s)(3.16×107s) = 7.6×1020m.
The mass of the galaxy is
M= (1.4×1011)(1.99×1030kg) = 2.8×1041kg.
The escape velocity is
v=p2GM/r =q2(6.67×1011N·m2/kg2)(2.8×1041kg)/(7.6×1020m) = 2.2×105m/s.
E14-14 Staying in a circular orbit requires the centripetal force be equal to the gravitational force,
so
mvorb2/r =GMm/r2,
or mvorb2=GMm/r. But GMm/r is the gravitational potential energy; to escape one requires a
kinetic energy
mvesc2/2 = GMm/r =mvorb2,
which has solution vesc =2vorb.
166
E14-15 (a) Near the surface of the Earth the total energy is
E=K+U=1
2m2pgRE2GMEm
RE
but
g=GM
RE2,
so the total energy is
E= 2mgREGMEm
RE
,
= 2mGM
RE2REGMEm
RE
,
=GMEm
RE
This is a positive number, so the rocket will escape.
(b) Far from earth there is no gravitational potential energy, so
1
2mv2=GMEm
RE
=GME
RE2mRE=gmRE,
with solution v=2gRE.
E14-16 The rotational acceleration of the sun is related to the galactic acceleration of free fall by
4π2mr/T 2=GNm2/r2,
where Nis the number of “sun” sized stars of mass m,ris the size of the galaxy, Tis period of
revolution of the sun. Then
N=4π2r3
GmT 2=4π2(2.2×1020m)3
(6.67×1011N·m2/kg2)(2.0×1030kg)(7.9×1015s)2= 5.1×1010.
E14-17 Energy conservation is Ki+Ui=Kf+Uf, but at the highest point Kf= 0, so
Uf=Ki+Ui,
GMEm
R=1
2mv2
0GMEm
RE
,
1
R=1
RE1
2GME
v2
0,
1
R=1
(6.37×106m) (9.42×103m/s)2)
2(6.67×1011N·m2/kg2)(5.98×1024kg) ,
R= 2.19×107m.
The distance above the Earth’s surface is 2.19×107m6.37×106m=1.55×106m.
167
E14-18 (a) Free-fall acceleration is g=GM/r2. Escape speed is v=p2GM/r. Then v=2gr =
p2(1.30 m/s2)(1.569×106m) = 2.02×103m/s.
(b) Uf=Ki+Ui. But U/m =g0r2
0/r, so
1
rf
=1
(1.569×106m) (1.01×103m/s)2
2(1.30 m/s2)(1.569×106m)2=1
2.09×106m.
That’s 523 km above the surface.
(c) Kf=UiUf. But U/m =g0r2
0/r, so
v=p2(1.30 m/s2)(1.569×106m)2[1/(1.569×106m) 1/(2.569×106m)] = 1260 m/s.
(d) M=gr2/G, or
M= (1.30 m/s2)(1.569×106m)2/(6.67×1011N·m2/kg2) = 4.8×1022kg.
E14-19 (a) Apply ∆K=U. Then mv2=Gm2(1/r21/r1), so
v=s(6.67×1011N·m2/kg2)(1.56×1030kg) 1
(4.67×104m) 1
(9.34×104m) = 3.34×107m/s.
(b) Apply ∆K=U. Then mv2=Gm2(1/r21/r1), so
v=s(6.67×1011N·m2/kg2)(1.56×1030kg) 1
(1.26×104m) 1
(9.34×104m) = 5.49×107m/s.
E14-20 Call the particles 1 and 2. Then conservation of momentum requires the particle to have
the same momentum of the same magnitude, p=mv1=M v2. The momentum of the particles is
given by
1
2mp2+1
2Mp2=GMm
d,
m+M
mM p2= 2GMm/d,
p=mMp2G/d(m+M).
Then vrel =|v1|+|v2|is equal to
vrel =mMp2G/d(m+M)1
m+1
M,
=mMp2G/d(m+M)m+M
mM ,
=p2G(m+M)/d
E14-21 The maximum speed is mv2=Gm2/d, or v=pGm/d.
E14-22 T2
1/r3
1=T2
2/r3
2, or
T2=T1(r2/r1)3/2= (1.00 y)(1.52)3/2= 1.87 y.
168
E14-23 We can use Eq. 14-23 to find the mass of Mars; all we need to do is rearrange to solve
for M
M=4π2r3
GT 2=4π2(9.4×106m)3
(6.67×1011N·m2/kg2)(2.75×104s)2= 6.5×1023kg.
E14-24 Use GM/r2= 4π2r/T 2, so M= 4π2r3/GT 2, and
M=4π2(3.82×108m)3
(6.67×1011N·m2/kg2)(27.3×86400 s)2= 5.93×1024kg.
E14-25 T2
1/r3
1=T2
2/r3
2, or
T2=T1(r2/r1)3/2= (1.00 month)(1/2)3/2= 0.354 month.
E14-26 Geosynchronous orbit was found in Sample Problem 14-8 to be 4.22×107m. The latitude
is given by
θ= arccos(6.37×106m/4.22×107m) = 81.3.
E14-27 (b) Make the assumption that the altitude of the satellite is so low that the radius of
the orbit is effectively the radius of the moon. Then
T2=4π2
GM r3,
=4π2
(6.67×1011N·m2/kg2)(7.36×1022kg) (1.74×106m)3= 4.24×107s2.
So T= 6.5×103s.
(a) The speed of the satellite is the circumference divided by the period, or
v=2πr
T=2π(1.74×106m)
(6.5×103s) = 1.68×103m/s.
E14-28 The total energy is GMm/2a. Then
1
2mv2GMm
r=GMm
2a,
so
v2=GM 2
r1
a.
E14-29 ra=a(1 + e), so from Ex. 14-28,
va=sGM 2
a(1 + e)1
a;
rp=a(1 e), so from Ex. 14-28,
vp=sGM 2
a(1 e)1
a;
Dividing one expression by the other,
vp=vas2/(1 e)1
2/(1 + e)1= (3.72 km/s)s2/0.12 1
2/1.88 1= 58.3 km/s.
169
E14-30 (a) Convert.
G=6.67×1011 m3
kg ·s21.99×1030kg
MS3.156×107s
y2AU
1.496×1011m3
,
which is G= 39.49 AU3/MS2·y2.
(b) Here is a hint: 4π2= 39.48. Kepler’s law then looks like
T2=MS2·y2
AU3r3
M.
E14-31 Kepler’s third law states T2r3, where ris the mean distance from the Sun and Tis
the period of revolution. Newton was in a position to find the acceleration of the Moon toward the
Earth by assuming the Moon moved in a circular orbit, since ac=v2/r = 4π2r/T 2. But this means
that, because of Kepler’s law, acr/T 21/r2.
E14-32 (a) The force of attraction between the two bodies is
F=GMm
(r+R)2.
The centripetal acceleration for the body of mass mis
rω2=GM
(r+R)2,
ω2=GM
r3(1 + R/r)2,
T2=4π2
GM r3(1 + R/r)2.
(b) Note that r=Md/(m+M) and R=md/(m+M). Then R/r =m/M, so the correction is
(1 + 5.94×1024/1.99×1030)2= 1.000006 for the Earth/Sun system and 1.025 for the Earth/Moon
system.
E14-33 (a) Use the results of Exercise 14-32. The center of mass is located a distance r=
2md/(m+ 2m)=2d/3 from the star of mass mand a distance R=d/3 from the star of mass 2m.
The period of revolution is then given by
T2=4π2
G(2m)2
3d31 + d/3
2d/32
=4π2
3Gmd3.
(b) Use Lm=mr2ω, then
Lm
LM
=mr2
MR2=m(2d/3)2
(2m)(d/3)2= 2.
(c) Use K=Iω2/2 = mr2ω2/2. Then
Km
KM
=mr2
MR2=m(2d/3)2
(2m)(d/3)2= 2.
170
E14-34 Since we don’t know which direction the orbit will be, we will assume that the satellite
on the surface of the Earth starts with zero kinetic energy. Then Ei=Ui.
U=UfUito get the satellite up to the specified altitude. K=Kf=Uf/2. We want
to know if ∆UKis positive (more energy to get it up) or negative (more energy to put it in
orbit). Then we are interested in
UK= 3Uf/2Ui=GMm 1
ri3
2rf.
The “break-even” point is when rf= 3ri/2 = 3(6400 km)/2 = 9600 km, which is 3200 km above the
Earth.
(a) More energy to put it in orbit.
(b) Same energy for both.
(c) More energy to get it up.
E14-35 (a) The approximate force of gravity on a 2000 kg pickup truck on Eros will be
F=GMm
r2=(6.67×1011N·m2/kg2)(5.0×1015 kg)(2000 kg)
(7000 m)2= 13.6 N.
(b) Use
v=rGM
r=s(6.67×1011N·m2/kg2)(5.0×1015 kg)
(7000 m) = 6.9 m/s.
E14-36 (a) U=GMm/r. The variation is then
U= (6.67×1011N·m2/kg2)(1.99×1030kg)(5.98×1024kg) 1
(1.47×1011m) 1
(1.52×1011m)
= 1.78×1032J.
(b) ∆K+ ∆U= ∆E= 0, so |K|= 1.78×1032J.
(c) ∆E= 0.
(d) Since ∆l= 0 and l=mvr, we have
vpva=vp1rp
ra=vp1(1.47×1011m)
(1.52×1011m) = 3.29×102vp.
But vpvav = 2π(1.5×1011m)/(3.16×107s) = 2.98×104m/s. Then ∆v= 981 m/s.
E14-37 Draw a triangle. The angle made by Chicago, Earth center, satellite is 47.5. The distance
from Earth center to satellite is 4.22×107m. The distance from Earth center to Chicago is 6.37×106m.
Applying the cosine law we find the distance from Chicago to the satellite is
p(4.22×107m)2+ (6.37×106m)22(4.22×107m)(6.37×106m) cos(47.5) = 3.82×107m.
Applying the sine law we find the angle made by Earth center, Chicago, satellite to be
arcsin (4.22×107m)
(3.82×107m) sin(47.5)= 126.
That’s 36above the horizontal.
171
E14-38 (a) The new orbit is an ellipse with eccentricity given by r=a0(1 + e). Then
e=r/a01 = (6.64×106m)/(6.52×106m) 1 = 0.0184.
The distance at P0is given by rP0=a0(1 e). The potential energy at P0is
UP0=UP
1 + e
1e= 2(9.76×1010J) 1+0.0184
10.0184 =2.03×1011J.
The kinetic energy at P0is then
KP0= (9.94×1010J) (2.03×1011J) = 1.04×1011J.
That would mean v=p2(1.04×1011J)/(3250kg) = 8000 m/s.
(b) The average speed is
v=2π(6.52×106m)
(5240 s) = 7820 m/s.
E14-39 (a) The Starshine satellite was approximately 275 km above the surface of the Earth on
1 January 2000. We can find the orbital period from Eq. 14-23,
T2=4π2
GM r3,
=4π2
(6.67×1011N·m2/kg2)(5.98×1024kg) (6.65×106m)3= 2.91×107s2,
so T= 5.39×103s.
(b) Equation 14-25 gives the total energy of the system of a satellite of mass min a circular orbit
of radius raround a stationary body of mass Mm,
E=GMm
2r.
We want the rate of change of this with respect to time, so
dE
dt =GMm
2r2
dr
dt
We can estimate the value of dr/dt from the diagram. I’ll choose February 1 and December 1 as my
two reference points.
dr
dt t=t0r
t=(240 km) (300 km)
(62 days) ≈ −1 km/day
The rate of energy loss is then
dE
dt =(6.67×1011N·m2/kg2)(5.98×1024kg)(39 kg)
2(6.65×106m)21000 m
8.64×104s=2.0 J/s.
P14-1 The object on the top experiences a force down from gravity W1and a force down from
the tension in the rope T. The object on the bottom experiences a force down from gravity W2and
a force up from the tension in the rope.
In either case, the magnitude of Wiis
Wi=GMm
r2
i
172
where riis the distance of the ith object from the center of the Earth. While the objects fall they
have the same acceleration, and since they have the same mass we can quickly write
GMm
r2
1
+T=GMm
r2
2T,
or
T=GMm
2r2
2GMm
2r2
1
,
=GMm
21
r2
11
r2
2,
=GMm
2
r2
2r2
1
r2
1r2
2
.
Now r1r2Rin the denominator, but r2=r1+l, so r2
2r2
12Rl in the numerator. Then
TGMml
R3.
P14-2 For a planet of uniform density, g=GM/r2=G(4πρr3/3)/r2= 4πGρr/3. Then if ρis
doubled while ris halved we find that gwill be unchanged.
P14-3 (a) F=GMm/r2,a=F/m =GM/r.
(b) The acceleration of the Earth toward the center of mass is aE=F/M =Gm/r2. The
relative acceleration is then GM/r +Gm/r =G(m+M)/r. Only if Mmcan we assume that a
is independent of mrelative to the Earth.
P14-4 (a) g=GM/r2,δg =(2GM/r3)δr. In this case δr =hand M= 4πρr3/3. Then
δW =m δg = 8πGρmh/3.
(b) ∆W/W = ∆g/g = 2h/r. Then an error of one part in a million will occur when his one
part in two million of r, or 3.2 meters.
P14-5 (a) The magnitude of the gravitational force from the Moon on a particle at Ais
FA=GMm
(rR)2,
where the denominator is the distance from the center of the moon to point A.
(b) At the center of the Earth the gravitational force of the moon on a particle of mass mis
FC=GMm/r2.
(c) Now we want to know the difference between these two expressions:
FAFC=GMm
(rR)2GMm
r2,
=GMm r2
r2(rR)2(rR)2
r2(rR)2,
=GMm r2(rR)2
r2(rR)2,
=GMm R(2rR)
r2(rR)2.
173
To simplify assume Rrand then substitute (rR)r. The force difference simplifies to
FT=GMm R(2r)
r2(r)2=2GMmR
r3
(d) Repeat part (c) except we want r+Rinstead of rR. Then
FAFC=GMm
(r+R)2GMm
r2,
=GMm r2
r2(r+R)2(r+R)2
r2(r+R)2,
=GMm r2(r+R)2
r2(r+R)2,
=GMm R(2r+R)
r2(r+R)2.
To simplify assume Rrand then substitute (r+R)r. The force difference simplifies to
FT=GMmR(2r)
r2(r)2=2GMmR
r3
The negative sign indicates that this “apparent” force points away from the moon, not toward it.
(e) Consider the directions: the water is effectively attracted to the moon when closer, but
repelled when farther.
P14-6 Fnet =mrωs2, where ωsis the rotational speed of the ship. But since the ship is moving
relative to the earth with a speed v, we can write ωs=ω±v/r, where the sign is positive if the ship
is sailing east. Then Fnet =mr(ω±v/r)2.
The scale measures a force Wwhich is given by mg Fnet, or
W=mg mr(ω±v/r)2.
Note that W0=m(grω2). Then
W=W0
gr(ω±v/r)2
grω2,
W01±2ωv
1rω2,
W0(1 ±2vω/g).
P14-7 (a) a=GM/r22.ωis the rotational speed of the Earth. Since Frank observes a=g/2
we have
g/2 = GM/r22,
r2= (2GM 2r3ω2)/g,
r=p2(GM r3ω2)/g
Note that
GM = (6.67×1011N·m2/kg2)(5.98×1024kg) = 3.99×1014m3/s2
while
r3ω2= (6.37×106m)3(2π/86400 s)2= 1.37×1012m3/s2.
174
Consequently, r3ω2can be treated as a perturbation of GM bear the Earth. Solving iteratively,
r0=p2[(3.99×1014m3/s2)(6.37×106m)3(2π/86400 s)2]/(9.81 m/s2) = 9.00×106m,
r1=p2[(3.99×1014m3/s2)(9.00×106m)3(2π/86400 s)2]/(9.81 m/s2) = 8.98×106m,
which is close enough for me. Then h= 8.98×106m6.37×106m = 2610 km.
(b) ∆E=EfEi=Uf/2Ui. Then
E= (6.67×1011N·m2/kg2)(5.98×1024kg)(100 kg) 1
(6.37×106m) 1
2(8.98×106m) = 4.0×109J.
P14-8 (a) Equate centripetal force with the force of gravity.
4π2mr
T2=GMm
r2,
4π2
T2=G(4/3)πr3ρ
r3,
T=r3π
(b) T=q3π/(6.7×1011N·m2/kg2)(3.0×103kg/m3) = 6800 s.
P14-9 (a) One can find δg by pretending the Earth is not there, but the material in the hole is.
Concentrate on the vertical component of the resulting force of attraction. Then
δg =GM
r2
d
r,
where ris the straight line distance from the prospector to the center of the hole and Mis the mass
of material that would fill the hole. A few substitutions later,
δg =4πGρR3d
3(d2+x2)3.
(b) Directly above the hole x= 0, so a ratio of the two readings gives
(10.0 milligals)
(14.0 milligals) =d2
d2+ (150 m)23/2
or
(0.800)(d2+ 2.25×104m2) = d2,
which has solution d= 300 m. Then
R3=3(14.0×105m/s2)(300 m)2
4π(6.7×1011N·m2/kg2)(2800 kg/m3),
so R= 250 m. The top of the cave is then 300 m 250 m = 50 m beneath the surface.
(b) All of the formulae stay the same except replace ρwith the difference between rock and water.
ddoesn’t change, but Rwill now be given by
R3=3(14.0×105m/s2)(300 m)2
4π(6.7×1011N·m2/kg2)(1800 kg/m3),
so R= 292 m, and then the cave is located 300 m 292 m = 7 m beneath the surface.
175
P14-10 g=GM/r2, where Mis the mass enclosed in within the sphere of radius r. Then
dg = (G/r2)dM 2(GM/r3)dr, so that gis locally constant if dM/dr = 2M/r. Expanding,
4πr2ρl= 8πr2ρ/3,
ρl= 2ρ/3.
P14-11 The force of gravity on the small sphere of mass mis equal to the force of gravity from
a solid lead sphere minus the force which would have been contributed by the smaller lead sphere
which would have filled the hole. So we need to know about the size and mass of the lead which was
removed to make the hole.
The density of the lead is given by
ρ=M
4
3πR3
The hole has a radius of R/2, so if the density is constant the mass of the hole will be
Mh=ρV =M
4
3πR34
3πR
23
=M
8
The “hole” is closer to the small sphere; the center of the hole is dR/2 away. The force of the
whole lead sphere minus the force of the “hole” lead sphere is
GMm
d2G(M/8)m
(dR/2)2
P14-12 (a) Use v=ωR2r2, where ω=pGME/R3. Then
T=ZT
0
dt =Z0
R
dr
dr/dt =Z0
R
dr
v,
=Z0
R
dr
ωR2r2,
=π
2ω
Knowing that
ω=s(6.67×1011N·m2/kg2)(5.98×1024kg)
(6.37×106m)3= 1.24×103/s,
we can find T= 1260 s = 21 min.
(b) Same time, 21 minutes. To do a complete journey would require four times this, or 2π.
That’s 84 minutes!
(c) The answers are the same.
P14-13 (a) g=GM/r2and M= 1.93×1024kg + 4.01×1024kg + 3.94×1022kg = 5.98×1024kg so
g= (6.67×1011N·m2/kg2)(5.98×1024kg)/(6.37×106m)2= 9.83 m/s2.
(b) Now M= 1.93×1024kg + 4.01×1024kg = 5.94×1024kg so
g= (6.67×1011N·m2/kg2)(5.94×1024kg)/(6.345×106m)2= 9.84 m/s2.
(c) For a uniform body, g= 4πGρr/3 = GM r/R3, so
g= (6.67×1011N·m2/kg2)(5.98×1024kg)(6.345×106m)/(6.37×106m)3= 9.79 m/s2.
176
P14-14 (a) Use g=GM/r2, then
g= (6.67×1011N·m2/kg2)(1.93×1024kg)/(3.490×106m)2= 106 m/s2.
The variation with depth is linear if core has uniform density.
(b) In the mantle we have g=G(Mc+M)/r2, where Mis the amount of the mass of the mantle
which is enclosed in the sphere of radius r. The density of the core is
ρc=3(1.93×1024kg)
4π(3.490×106m)3= 1.084×104kg/m3.
The density of the mantle is harder to find,
ρc=3(4.01×1024kg)
4π[(6.345×106m)3(3.490×106m)3= 4.496×103kg/m3.
We can pretend that the core is made up of a point mass at the center and the rest has a density
equal to that of the mantle. That point mass would be
Mp=4π(3.490×106m)3(1.084×104kg/m34.496×103kg/m3)
3= 1.130×1024kg.
Then
g=GMp/r2+ 4πGρmr/3.
Find dg/dr, and set equal to zero. This happens when
2Mp/r3= 4πρm/3,
or r= 4.93×106m. Then g= 9.29 m/s2. Since this is less than the value at the end points it must
be a minimum.
P14-15 (a) We will use part of the hint, but we will integrate instead of assuming the bit about
gav; doing it this way will become important for later chapters. Consider a small horizontal slice of
the column of thickness dr. The weight of the material above the slice exerts a force F(r) on the
top of the slice; there is a force of gravity on the slice given by
dF =GM(r)dm
r2,
where M(r) is the mass contained in the sphere of radius r,
M(r) = 4
3πr3ρ.
Lastly, the mass of the slice dm is related to the thickness and cross sectional area by dm =ρA dr.
Then
dF =4πGAρ2
3r dr.
Integrate both sides of this expression. On the left the limits are 0 to Fcenter, on the right the limits
are Rto 0; we need to throw in an extra negative sign because the force increases as rdecreases.
Then
F=2
3πGAρ2R2.
Divide both sides by Ato get the compressive stress.
177
(b) Put in the numbers!
S=2
3π(6.67×1011N·m2/kg2)(4000 kg/m3)2(3.0×105m)2= 2.0×108N/m2.
(c) Rearrange, and then put in numbers;
R=s3(4.0×107N/m2)
2π(6.67×1011N·m2/kg2)(3000 kg/m3)2= 1.8×105m.
P14-16 The two mass increments each exert a vertical and a horizontal force on the particle, but
the horizontal components will cancel. The vertical component is proportional to the sine of the
angle, so that
dF =2Gm dm
r2
y
r=2Gmλ dx
r2
y
r,
where r2=x2+y2. We will eventually integrate from 0 to , so
F=Z
0
2Gmλ dx
r2
y
r,
= 2Gmλy Z
0
dx
(x2+y2)3/2,
=2Gmλ
y.
P14-17 For any arbitrary point Pthe cross sectional area which is perpendicular to the axis
dA0=r2dΩ is not equal to the projection dA onto the surface of the sphere. It depends on the angle
that the axis makes with the normal, according to dA0= cos θdA. Fortunately, the angle made at
point 1 is identical to the angle made at point 2, so we can write
d1=d2,
dA1/r2
1=dA2/r2
2
But the mass of the shell contained in dA is proportional to dA, so
r2
1dm1=r2
2dm2,
Gm dm1/r2
1=Gm dm2/r2
2.
Consequently, the force on an object at point Pis balanced by both cones.
(b) Evaluate RdΩ for the top and bottom halves of the sphere. Since every dΩ on the top is
balanced by one on the bottom, the net force is zero.
P14-18
P14-19 Assume that the small sphere is always between the two spheres. Then
W= ∆U1+ ∆U2,
= (6.67×1011N·m2/kg2)(0.212 kg) [(7.16 kg) (2.53 kg)] 1
(0.420 m) 1
(1.14 m),
= 9.85×1011J.
178
P14-20 Note that 1
2mvesc2=U0, where U0is the potential energy at the burn-out height.
Energy conservation gives
K=K0+U0,
1
2mv2=1
2mv2
01
2mvesc2,
v=qv2
0vesc2.
P14-21 (a) The force of one star on the other is given by F=Gm2/d2, where dis the distance
between the stars. The stars revolve around the center of mass, which is halfway between the stars
so r=d/2 is the radius of the orbit of the stars. If ais the centripetal acceleration of the stars, the
period of revolution is then
T=r4π2r
a=r42r
F=r16π2r3
Gm .
The numerical value is
T=s16π2(1.12×1011m)3
(6.67×1011N·m2/kg2)(3.22×1030kg) = 3.21×107s=1.02 y.
(b) The gravitational potential energy per kilogram midway between the stars is
2Gm
r=2(6.67×1011N·m2/kg2)(3.22×1030kg)
(1.12×1011m) =3.84×109J/kg.
An object of mass Mat the center between the stars would need (3.84×109J/kg)Mkinetic energy
to escape, this corresponds to a speed of
v=p2K/M =p2(3.84×109J/kg) = 8.76×104m/s.
P14-22 (a) Each differential mass segment on the ring contributes the same amount to the force
on the particle,
dF =Gm dm
r2
x
r,
where r2=x2+R2. Since the differential mass segments are all equal distance, the integration is
trivial, and the net force is
F=GMmx
(x2+R2)3/2.
(b) The potential energy can be found by integrating with respect to x,
U=Z
0
F dx =Z
0
GMmx
(x2+R2)3/2dx =GMm
R.
Then the particle of mass mwill pass through the center of the ring with a speed v=p2∆U/m =
p2GM/R.
179
P14-23 (a) Consider the following diagram.
R
θ
α
R
r
F
FR
The distance ris given by the cosine law to be
r2=R2+R22R2cos θ= 2R2(1 cos θ).
The force between two particles is then F=Gm2/r2.Each particle has a symmetric partner, so
only the force component directed toward the center contributes. If we call this the Rcomponent
we have
FR=Fcos α=Fcos(90θ/2) = Fsin(θ/2).
Combining,
FR=Gm2
2R2
sin(θ/2)
1cos θ.
But each of the other particles contributes to this force, so
Fnet =Gm2
2R2X
i
sin(θi/2)
1cos θi
When there are only 9 particles the angles are in steps of 40; the θiare then 40, 80, 120, 160,
200, 240, 280, and 320. With a little patience you will find
X
i
sin(θi/2)
1cos θi
= 6.649655,
using these angles. Then Fnet = 3.32Gm2/R2.
(b) The rotational period of the ring would need to be
T=r4π2R
a=r42R
F=r16π2R3
3.32Gm.
P14-24 The potential energy of the system is U=Gm2/r. The kinetic energy is mv2. The total
energy is E=Gm2/d. Then
dr
dt = 2pGm(1/r 1/d),
180
so the time to come together is
T=Z0
d
dr
2pGm(1/r 1/d)=rd3
4Gm Z1
0rx
1xdx =π
4rd3
Gm.
P14-25 (a) E=U/2 for each satellite, so the total mechanical energy is GMm/r.
(b) Now there is no K, so the total mechanical energy is simply U=2GMm/r. The factor of
2 is because there are two satellites.
(c) The wreckage falls vertically onto the Earth.
P14-26 Let ra=a(1 + e) and rp=a(1 e). Then ra+re= 2aand rarp= 2ae. So the answer
is
2(0.0167)(1.50×1011m) = 5.01×109m,
or 7.20 solar radii.
P14-27
P14-28 The net force on an orbiting star is
F=Gm M
r2+m4r2.
This is the centripetal force, and is equal to 4π2mr/T 2. Combining,
4π2
T2=G
4r3(4M+m),
so T= 4πpr3/[G(4M+m)].
P14-29 (a) v=pGM/r, so
v=q(6.67×1011N·m2/kg2)(5.98×1024kg)/(7.01×106m) = 7.54×103m/s.
(b) T= 2π(7.01×106m)/(7.54×103m/s) = 5.84×103s.
(c) Originally E0=U/2, or
E=(6.67×1011N·m2/kg2)(5.98×1024kg)(220kg)
2(7.01×106m) =6.25×109J.
After 1500 orbits the energy is now 6.25×109J(1500)(1.40×105J) = 6.46×109J. The new
distance from the Earth is then
r=(6.67×1011N·m2/kg2)(5.98×1024kg)(220kg)
2(6.46×109J) = 6.79×106m.
The altitude is now 6790 6370 = 420 km.
(d) F= (1.40×105J)/(2π7.01×106m) = 3.2×103N.
(e) No.
181
P14-30 Let the satellite Sbe directly overhead at some time. The magnitude of the speed is
equal to that of a geosynchronous satellite Twhose orbit is not inclined, but since there are both
parallel and perpendicular components to the motion of Sit will appear to move north while “losing
ground” compared to T. Eventually, though, it must pass overhead again in 12 hours. When Sis as
far north as it will go (6 hours) it has a velocity which is parallel to T, but it is located in a region
where the required speed to appear fixed is slower. Hence, it will appear to be “gaining ground”
against the background stars. Consequently, the motion against the background stars appears to be
a figure 8.
P14-31 The net force of gravity on one star because of the other two is
F=2GM2
L2cos(30).
The stars orbit about a point r=L/2 cos(30) from any star. The orbital speed is then found from
Mv2
r=Mv2
L/2 cos(30)=2GM2
L2cos(30),
or v=pGM/L.
P14-32 A parabolic path will eventually escape; this means that the speed of the comet at any
distance is the escape speed for that distance, or v=p2GM/r. The angular momentum is constant,
and is equal to
l=mvArA=mp2GMrA.
For a parabolic path, r= 2rA/(1 + cos θ). Combining with Eq. 14-21 and the equation before that
one we get
dt =2GMrA
4rA2(1 + cos θ)2.
The time required is the integral
T=r8rA3
GM Zπ/2
0
(1 + cos θ)2=r8rA3
GM 2
3.
Note that prA3/GM is equal to 1/2πyears. Then the time for the comet to move is
T=1
2π82
3y = 0.300 y.
P14-33 There are three forces on loose matter (of mass m0) sitting on the moon: the force of
gravity toward the moon, Fm=Gmm0/a2,the force of gravity toward the planet, FM=GM m0/(r
a)2,and the normal force Nof the moon pushing the loose matter away from the center of the moon.
The net force on this loose matter is FM+NFm, this value is exactly equal to the centripetal
force necessary to keep the loose matter moving in a uniform circle. The period of revolution of the
loose matter is identical to that of the moon,
T= 2πpr3/GM,
but since the loose matter is actually revolving at a radial distance rathe centripetal force is
Fc=4π2m0(ra)
T2=GMm0(ra)
r3.
182
Only if the normal force is zero can the loose matter can lift off, and this will happen when Fc=
FMFm, or
M(ra)
r3=M
(ra)2m
a2,
=Ma2m(ra)2
a2(ra)2,
Ma2(ra)3=Mr3a2mr3(ra)2,
3r2a3+ 3ra4a4=m
Mr5+ 2r4ar3a2
Let r=ax, then xis dimensionless; let β=m/M , then βis dimensionless. The expression then
simplifies to
3x2+ 3x1 = β(x5+ 2x4x3).
If we assume than xis very large (ra) then only the largest term on each side survives. This means
3x2βx5,or x= (3)1/3.In that case, r=a(3M/m)1/3. For the Earth’s moon rc= 1.1×107m,
which is only 4,500 km away from the surface of the Earth. It is somewhat interesting to note that
the radius ris actually independent of both aand mif the moon has a uniform density!
183
E15-1 The pressure in the syringe is
p=(42.3 N)
π(1.12×102m/s)2= 4.29×105Pa.
E15-2 The total mass of fluid is
m= (0.5×103m3)(2600 kg/m3)+(0.25×103m3)(1000 kg/m3)+(0.4×103m3)(800 kg/m3) = 1.87 kg.
The weight is (18.7 kg)(9.8 m/s2) = 18 N.
E15-3 F=Ap, so
F= (3.43 m)(2.08 m)(1.00 atm 0.962 atm)(1.01×105Pa/atm) = 2.74×104N.
E15-4 BV/V =p;V=L3; ∆VL2L/3. Then
p= (140×109Pa)(5×103m)
3(0.85 m) = 2.74×109Pa.
E15-5 There is an inward force F1pushing the lid closed from the pressure of the air outside the
box; there is an outward force F2pushing the lid open from the pressure of the air inside the box.
To lift the lid we need to exert an additional outward force F3to get a net force of zero.
The magnitude of the inward force is F1=PoutA, where Ais the area of the lid and Pout is
the pressure outside the box. The magnitude of the outward force F2is F2=PinA. We are told
F3= 108 lb. Combining,
F2=F1F3,
PinA=PoutAF3,
Pin =Pout F3/A,
so Pin = (15 lb/in2(108 lb)/(12 in2) = 6.0 lb/in2.
E15-6 h= ∆p/ρg, so
h=(0.05 atm)(1.01×105Pa/atm)
(1000 kg/m3)(9.8 m/s2)= 0.52 m.
E15-7 p= (1060 kg/m3)(9.81 m/s2)(1.83 m) = 1.90×104Pa.
E15-8 p= (1024 kg/m3)(9.81 m/s2)(118 m) = 1.19×106Pa.Add this to p0; the total pressure is
then 1.29×106Pa.
E15-9 The pressure differential assuming we don’t have a sewage pump:
p2p1=ρg (y2y1) = (926 kg/m3)(9.81 m/s2)(8.16 m 2.08 m) = 5.52×104Pa.
We need to overcome this pressure difference with the pump.
E15-10 (a) p= (1.00 atm)e5.00/8.55 = 0.557 atm.
(b) h= (8.55 km) ln(1.00/0.500) = 5.93 km.
184
E15-11 The mercury will rise a distance aon one side and fall a distance aon the other so that
the difference in mercury height will be 2a. Since the masses of the “excess” mercury and the water
will be proportional, we have 2m=w, so
a=(0.112m)(1000 kg/m3)
2(13600 kg/m3)= 4.12×103m.
E15-12 (a) The pressure (due to the water alone) at the bottom of the pool is
P= (62.45 lb/ft3)(8.0 ft) = 500 lb/ft2.
The force on the bottom is
F= (500 lb/ft2)(80 ft)(30 ft) = 1.2×106lb.
The average pressure on the side is half the pressure on the bottom, so
F= (250 lb/ft2)(80 ft)(8.0 ft) = 1.6×105lb.
The average pressure on the end is half the pressure on the bottom, so
F= (250 lb/ft2)(30 ft)(8.0 ft) = 6.0×104lb.
(b) No, since that additional pressure acts on both sides.
E15-13 (a) Equation 15-8 can be used to find the height y2of the atmosphere if the density is
constant. The pressure at the top of the atmosphere would be p2= 0, and the height of the bottom
y1would be zero. Then
y2= (1.01×105Pa)/(1.21 kg/m3)(9.81 m/s2)= 8.51×103m.
(b) We have to go back to Eq. 15-7 for an atmosphere which has a density which varies linearly
with altitude. Linear variation of density means
ρ=ρ01y
ymax
Substitute this into Eq. 15-7,
p2p1=Zymax
0
ρg dy,
=Zymax
0
ρ0g1y
ymax dy,
=ρ0gyy2
2ymax
ymax
0
,
=ρgymax/2.
In this case we have ymax = 2p1/(ρg), so the answer is twice that in part (a), or 17 km.
E15-14 P= (1000 kg/m3)(9.8 m/s2)(112 m) = 1.1×106Pa.The force required is then F=
(1.1×106Pa)(1.22 m)(0.590 m) = 7.9×105N.
185
E15-15 (a) Choose any infinitesimally small spherical region where equal volumes of the two fluids
are in contact. The denser fluid will have the larger mass. We can treat the system as being a sphere
of uniform mass with a hemisphere of additional mass being superimposed in the region of higher
density. The orientation of this hemisphere is the only variable when calculating the potential energy.
The center of mass of this hemisphere will be a low as possible only when the surface is horizontal.
So all two-fluid interfaces will be horizontal.
(b) If there exists a region where the interface is not horizontal then there will be two different
values for ∆p=ρgh, depending on the path taken. This means that there will be a horizontal
pressure gradient, and the fluid will flow along that gradient until the horizontal pressure gradient
is equalized.
E15-16 The mass of liquid originally in the first vessel is m1=ρAh1; the center of gravity is
at h1/2, so the potential energy of the liquid in the first vessel is originally U1=ρgAh2
1/2. A
similar expression exists for the liquid in the second vessel. Since the two vessels have the same
cross sectional area the final height in both containers will be hf= (h1+h2)/2. The final potential
energy of the liquid in each container will be Uf=ρgA(h1+h2)2/8. The work done by gravity is
then
W=U1+U22Uf,
=ρgA
42h2
1+ 2h2
2(h2
1+ 2h1h2+h2
2),
=ρgA
4(h1h2)2.
E15-17 There are three force on the block: gravity (W=mg), a buoyant force B0=mwg, and a
tension T0. When the container is at rest all three forces balance, so B0WT0= 0. The tension
in this case is T0= (mwm)g.
When the container accelerates upward we now have BWT=ma. Note that neither the
tension nor the buoyant force stay the same; the buoyant force increases according to B=mw(g+a).
The new tension is then
T=mw(g+a)mg ma = (mwm)(g+a) = T0(1 + a/g).
E15-18 (a) F1/d2
1=F2/d2
2, so
F2= (18.6 kN)(3.72 cm)2/(51.3 cm)2= 97.8 N.
(b) F2h2=F1h1, so
h2= (1.65 m)(18.6 kN)/(97.8 N) = 314 m.
E15-19 (a) 35.6 kN; the boat doesn’t get heavier or lighter just because it is in different water!
(b) Yes.
V=(35.6×103N)
(9.81 m/s2)1
(1024 kg/m3)1
(1000 kg/m3)=8.51×102m3.
E15-20 (a) ρ2=ρ1(V1/V2) = (1000 kg/m3)(0.646) = 646 kg/m3.
(b) ρ2=ρ1(V1/V2) = (646 kg/m3)(0.918)1= 704 kg/m3.
186
E15-21 The can has a volume of 1200 cm3, so it can displace that much water. This would
provide a buoyant force of
B=ρV g = (998kg/m3)(1200×106m3)(9.81 m/s2) = 11.7 N.
This force can then support a total mass of (11.7 N)/(9.81 m/s2) = 1.20 kg. If 130 g belong to the
can, then the can will be able to carry 1.07 kg of lead.
E15-22 ρ2=ρ1(V1/V2) = (0.98 g/cm3)(2/3)1= 1.47 g/cm3.
E15-23 Let the object have a mass m. The buoyant force of the air on the object is then
Bo=ρa
ρo
mg.
There is also a buoyant force on the brass, equal to
Bb=ρa
ρb
mg.
The fractional error in the weighing is then
BoBb
mg =(0.0012 g/cm3)
(3.4 g/cm3)(0.0012 g/cm3)
(8.0 g/cm3)= 2.0×104
E15-24 The volume of iron is
Vi= (6130 N)/(9.81 m/s2)(7870 kg/m3) = 7.94×102m3.
The buoyant force of water is 6130 N 3970 N = 2160 N. This corresponds to a volume of
Vw= (2160 N)/(9.81 m/s2)(1000 kg/m3) = 2.20×101m3.
The volume of air is then 2.20×101m37.94×102m3= 1.41×101m3.
E15-25 (a) The pressure on the top surface is p=p0+ρgL/2. The downward force is
Ft= (p0+ρgL/2)L2,
=(1.01×105Pa) + (944 kg/m3)(9.81 m/s2)(0.608 m)/2(0.608 m)2= 3.84×104N.
(b) The pressure on the bottom surface is p=p0+ 3ρgL/2. The upward force is
Fb= (p0+ 3ρgL/2)L2,
=(1.01×105Pa) + 3(944 kg/m3)(9.81 m/s2)(0.608 m)/2(0.608 m)2= 4.05×104N.
(c) The tension in the wire is given by T=W+FtFb, or
T= (4450 N) + (3.84×104N) (4.05×104N) = 2350 N.
(d) The buoyant force is
B=L3ρg = (0.6083)(944 kg/m3)(9.81 m/s2) = 2080 N.
187
E15-26 The fish has the (average) density of water if
ρw=mf
Vc+Va
or
Va=mf
ρwVc.
We want the fraction Va/(Vc+Va), so
Va
Vc+Va
= 1 ρw
Vc
mf
,
= 1 ρwc= 1 (1.024 g/cm3)/(1.08 g/cm3) = 5.19×102.
E15-27 There are three force on the dirigible: gravity (W=mgg), a buoyant force B=mag,
and a tension T. Since these forces must balance we have T=BW. The masses are related to
the densities, so we can write
T= (ρaρg)V g = (1.21 kg/m30.796 kg/m3)(1.17×106m3)(9.81m/s2) = 4.75×106N.
E15-28 m= ∆ρV , so
m= [(0.160 kg/m3)(0.0810 kg/m3)](5000 m3) = 395 kg.
E15-29 The volume of one log is π(1.05/2 ft)2(5.80 ft) = 5.02 ft3. The weight of the log is
(47.3 lb/ft3)(5.02 ft3) = 237 lb. Each log if completely submerged will displace a weight of wa-
ter (62.4 lb/ft3)(5.02 ft3) = 313 lb. So each log can support at most 313 lb 237 lb = 76 lb. The
three children have a total weight of 247 lb, so that will require 3.25 logs. Round up to four.
E15-30 (a) The ice will hold up the automobile if
ρw>ma+mi
Vi
=ma
At +ρi.
Then
A=(1120 kg)
(0.305 m)[(1000 kg/m3)(917 kg/m3)] = 44.2 m2.
E15-31 If there were no water vapor pressure above the barometer then the height of the water
would be y1=p/(ρg), where p=p0is the atmospheric pressure. If there is water vapor where there
should be a vacuum, then pis the difference, and we would have y2= (p0pv)/(ρg). The relative
error is
(y1y2)/y1= [p0/(ρg)(p0pv)/(ρg)] /[p0/(ρg)] ,
=pv/p0= (3169 Pa)/(1.01×105Pa) = 3.14 %.
E15-32 ρ= (1.01×105Pa)/(9.81 m/s2)(14 m) = 740 kg/m3.
E15-33 h= (90)(1.01×105Pa)/(8.60 m/s2)(1.36×104kg/m3) = 78 m.
E15-34 U= 2(4.5×102N/m)4π(2.1×102m)2= 5.0×104J.
188
E15-35 The force required is just the surface tension times the circumference of the circular
patch. Then
F= (0./072 N/m)2π(0.12 m) = 5.43×102N.
E15-36 U= 2(2.5×102N/m)4π(1.4×102m)2= 1.23×104J.
P15-1 (a) One can replace the two hemispheres with an open flat end with two hemispheres with
a closed flat end. Then the area of relevance is the area of the flat end, or πR2. The net force from
the pressure difference is ∆pA = ∆R2; this much force must be applied to pull the hemispheres
apart.
(b) F=π(0.9)(1.01×105Pa)(0.305 m)2= 2.6×104N.
P15-2 The pressure required is 4×109Pa. This will happen at a depth
h=(4×109Pa)
(9.8 m/s2)(3100 kg/m3)= 1.3×105m.
P15-3 (a) The resultant force on the wall will be
F=Z Z P dx dy,
=Z(ρgy)W dy,
=ρgD2W/2.
(b) The torque will is given by τ=F(Dy) (the distance is from the bottom) so if we generalize,
τ=Z Z P y dx dy,
=Z(ρg(Dy)) yW dy,
=ρgD3W/6.
(c) Dividing to find the location of the equivalent resultant force,
d=τ/F = (ρgD3W/6)/(ρgD2W/2) = D/3,
this distance being measured from the bottom.
P15-4 p=ρgy =ρg(3.6 m); the force on the bottom is F=pA =ρg(3.6 m)π(0.60 m)2= 1.296πρg.
The volume of liquid is
V= (1.8 m) π(0.60 m) + 4.6×104m2= 2.037 m3
The weight is W=ρg(2.037 m3). The ratio is 2.000.
P15-5 The pressure at bis ρc(3.2×104m)+ρmy. The pressure at ais ρc(3.8×104m+d)+ρm(yd).
Set these quantities equal to each other:
ρc(3.8×104m + d) + ρm(yd) = ρc(3.2×104m) + ρmy,
ρc(6×103m + d) = ρmd,
d=ρc(6×103m)/(ρmρc),
= (2900 kg/m3)(6×103m)/(400 kg/m3) = 4.35×104m.
189
P15-6 (a) The pressure (difference) at a depth yis ∆p=ρgy. Since ρ=m/V , then
ρ≈ −m
V
V
V=ρs
p
B.
Then
ρρs+ ∆ρ=ρs+ρs2gy
B.
(b) ∆ρ/ρs=ρsgy/B, so
ρ/ρ (1000 kg/m3)(9.8 m/s2)(4200 m)/(2.2×109Pa) = 1.9 %.
P15-7 (a) Use Eq. 15-10, p= (p00)ρ, then Eq. 15-13 will look like
(p00)ρ= (p00)ρ0eh/a.
(b) The upward velocity of the rocket as a function of time is given by v=art. The height of
the rocket above the ground is given by y=1
2art2. Combining,
v=arr2y
ar
=p2yar.
Put this into the expression for drag, along with the equation for density variation with altitude;
D=CAρv2=C0ey/a2yar.
Now take the derivative with respect to y,
dD/dy = (1/a)C0ey/a(2yar) + C0ey/a(2ar).
This will vanish when y=a, regardless of the acceleration ar.
P15-8 (a) Consider a slice of cross section Aa depth hbeneath the surface. The net force on the
fluid above the slice will be
Fnet =ma =ρhAg,
Since the weight of the fluid above the slice is
W=mg =ρhAg,
then the upward force on the bottom of the fluid at the slice must be
W+Fnet =ρhA(g+a),
so the pressure is p=F/A =ρh(g+a).
(b) Change ato a.
(c) The pressure is zero (ignores atmospheric contributions.)
P15-9 (a) Consider a portion of the liquid at the surface. The net force on this portion is ~
F=
m~
a=maˆ
i. The force of gravity on this portion is ~
W=mgˆ
j. There must then be a buoyant
force on the portion with direction ~
B=~
F~
W=m(aˆ
i+gˆ
j). The buoyant force makes an angle
θ= arctan(a/g) with the vertical. The buoyant force must be perpendicular to the surface of the
fluid; there are no pressure-related forces which are parallel to the surface. Consequently, the surface
must make an angle θ= arctan(a/g) with the horizontal.
(b) It will still vary as ρgh; the derivation on page 334 is still valid for vertical displacements.
190
P15-10 dp/dr =ρg, but now g= 4πGρr/3. Then
Zp
0
dp =4
3π2Zr
R
r dr,
p=2
3π2R2r2.
P15-11 We can start with Eq. 15-11, except that we’ll write our distance in terms of rinstead
if y. Into this we can substitute our expression for g,
g=g0
R2
r2.
Substituting, then integrating,
dp
p=gρ0
p0
dr,
dp
p=g0ρ0R2
p0
dr
r2,
Zp
p0
dp
p=Zr
R
g0ρ0R2
p0
dr
r2,
ln p
p0
=g0ρ0R2
p01
r1
R
If k=g0ρ0R2/p0, then
p=p0ek(1/r1/R).
P15-12 (a) The net force on a small volume of the fluid is dF =rω2dm directed toward the
center. For radial displacements, then, dF/dr =rω2dm/dr or dp/dr =2ρ.
(b) Integrating outward,
p=pc+Zr
0
ρω2r dr =pc+1
2ρr2ω2.
(c) Do part (d) first.
(d) It will still vary as ρgh; the derivation on page 334 is still valid for vertical displacements.
(c) The pressure anywhere in the liquid is then given by
p=p0+1
2ρr2ω2ρgy,
where p0is the pressure on the surface, yis measured from the bottom of the paraboloid, and ris
measured from the center. The surface is defined by p=p0, so
1
2ρr2ω2ρgy = 0,
or y=r2ω2/2g.
P15-13 The total mass of the shell is m=ρwπdo3/3, or it wouldn’t barely float. The mass of iron
in the shell is m=ρiπ(do3di3)/3, so
di3=ρiρw
ρi
do3,
so
di=3
s(7870 kg/m3)(1000 kg/m3)
(7870 kg/m3)(0.587 m) = 0.561 m.
191
P15-14 The wood will displace a volume of water equal to (3.67 kg)/(594 kg/m3)(0.883) = 5.45×
103m3in either case. That corresponds to a mass of (1000 kg/m3)(5.45×103m3)=5.45 kg that
can be supported.
(a) The mass of lead is 5.45 kg 3.67 kg = 1.78 kg.
(b) When the lead is submerged beneath the water it displaces water, which affects the “apparent”
mass of the lead. The true weight of the lead is mg, the buoyant force is (ρwl)mg, so the apparent
weight is (1 ρwl)mg. This means the apparent mass of the submerged lead is (1 ρwl)m.
This apparent mass is 1.78 kg, so the true mass is
m=(11400 kg/m3)
(11400 kg/m3)(1000 kg)(1.78 kg) = 1.95 kg.
P15-15 We initially have
1
4=ρo
ρmercury
.
When water is poured over the object the simple relation no longer works.
Once the water is over the object there are two buoyant forces: one from mercury, F1, and one
from the water, F2. Following a derivation which is similar to Sample Problem 15-3, we have
F1=ρ1V1gand F2=ρ2V2g
where ρ1is the density of mercury, V1the volume of the object which is in the mercury, ρ2is the
density of water, and V2is the volume of the object which is in the water. We also have
F1+F2=ρoVogand V1+V2=Vo
as expressions for the net force on the object (zero) and the total volume of the object. Combining
these four expressions,
ρ1V1+ρ2V2=ρoVo,
or
ρ1V1+ρ2(VoV1) = ρoVo,
(ρ1ρ2)V1= (ρoρ2)Vo,
V1
Vo
=ρoρ2
ρ1ρ2
.
The left hand side is the fraction that is submerged in the mercury, so we just need to substitute
our result for the density of the material from the beginning to solve the problem. The fraction
submerged after adding water is then
V1
Vo
=ρoρ2
ρ1ρ2
,
=ρ1/4ρ2
ρ1ρ2
,
=(13600 kg/m3)/4(998 kg/m3)
(13600 kg/m3)(998 kg/m3)= 0.191.
P15-16 (a) The car floats if it displaces a mass of water equal to the mass of the car. Then
V= (1820 kg)/(1000 kg/m3) = 1.82 m3.
(b) The car has a total volume of 4.87 m3+0.750 m3+0.810 m3= 6.43 m3. It will sink if the total
mass inside the car (car + water) is then (6.43 m3)(1000 kg/m3) = 6430 kg. So the mass of the water
in the car is 6430 kg1820 kg = 4610 kg when it sinks. That’s a volume of (4610 kg)/(1000 kg/m3) =
4.16 m3.
192
P15-17 When the beaker is half filled with water it has a total mass exactly equal to the maximum
amount of water it can displace. The total mass of the beaker is the mass of the beaker plus the
mass of the water inside the beaker. Then
ρw(mgg+Vb) = mg+ρwVb/2,
where mggis the volume of the glass which makes up the beaker. Rearrange,
ρg=mg
mgwVb/2=(0.390 kg)
(0.390 kg)/(1000 kg/m3)(5.00×104m3)/2= 2790 kg/m3.
P15-18 (a) If each atom is a cube then the cube has a side of length
l=3
p(6.64×1027 kg)/(145 kg/m3) = 3.58×1010m.
Then the atomic surface density is l2= (3.58×1010m)2= 7.8×1018/m2.
(b) The bond surface density is twice the atomic surface density. Show this by drawing a square
array of atoms and then joining each adjacent pair with a bond. You will need twice as many bonds
as there are atoms. Then the energy per bond is
(3.5×104N/m)
2(7.8×1018/m2)(1.6×1019 J/eV) = 1.4×104eV.
P15-19 Pretend the bubble consists of two hemispheres. The force from surface tension holding
the hemispheres together is F= 2γL = 4πrγ. The “extra” factor of two occurs because each
hemisphere has a circumference which “touches” the boundary that is held together by the surface
tension of the liquid. The pressure difference between the inside and outside is ∆p=F/A, where A
is the area of the flat side of one of the hemispheres, so ∆p= (4π)/(πr2) = 4γ/r.
P15-20 Use the results of Problem 15-19. To get a numerical answer you need to know the surface
tension; try γ= 2.5×102N/m. The initial pressure inside the bubble is pi=p0+ 4γ/ri. The final
pressure inside the bell jar is p=pf4γ/rf. The initial and final pressure inside the bubble are
related by piri3=pfrf3. Now for numbers:
pi= (1.00×105Pa) + 4(2.5×102N/m)/(1.0×103m) = 1.001×105Pa.
pf= (1.0×103m/1.0×102m)3(1.001×105Pa) = 1.001×102Pa.
p= (1.001×102Pa) 4(2.5×102N/m)/(1.0×102m) = 90.1Pa.
P15-21 The force on the liquid in the space between the rod and the cylinder is F=γL =
2πγ(R+r). This force can support a mass of water m=F/g. This mass has a volume V=m/ρ.
The cross sectional area is π(R2r2), so the height hto which the water rises is
h=2πγ(R+r)
ρgπ(R2r2)=2γ
ρg(Rr),
=2(72.8×103N/m)
(1000 kg/m3)(9.81 m/s2)(4.0×103m) = 3.71×103m.
193
P15-22 (a) Refer to Problem 15-19. The initial pressure difference is
4(2.6×102N/m)/(3.20×102m) = 3.25Pa.
(b) The final pressure difference is
4(2.6×102N/m)/(5.80×102m) = 1.79Pa.
(c) The work done against the atmosphere is pV, or
(1.01×105Pa)4π
3[(5.80×102m)3(3.20×102m)3] = 68.7 J.
(d) The work done in stretching the bubble surface is γA, or
(2.60×102N/m)4π[(5.80×102m)2(3.20×102m)2] = 7.65×104J.
194
E16-1 R=Av =πd2v/4 and V=Rt, so
t=4(1600 m3)
π(0.345 m)2(2.62 m) = 6530 s
E16-2 A1v1=A2v2,A1=πd2
1/4 for the hose, and A2=Nπd2
2for the sprinkler, where N= 24.
Then
v2=(0.75 in)2
(24)(0.050 in)2(3.5 ft/s) = 33 ft/s.
E16-3 We’ll assume that each river has a rectangular cross section, despite what the picture
implies. The cross section area of the two streams is then
A1= (8.2 m)(3.4 m) = 28 m2and A2= (6.8 m)(3.2 m) = 22 m2.
The volume flow rate in the first stream is
R1=A1v1= (28 m2)(2.3 m/s) = 64 m3/s,
while the volume flow rate in the second stream is
R2=A2v2= (22 m2)(2.6 m/s) = 57 m3/s.
The amount of fluid in the stream/river system is conserved, so
R3=R1+R2= (64 m3/s) + (57 m3/s) = 121 m3/s.
where R3is the volume flow rate in the river. Then
D3=R3/(v3W3) = (121 m3/s)/[(10.7 m)(2.9 m/s)] = 3.9 m.
E16-4 The speed of the water is originally zero so both the kinetic and potential energy is zero.
When it leaves the pipe at the top it has a kinetic energy of 1
2(5.30 m/s)2= 14.0 J/kg and a
potential energy of (9.81 m/s2)(2.90 m) = 28.4 J/kg. The water is flowing out at a volume rate of
R= (5.30 m/s)π(9.70×103m)2= 1.57×103m3/s. The mass rate is ρR = (1000 kg/m3)(1.57×
103m3/s) = 1.57 kg/s.
The power supplied by the pump is (42.8 J/kg)(1.57 kg/s) = 67.2W.
E16-5 There are 8500 km2which collects an average of (0.75)(0.48 m/y), where the 0.75 reflects
the fact that 1/4 of the water evaporates, so
R=8500(103m)2(0.75)(0.48 m/y) 1 y
365 ×24 ×60 ×60 s= 97 m3/s.
Then the speed of the water in the river is
v=R/A = (97 m3/s)/[(21 m)(4.3 m)] = 1.1 m/s.
E16-7 (a) ∆p=ρg(y1y2) + ρ(v2
1v2
2)/2. Then
p= (62.4 lb/ft3)(572 ft)+ [(62.4 lb/ft3)/(32 ft/s2)][(1.33 ft/s)2(31.0 ft/s)2]/2 = 3.48×104lb/ft2.
(b) A2v2=A1v1, so
A2= (7.60 ft2)(1.33 ft/s)/(31.0 ft/s) = 0.326 ft2.
195
E16-8 (a) A2v2=A1v1, so
v2= (2.76 m/s)[(0.255 m)2(0.0480 m)2]/(0.255 m)2= 2.66 m/s.
(b) ∆p=ρ(v2
1v2
2)/2,
p= (1000 kg/m3)[(2.66 m/s)2(2.76 m/s)2]/2 = 271 Pa
E16-9 (b) We will do part (b) first.
R= (100 m2)(1.6 m/y) 1 y
365 ×24 ×60 ×60 s= 5.1×106m3/s.
(b) The speed of the flow Rthrough a hole of cross sectional area awill be v=R/a.p=p0+ρgh,
where h= 2.0 m is the depth of the hole. Bernoulli’s equation can be applied to find the speed of
the water as it travels a horizontal stream line out the hole,
p0+1
2ρv2=p,
where we drop any terms which are either zero or the same on both sides. Then
v=p2(pp0)=p2gh =p2(9.81 m/s2)(2.0 m) = 6.3 m/s.
Finally, a= (5.1×106m3/s)/(6.3 m/s) = 8.1×107m2, or about 0.81 mm2.
E16-10 (a) v2= (A1/A2)v1= (4.20 cm2)(5.18 m/s)/(7.60 cm2) = 2.86 m/s.
(b) Use Bernoulli’s equation:
p2+ρgy2+1
2ρv2
2=p1+ρgy1+1
2ρv2
1.
Then
p2= (1.52×105Pa) + (1000 kg/m3)(9.81 m/s2)(9.66 m) + 1
2(5.18 m/s)21
2(2.86 m/s)2,
= 2.56×105Pa.
E16-11 (a) The wind speed is (110 km/h)(1000 m/km)/(3600 s/h) = 30.6 m/s. The pressure
difference is then
p=1
2(1.2 kg/m3)(30.6 m/s)2= 562 Pa.
(b) The lifting force would be F= (562 Pa)(93 m2) = 52000 N.
E16-12 The pressure difference is
p=1
2(1.23 kg/m3)(28.0 m/s)2= 482 N.
The net force is then F= (482 N)(4.26 m)(5.26 m) = 10800 N.
196
E16-13 The lower pipe has a radius r1= 2.52 cm, and a cross sectional area of A1=πr2
1. The
speed of the fluid flow at this point is v1. The higher pipe has a radius of r2= 6.14 cm, a cross
sectional area of A2=πr2
2, and a fluid speed of v2. Then
A1v1=A2v2or r2
1v1=r2
2v2.
Set y1= 0 for the lower pipe. The problem specifies that the pressures in the two pipes are the
same, so
p0+1
2ρv2
1+ρgy1=p0+1
2ρv2
2+ρgy2,
1
2v2
1=1
2v2
2+gy2,
We can combine the results of the equation of continuity with this and get
v2
1=v2
2+ 2gy2,
v2
1=v1r2
1/r2
22+ 2gy2,
v2
11r4
1/r4
2= 2gy2,
v2
1= 2gy2/1r4
1/r4
2.
Then
v2
1= 2(9.81 m/s2)(11.5 m)/1(0.0252 m)4/(0.0614 m)4= 232 m2/s2
The volume flow rate in the bottom (and top) pipe is
R=πr2
1v1=π(0.0252 m)2(15.2 m/s) = 0.0303 m3/s.
E16-14 (a) As instructed,
p0+1
2ρv2
1+ρgy1=p0+1
2ρv2
3+ρgy3,
0 = 1
2v2
3+g(y3y1),
But y3y1=h, so v3=2gh.
(b) habove the hole. Just reverse your streamline!
(c) It won’t come out as fast and it won’t rise as high.
E16-15 Sea level will be defined as y= 0, and at that point the fluid is assumed to be at rest.
Then
p0+1
2ρv2
1+ρgy1=p0+1
2ρv2
2+ρgy2,
0 = 1
2v2
2+gy2,
where y2=200 m. Then
v2=p2gy2=p2(9.81 m/s2)(200 m) = 63 m/s.
197
E16-16 Assume streamlined flow, then
p1+1
2ρv2
1+ρgy1=p2+1
2ρv2
2+ρgy2,
(p1p2)+g(y1y2) = 1
2v2
2.
Then upon rearranging
v2=p2 [(2.1)(1.01×105Pa)/(1000 kg/m3) + (9.81 m/s2)(53.0 m)] = 38.3 m/s.
E16-17 (a) Points 1 and 3 are both at atmospheric pressure, and both will move at the same
speed. But since they are at different heights, Bernoulli’s equation will be violated.
(b) The flow isn’t steady.
E16-18 The atmospheric pressure difference between the two sides will be ∆p=1
2ρav2.The height
difference in the U-tube is given by ∆p=ρwgh. Then
h=(1.20 kg/m3)(15.0 m/s)2
2(1000 kg/m3)(9.81 m/s2)= 1.38×102m.
E16-19 (a) There are three forces on the plug. The force from the pressure of the water, F1=
P1A, the force from the pressure of the air, F2=P2A, and the force of friction, F3. These three
forces must balance, so F3=F1F2,or F3=P1AP2A. But P1P2is the pressure difference
between the surface and the water 6.15 m below the surface, so
F3= ∆P A =ρgyA,
=(998 kg/m3)(9.81 m/s2)(6.15 m)π(0.0215 m)2,
= 87.4 N
(b) To find the volume of water which flows out in three hours we need to know the volume
flow rate, and for that we need both the cross section area of the hole and the speed of the flow.
The speed of the flow can be found by an application of Bernoulli’s equation. We’ll consider the
horizontal motion only— a point just inside the hole, and a point just outside the hole. These points
are at the same level, so
p1+1
2ρv2
1+ρgy1=p2+1
2ρv2
2+ρgy2,
p1=p2+1
2ρv2
2.
Combine this with the results of Pascal’s principle above, and
v2=p2(p1p2)=p2gy =p2(9.81 m/s2)(6.15 m) = 11.0 m/s.
The volume of water which flows out in three hours is
V=Rt = (11.0 m/s)π(0.0215 m)2(3 ×3600 s) = 173 m3.
E16-20 Apply Eq. 16-12:
v1=p2(9.81 m/s2)(0.262 m)(810 kg/m3)/(1.03 kg/m3) = 63.6 m/s.
198
E16-21 We’ll assume that the central column of air down the pipe exerts minimal force on the
card when it is deflected to the sides. Then
p1+1
2ρv2
1+ρgy1=p2+1
2ρv2
2+ρgy2,
p1=p2+1
2ρv2
2.
The resultant upward force on the card is the area of the card times the pressure difference, or
F= (p1p2)A=1
2ρAv2.
E16-22 If the air blows uniformly over the surface of the plate then there can be no torque about
any axis through the center of mass of the plate. Since the weight also doesn’t introduce a torque,
then the hinge can’t exert a force on the plate, because any such force would produce an unbalanced
torque. Consequently mg = ∆pA. ∆p=ρv2/2, so
v=r2mg
ρA =s2(0.488 kg)(9.81 m/s2)
(1.21 kg/m3)(9.10×102m)2= 30.9 m/s.
E16-23 Consider a streamline which passes above the wing and a streamline which passes beneath
the wing. Far from the wing the two streamlines are close together, move with zero relative velocity,
and are effectively at the same pressure. So we can pretend they are actually one streamline. Then,
since the altitude difference between the two points above and below the wing (on this new, single
streamline) is so small we can write
p=1
2ρ(vt2vu2)
The lift force is then
L= ∆p A =1
2ρA(vt2vu2)
E16-24 (a) From Exercise 16-23,
L=1
2(1.17 kg/m3)(2)(12.5 m2)(49.8 m/s)2(38.2 m/s)2= 1.49×104N.
The mass of the plane must be m=L/g = (1.49×104N)/(9.81 m/s2) = 1520 kg.
(b) The lift is directed straight up.
(c) The lift is directed 15off the vertical toward the rear of the plane.
(d) The lift is directed 15off the vertical toward the front of the plane.
E16-25 The larger pipe has a radius r1= 12.7 cm, and a cross sectional area of A1=πr2
1. The
speed of the fluid flow at this point is v1. The smaller pipe has a radius of r2= 5.65 cm, a cross
sectional area of A2=πr2
2, and a fluid speed of v2. Then
A1v1=A2v2or r2
1v1=r2
2v2.
Now Bernoulli’s equation. The two pipes are at the same level, so y1=y2. Then
p1+1
2ρv2
1+ρgy1=p2+1
2ρv2
2+ρgy2,
p1+1
2ρv2
1=p2+1
2ρv2
2.
199
Combining this with the results from the equation of continuity,
p1+1
2ρv2
1=p2+1
2ρv2
2,
v2
1=v2
2+2
ρ(p2p1),
v2
1=v1
r2
1
r2
22
+2
ρ(p2p1),
v2
11r4
1
r4
2=2
ρ(p2p1),
v2
1=2 (p2p1)
ρ(1 r4
1/r4
2).
It may look a mess, but we can solve it to find v1,
v1=s2(32.6×103Pa 57.1×103Pa)
(998 kg/m3)(1 (0.127 m)4/(0.0565 m)4)= 1.41 m/s.
The volume flow rate is then
R=Av =π(0.127 m)2(1.41 m/s) = 7.14×103m3/s.
That’s about 71 liters/second.
E16-26 The lines are parallel and equally spaced, so the velocity is everywhere the same. We can
transform to a reference frame where the liquid appears to be at rest, so the Pascal’s equation would
apply, and p+ρgy would be a constant. Hence,
p0=p+ρgy +1
2ρv2
is the same for all streamlines.
E16-27 (a) The “particles” of fluid in a whirlpool would obey conservation of angular momentum,
meaning a particle off mass mwould have l=mvr be constant, so the speed of the fluid as a function
of radial distance from the center would be given by k=vr, where kis some constant representing
the angular momentum per mass. Then v=k/r.
(b) Since v=k/r and v= 2πr/T , the period would be Tr2.
(c) Kepler’s third law says Tr3/2.
E16-28 Rc= 2000. Then
v < Rcη
ρD =(2000)(4.0×103N·s/m2)
(1060 kg/m3)(3.8×103m) = 2.0 m/s.
E16-29 (a) The volume flux is given; from that we can find the average speed of the fluid in the
pipe.
v=5.35 ×102L/min
π(1.88 cm)2= 4.81×103L/cm2·min.
But 1 L is the same as 1000 cm3and 1 min is equal to 60 seconds, so v= 8.03×104m/s.
200
Reynold’s number from Eq. 16-22 is then
R=ρDv
η=(13600 kg/m3)(0.0376 m)(8.03×104m/s)
(1.55 ×103N·s/m2)= 265.
This is well below the critical value of 2000.
(b) Poiseuille’s Law, Eq. 16-20, can be used to find the pressure difference between the ends of
the pipe. But first, note that the mass flux dm/dt is equal to the volume rate times the density
when the density is constant. Then ρ dV /dt =dm/dt, and Poiseuille’s Law can be written as
δp =8ηL
πR4
dV
dt =8(1.55 ×103N·s/m2)(1.26 m)
π(1.88×102m)4(8.92×107m3/s) = 0.0355 Pa.
P16-1 The volume of water which needs to flow out of the bay is
V= (6100 m)(5200 m)(3 m) = 9.5×107m3
during a 6.25 hour (22500 s) period. The average speed through the channel must be
v=(9.5×107m3)
(22500 s)(190 m)(6.5 m) = 3.4 m/s.
P16-2 (a) The speed of the fluid through either hole is v=2gh. The mass flux through a hole
is Q=ρAv, so ρ1A1=ρ2A2. Then ρ12=A2/A1= 2.
(b) R=Av, so R1/R2=A1/A2= 1/2.
(c) If R1=R2then A12gh1=A22gh2. Then
h2/h1= (A1/A2)2= (1/2)2= 1/4.
So h2=h1/4.
P16-3 (a) Apply Torricelli’s law (Exercise 16-14): v=2gh. The speed vis a horizontal velocity,
and serves as the initial horizontal velocity of the fluid “projectile” after it leaves the tank. There
is no initial vertical velocity.
This fluid “projectile” falls through a vertical distance Hhbefore splashing against the ground.
The equation governing the time tfor it to fall is
(Hh) = 1
2gt2,
Solve this for the time, and t=p2(Hh)/g. The equation which governs the horizontal distance
traveled during the fall is x=vxt, but vx=vand we just found t, so
x=vxt=p2gh p2(Hh)/g = 2ph(Hh).
(b) How many values of hwill lead to a distance of x? We need to invert the expression, and
we’ll start by squaring both sides
x2= 4h(Hh) = 4hH 4h2,
and then solving the resulting quadratic expression for h,
h=4H±16H216x2
8=1
2H±pH2x2.
201
For values of xbetween 0 and Hthere are two real solutions, if x=Hthere is one real solution,
and if x > H there are no real solutions.
If h1is a solution, then we can write h1= (H+ ∆)/2, where ∆ = 2h1Hcould be positive or
negative. Then h2= (H+ ∆)/2 is also a solution, and
h2= (H+ 2h12H)/2 = h1H
is also a solution.
(c) The farthest distance is x=H, and this happens when h=H/2, as we can see from the
previous section.
P16-4 (a) Apply Torricelli’s law (Exercise 16-14): v=p2g(d+h2), assuming that the liquid
remains in contact with the walls of the tube until it exits at the bottom.
(b) The speed of the fluid in the tube is everywhere the same. Then the pressure difference at
various points are only functions of height. The fluid exits at C, and assuming that it remains in
contact with the walls of the tube the pressure difference is given by ∆p=ρ(h1+d+h2), so the
pressure at Bis
p=p0ρ(h1+d+h2).
(c) The lowest possible pressure at Bis zero. Assume the flow rate is so slow that Pascal’s
principle applies. Then the maximum height is given by 0 = p0+ρgh1, or
h1= (1.01×105Pa)/[(9.81 m/s2)(1000 kg/m3)] = 10.3 m.
P16-5 (a) The momentum per kilogram of the fluid in the smaller pipe is v1. The momentum per
kilogram of the fluid in the larger pipe is v2. The change in momentum per kilogram is v2v1.
There are ρa2v2kilograms per second flowing past any point, so the change in momentum per
second is ρa2v2(v2v1). The change in momentum per second is related to the net force according
to F= ∆p/t, so F=ρa2v2(v2v1). But Fp/a2, so p1p2ρv2(v2v1).
(b) Applying the streamline equation,
p1+ρgy1+1
2ρv2
1=p2+ρgy2+1
2ρv2
2,
1
2ρ(v2
1v2
2) = p2p1
(c) This question asks for the loss of pressure beyond that which would occur from a gradually
widened pipe. Then we want
p=1
2ρ(v2
1v2
2)ρv2(v1v2),
=1
2ρ(v2
1v2
2)ρv2v1+ρv2
2,
=1
2ρ(v2
12v1v2+v2
2) = 1
2ρ(v1v2)2.
P16-6 The juice leaves the jug with a speed v=2gy, where yis the height of the juice in the
jug. If Ais the cross sectional area of the base of the jug and athe cross sectional area of the hole,
then the juice flows out the hole with a rate dV /dt =va =a2gy, which means the level of jug
varies as dy/dt =(a/A)2gy. Rearrange and integrate,
Zh
y
dy/y=Zt
0p2g(a/A)dt,
202
2(hy) = p2gat/A.
A
as2h
g!(hy) = t
When y= 14h/15 we have t= 12.0 s. Then the part in the parenthesis on the left is 3.539×102s.
The time to empty completely is then 354 seconds, or 5 minutes and 54 seconds. But we want the
remaining time, which is 12 seconds less than this.
P16-7 The greatest possible value for vwill be the value over the wing which results in an air
pressure of zero. If the air at the leading edge is stagnant (not moving) and has a pressure of p0,
then Bernoulli’s equation gives
p0=1
2ρv2,
or v=p2p0=p2(1.01×105Pa)/(1.2 kg/m3) = 410 m/s. This value is only slightly larger than
the speed of sound; they are related because sound waves involve the movement of air particles which
“shove” other air particles out of the way.
P16-8 Bernoulli’s equation together with continuity gives
p1+1
2ρv2
1=p2+1
2ρv2
2,
p1p2=1
2ρv2
2v2
1,
=1
2ρA2
1
A2
2
v2
1v2
1,
=v2
1
2A2
2
ρA2
1A2
2.
But p1p2= (ρ0ρ)gh. Note that we are not assuming ρis negligible compared to ρ0. Combining,
v1=A2s2(ρ0ρ)gh
ρ(A2
1A2
2).
P16-9 (a) Bernoulli’s equation together with continuity gives
p1+1
2ρv2
1=p2+1
2ρv2
2,
p1=1
2ρv2
2v2
1,
=1
2ρA2
1
A2
2
v2
1v2
1,
=v2
1ρ(4.75)21/2.
Then
v1=s2(2.12)(1.01×105Pa)
(1000 kg/m3)(21.6) = 4.45 m/s,
and then v2= (4.75)(4.45 m/s) = 21.2 m/s.
(b) R=π(2.60×102m)2(4.45 m/s) = 9.45×103m3/s.
203
P16-10 (a) For Fig. 16-13 the velocity is constant, or ~
v=vˆ
i.d~
s=ˆ
idx +ˆ
jdy. Then
I~
v·d~
s=vIdx = 0,
because Hdx = 0.
(b) For Fig. 16-16 the velocity is ~
v= (k/r)ˆ
r.d~
s=ˆ
rdr +ˆ
θr dφ. Then
I~
v·d~
s=vIdr = 0,
because Hdr = 0.
P16-11 (a) For an element of the fluid of mass dm the net force as it moves around the circle is
dF = (v2/r)dm.dm/dV =ρand dV =A dr and dF/A =dp. Then dp/dr =ρv2/r.
(b) From Bernoulli’s equation p+ρv2/2 is a constant. Then
dp
dr +ρv dv
dr = 0,
or v/r +dv/dr = 0, or d(vr) = 0. Consequently vr is a constant.
(c) The velocity is ~
v= (k/r)ˆ
r.d~
s=ˆ
rdr +ˆ
θr dφ. Then
I~
v·d~
s=vIdr = 0,
because Hdr = 0. This means the flow is irrotational.
P16-12 F/A =ηv/D, so
F/A = (4.0×1019N·s/m2)(0.048 m/3.16×107s)/(1.9×105m) = 3.2×105Pa.
P16-13 A flow will be irrotational if and only if H~
v·d~
s= 0 for all possible paths. It is fairly
easy to construct a rectangular path which is parallel to the flow on the top and bottom sides, but
perpendicular on the left and right sides. Then only the top and bottom paths contribute to the
integral. ~
vis constant for either path (but not the same), so the magnitude vwill come out of the
integral sign. Since the lengths of the two paths are the same but vis different the two terms don’t
cancel, so the flow is not irrotational.
P16-14 (a) The area of a cylinder is A= 2πrL. The velocity gradient if dv/dr. Then the retarding
force on the cylinder is F=η(2πrL)dv/dr.
(b) The force pushing a cylinder through is F0=Ap=πr2p.
(c) Equate, rearrange, and integrate:
πr2p=η(2πrL)dv
dr ,
pZR
r
r dr = 2ηL Zv
0
dv,
p1
2(R2r2)=2ηLv.
Then
v=p
4ηL(R2r2).
204
P16-15 The volume flux (called Rfto distinguish it from the radius R) through an annular ring
of radius rand width δr is
δRf=δA v = 2πr δr v,
where vis a function of rgiven by Eq. 16-18. The mass flux is the volume flux times the density,
so the total mass flux is
dm
dt =ρZR
0
δRf
δr dr,
=ρZR
0
2πr p
4ηL(R2r2)dr,
=πρp
2ηL ZR
0
(rR2r3)dr,
=πρp
2ηL (R4/2R4/4),
=πρpR4
8ηL .
P16-16 The pressure difference in the tube is ∆p= 4γ/r, where ris the (changing) radius of the
bubble. The mass flux through the tube is
dm
dt =4ρπR4γ
8ηLr ,
Ris the radius of the tube. dm =ρdV , and dV = 4πr2dr. Then
Zr2
r1
r3dr =Zt
0
R4γ
8ηL dt,
r4
1r4
2=ρR4γ
2ηL t,
Then
t=2(1.80×105N·s/m2)(0.112 m)
(0.54×103m)4(2.50×102N/m) [(38.2×103m)4(21.6×103m)4] = 3630 s.
205
E17-1 For a perfect spring |F|=k|x|.x= 0.157 m when 3.94 kg is suspended from it. There
would be two forces on the object— the force of gravity, W=mg, and the force of the spring, F.
These two force must balance, so mg =kx or
k=mg
x=(3.94 kg)(9.81 m/s2)
(0.157 m) = 0.246 N/m.
Now that we know k, the spring constant, we can find the period of oscillations from Eq. 17-8,
T= 2πrm
k= 2πs(0.520 kg)
(0.246 N/m) = 0.289 s.
E17-2 (a) T= 0.484 s.
(b) f= 1/T = 1/(0.484 s) = 2.07 s1.
(c) ω= 2πf = 13.0 rad/s.
(d) k=2= (0.512 kg)(13.0 rad/s)2= 86.5 N/m.
(e) vm=ωxm= (13.0 rad/s)(0.347 m) = 4.51 m/s.
(f) Fm=mam= (0.512 kg)(13.0 rad/s)2(0.347 m) = 30.0 N.
E17-3 am= (2πf)2xm. Then
f=p(9.81 m/s2)/(1.20×106m)/(2π) = 455 Hz.
E17-4 (a) ω= (2π)/(0.645 s) = 9.74 rad/s. k=2= (5.22 kg)(9.74 rad/s)2= 495 N/m.
(b) xm=vm= (0.153 m/s)/(9.74 rad/s) = 1.57×102m.
(c) f= 1/(0.645 s) = 1.55 Hz.
E17-5 (a) The amplitude is half of the distance between the extremes of the motion, so A= (2.00
mm)/2 = 1.00 mm.
(b) The maximum blade speed is given by vm=ωxm. The blade oscillates with a frequency of
120 Hz, so ω= 2πf = 2π(120 s1) = 754 rad/s,and then vm= (754 rad/s)(0.001 m) = 0.754 m/s.
(c) Similarly, am=ω2xm,am= (754 rad/s)2(0.001 m) = 568 m/s2.
E17-6 (a) k=2= (1460 kg/4)(2π2.95/s)2= 1.25×105N/m
(b) f=pk/m/2π=p(1.25×105N/m)/(1830 kg/4)/2π= 2.63/s.
E17-7 (a) x= (6.12 m) cos[(8.38 rad/s)(1.90 s) + 1.92 rad] = 3.27 m.
(b) v=(6.12 m)(8.38/s) sin[(8.38 rad/s)(1.90 s) + 1.92 rad] = 43.4 m/s.
(c) a=(6.12 m)(8.38/s)2cos[(8.38 rad/s)(1.90 s) + 1.92 rad] = 229 m/s2.
(d) f= (8.38 rad/s)/2π= 1.33/s.
(e) T= 1/f = 0.750 s.
E17-8 k= (50.0 lb)/(4.00 in) = 12.5 lb/in.
mg =(32 ft/s2)(12 in/ft)(12.5 lb/in)
[2π(2.00/s)]2= 30.4 lb.
E17-9 If the drive wheel rotates at 193 rev/min then
ω= (193 rev/min)(2πrad/rev)(1/60 s/min) = 20.2 rad/s,
then vm=ωxm= (20.2 rad/s)(0.3825 m) = 7.73 m/s.
206
E17-10 k= (0.325 kg)(9.81 m/s2)/(1.80×102m) = 177 N/m..
T= 2πrm
k= 2πs(2.14 kg)
(177 N/m) = 0.691 s.
E17-11 For the tides ω= 2π/(12.5 h). Half the maximum occurs when cos ωt = 1/2, or ωt =π/3.
Then t= (12.5 h)/6 = 2.08 h.
E17-12 The two will separate if the (maximum) acceleration exceeds g.
(a) Since ω= 2π/T = 2π/(1.18 s) = 5.32 rad/s the maximum amplitude is
xm= (9.81 m/s2)/(5.32 rad/s)2= 0.347 m.
(b) In this case ω=p(9.81 m/s2)/(0.0512 m) = 13.8 rad/s.Then f= (13.8 rad/s)/2π= 2.20/s.
E17-13 (a) ax/x =ω2. Then
ω=p(123 m/s)/(0.112 m) = 33.1 rad/s,
so f= (33.1 rad/s)/2π= 5.27/s.
(b) m=k2= (456 N/m)/(33.1 rad/s)2= 0.416 kg.
(c) x=xmcos ωt;v=xmωsin ωt. Combining,
x2+ (v)2=xm2cos2ωt +xm2sin2ωt =xm2.
Consequently,
xm=p(0.112 m)2+ (13.6 m/s)2/(33.1 rad/s)2= 0.426 m.
E17-14 x1=xmcos ωt,x2=xmcos(ωt +φ). The crossing happens when x1=xm/2, or when
ωt =π/3 (and other values!). The same constraint happens for x2, except that it is moving in the
other direction. The closest value is ωt +φ= 2π/3, or φ=π/3.
E17-15 (a) The net force on the three cars is zero before the cable breaks. There are three forces
on the cars: the weight, W, a normal force, N, and the upward force from the cable, F. Then
F=Wsin θ= 3mg sin θ.
This force is from the elastic properties of the cable, so
k=F
x=3mg sin θ
x
The frequency of oscillation of the remaining two cars after the bottom car is released is
f=1
2πrk
2m=1
2πr3mg sin θ
2mx =1
2πr3gsin θ
2x.
Numerically, the frequency is
f=1
2πr3gsin θ
2x=1
2πs3(9.81 m/s2) sin(26)
2(0.142 m) = 1.07 Hz.
(b) Each car contributes equally to the stretching of the cable, so one car causes the cable to
stretch 14.2/3 = 4.73 cm. The amplitude is then 4.73 cm.
207
E17-16 Let the height of one side over the equilibrium position be x. The net restoring force
on the liquid is 2ρAxg, where Ais the cross sectional area of the tube and gis the acceleration of
free-fall. This corresponds to a spring constant of k= 2ρAg. The mass of the fluid is m=ρAL.
The period of oscillation is
T= 2πrm
k=πs2L
g.
E17-17 (a) There are two forces on the log. The weight, W=mg, and the buoyant force B.
We’ll assume the log is cylindrical. If xis the length of the log beneath the surface and Athe
cross sectional area of the log, then V=Ax is the volume of the displaced water. Furthermore,
mw=ρwVis the mass of the displaced water and B=mwgis then the buoyant force on the log.
Combining,
B=ρwAgx,
where ρwis the density of water. This certainly looks similar to an elastic spring force law, with
k=ρwAg. We would then expect the motion to be simple harmonic.
(b) The period of the oscillation would be
T= 2πrm
k= 2πrm
ρwAg ,
where mis the total mass of the log and lead. We are told the log is in equilibrium when x=L= 2.56
m. This would give us the weight of the log, since W=Bis the condition for the log to float. Then
m=B
g=ρwAgL
g=ρAL.
From this we can write the period of the motion as
T= 2πsρAL
ρwAg = 2πpL/g = 2πs(2.56 m)
(9.81 m/s2)= 3.21 s.
E17-18 (a) k= 2(1.18 J)/(0.0984 m)2= 244 N/m.
(b) m= 2(1.18 J)/(1.22 m/s)2= 1.59 kg.
(c) f= [(1.22 m/s)/(0.0984 m)]/(2π) = 1.97/s.
E17-19 (a) Equate the kinetic energy of the object just after it leaves the slingshot with the
potential energy of the stretched slingshot.
k=mv2
x2=(0.130 kg)(11.2×103m/s)2
(1.53 m)2= 6.97×106N/m.
(b) N= (6.97×106N/m)(1.53 m)/(220 N) = 4.85×104people.
E17-20 (a) E=kxm2/2, U=kx2/2 = k(xm/2)2/2 = E/4. K=EU= 3E/4. The the energy
is 25% potential and 75% kinetic.
(b) If U=E/2 then kx2/2 = kxm2/4, or x=xm/2.
208
E17-21 (a) am=ω2xmso
ω=ram
xm
=s(7.93 ×103m/s2)
(1.86 ×103m) = 2.06 ×103rad/s
The period of the motion is then
T=2π
ω= 3.05×103s.
(b) The maximum speed of the particle is found by
vm=ωxm= (2.06 ×103rad/s)(1.86 ×103m) = 3.83 m/s.
(c) The mechanical energy is given by Eq. 17-15, except that we will focus on when vx=vm,
because then x= 0 and
E=1
2mvm2=1
2(12.3 kg)(3.83 m/s)2= 90.2 J.
E17-22 (a) f=pk/m/2π=p(988 N/m)/(5.13 kg)/2π= 2.21/s.
(b) Ui= (988 N/m)(0.535 m)2/2 = 141 J.
(c) Ki= (5.13 kg)(11.2 m/s)2/2 = 322 J.
(d) xm=p2E/k =p2(322 J + 141 J)/(988 N/m) = 0.968 m.
E17-23 (a) ω=p(538 N/m)/(1.26 kg) = 20.7 rad/s.
xm=p(0.263 m)2+ (3.72 m/s)2/(20.7 rad/s)2= 0.319 m.
(b) φ= arctan {−(3.72 m/s)/[(20.7 rad/s)(0.263 m)]}= 34.3.
E17-24 Before doing anything else apply conservation of momentum. If v0is the speed of the
bullet just before hitting the block and v1is the speed of the bullet/block system just after the two
begin moving as one, then v1=mv0/(m+M), where mis the mass of the bullet and Mis the mass
of the block.
For this system ω=pk/(m+M).
(a) The total energy of the oscillation is 1
2(m+M)v2
1, so the amplitude is
xm=rm+M
kv1=rm+M
k
mv0
m+M=mv0s1
k(m+M).
The numerical value is
xm= (0.050 kg)(150 m/s)s1
(500 N/m)(0.050 kg + 4.00 kg) = 0.167 m.
(b) The fraction of the energy is
(m+M)v2
1
mv2
0
=m+M
mm
m+M2
=m
m+M=(0.050 kg)
(0.050 kg + 4.00 kg) = 1.23×102.
E17-25 L= (9.82 m/s2)(1.00 s/2π)2= 0.249 m.
209
E17-26 T= (180 s)/(72.0). Then
g=2π(72.0)
(180 s) 2
(1.53 m) = 9.66 m/s.
E17-27 We are interested in the value of θmwhich will make the second term 2% of the first
term. We want to solve
0.02 = 1
22sin2θm
2,
which has solution
sin θm
2=0.08
or θm= 33.
(b) How large is the third term at this angle?
32
2242sin4θm
2=32
221
22sin2θm
22
=9
4(0.02)2
or 0.0009, which is very small.
E17-28 Since Tp1/g we have
Tp=Teqge/gp= (1.00 s)s(9.78 m/s2)
(9.834 m/s2)= 0.997 s.
E17-29 Let the period of the clock in Paris be T1. In a day of length D1= 24 hours it will
undergo n=D/T1oscillations. In Cayenne the period is T2.noscillations should occur in 24 hours,
but since the clock runs slow, D2is 24 hours + 2.5 minutes elapse. So
T2=D2/n = (D2/D1)T1= [(1442.5 min)/(1440.0 min)]T1= 1.0017T1.
Since the ratio of the periods is (T2/T1) = p(g1/g2), the g2in Cayenne is
g2=g1(T1/T2)2= (9.81 m/s2)/(1.0017)2= 9.78 m/s2.
E17-30 (a) Take the differential of
g=2π(100)
T2
(10 m) = 4π2×105m
T2,
so δg = (8π2×105m/T 3)δT . Note that Tis not the period here, it is the time for 100 oscillations!
The relative error is then δg
g=2δT
T.
If δg/g = 0.1% then δT/T = 0.05%.
(b) For g10 m/s2we have
T2π(100)p(10 m)/(10 m/s2) = 628 s.
Then δT (0.0005)(987 s) 300 ms.
210
E17-31 T= 2πp(17.3 m)/(9.81 m/s2) = 8.34 s.
E17-32 The spring will extend until the force from the spring balances the weight, or when Mg =
kh. The frequency of this system is then
f=1
2πrk
M=1
2πrMg/h
M=1
2πrg
h,
which is the frequency of a pendulum of length h. The mass of the bob is irrelevant.
E17-33 The frequency of oscillation is
f=1
2πrMgd
I,
where dis the distance from the pivot about which the hoop oscillates and the center of mass of the
hoop.
The rotational inertia Iis about an axis through the pivot, so we apply the parallel axis theorem.
Then
I=Md2+Icm =Md2+Mr2.
But dis r, since the pivot point is on the rim of the hoop. So I= 2Md2, and the frequency is
f=1
2πrMgd
2Md2=1
2πrg
2d=1
2πs(9.81 m/s2)
2(0.653 m) = 0.436 Hz.
(b) Note the above expression looks like the simple pendulum equation if we replace 2dwith l.
Then the equivalent length of the simple pendulum is 2(0.653 m) = 1.31 m.
E17-34 Apply Eq. 17-21:
I=T2κ
4π2=(48.7 s/20.0)2(0.513 N ·m)
4π2= 7.70 ×102kg ·m2.
E17-35 κ= (0.192 N ·m)/(0.850 rad) = 0.226 N ·m. I=2
5(95.2 kg)(0.148 m)2= 0.834 kg ·m2.
Then
T= 2πpI= 2πp(0.834 kg ·m2)/(0.226 N ·m) = 12.1 s.
E17-36 xis din Eq. 17-29. Since the hole is drilled off center we apply the principle axis theorem
to find the rotational inertia:
I=1
12ML2+Mx2.
Then
1
12ML2+Mx2=T2Mgx
4π2,
1
12(1.00 m)2+x2=(2.50 s)2(9.81 m/s2)
4π2x,
(8.33×102m2)(1.55 m)x+x2= 0.
This has solutions x= 1.49 m and x= 0.0557 m. Use the latter.
211
E17-37 For a stick of length Lwhich can pivot about the end, I=1
3ML2. The center of mass
of such a stick is located d=L/2 away from the end.
The frequency of oscillation of such a stick is
f=1
2πrMgd
I,
f=1
2πsMg (L/2)
1
3ML2,
f=1
2πr3g
2L.
This means that fis proportional to p1/L, regardless of the mass or density of the stick. The ratio
of the frequency of two such sticks is then f2/f1=pL1/L2, which in our case gives
f2=f1pL2/L1=f1p(L1)/(2L1/3) = 1.22f1.
E17-38 The rotational inertia of the pipe section about the cylindrical axis is
Icm =M
2r2
1+r2
2=M
2(0.102 m)2+ (0.1084 m)2= (1.11×102m2)M
(a) The total rotational inertia about the pivot axis is
I= 2Icm +M(0.102 m)2+M(0.3188 m)2= (0.134 m2)M.
The period of oscillation is
T= 2πs(0.134 m2)M
M(9.81 m/s2)(0.2104 m) = 1.60 s
(b) The rotational inertia of the pipe section about a diameter is
Icm =M
4r2
1+r2
2=M
4(0.102 m)2+ (0.1084 m)2= (5.54×103m2)M
The total rotational inertia about the pivot axis is now
I=M(1.11×102m2) + M(0.102 m)2+M(5.54×103m2) + M(0.3188 m)2= (0.129 m2)M
The period of oscillation is
T= 2πs(0.129 m2)M
M(9.81 m/s2)(0.2104 m) = 1.57 s.
The percentage difference with part (a) is (0.03 s)/(1.60 s) = 1.9%.
E17-39
E17-40
E17-41 (a) Since effectively x=y, the path is a diagonal line.
(b) The path will be an ellipse which is symmetric about the line x=y.
(c) Since cos(ωt + 90) = sin(ωt), the path is a circle.
212
E17-42 (a)
(b) Take two time derivatives and multiply by m,
~
F=mAω2ˆ
icos ωt + 9ˆ
jcos 3ωt.
(c) U=R~
F·d~
r, so
U=1
2mA2ω2cos2ωt + 9 cos23ωt.
(d) K=1
2mv2, so
K=1
2mA2ω2sin2ωt + 9 sin23ωt;
And then E=K+U= 5mA2ω2.
(e) Yes; the period is 2π/ω.
E17-43 The ωwhich describes the angular velocity in uniform circular motion is effectively the
same ωwhich describes the angular frequency of the corresponding simple harmonic motion. Since
ω=pk/m, we can find the effective force constant kfrom knowledge of the Moon’s mass and the
period of revolution.
The moon orbits with a period of T, so
ω=2π
T=2π
(27.3×24 ×3600 s) = 2.66×106rad/s.
This can be used to find the value of the effective force constant kfrom
k=2= (7.36×1022kg)(2.66×106rad/s)2= 5.21×1011N/m.
E17-44 (a) We want to know when ebt/2m= 1/3, or
t=2m
bln 3 = 2(1.52 kg)
(0.227 kg/s) ln 3 = 14.7 s
(b) The (angular) frequency is
ω0=s(8.13 N/m)
(1.52 kg) (0.227 kg/s)
2(1.52 kg) 2
= 2.31 rad/s.
The number of oscillations is then
(14.7 s)(2.31 rad/s)/2π= 5.40
E17-45 The first derivative of Eq. 17-39 is
dx
dt =xm(b/2m)ebt/2mcos(ω0t+φ) + xmebt/2m(ω0) sin(ω0t+φ),
=xmebt/2m((b/2m) cos(ω0t+φ) + ω0sin(ω0t+φ))
The second derivative is quite a bit messier;
d2
dx2=xm(b/2m)ebt/2m((b/2m) cos(ω0t+φ) + ω0sin(ω0t+φ))
xmebt/2m(b/2m)(ω0) sin(ω0t+φ)+(ω0)2cos(ω0t+φ),
=xmebt/2m(ω0b/m) sin(ω0t+φ)+(b2/4m2ω02) cos(ω0t+φ).
213
Substitute these three expressions into Eq. 17-38. There are, however, some fairly obvious simpli-
fications. Every one of the terms above has a factor of xm, and every term above has a factor of
ebt/2m, so simultaneously with the substitution we will cancel out those factors. Then Eq. 17-38
becomes
m(ω0b/m) sin(ω0t+φ)+(b2/4m2ω02) cos(ω0t+φ)
b[(b/2m) cos(ω0t+φ) + ω0sin(ω0t+φ)] + kcos(ω0t+φ) = 0
Now we collect terms with cosine and terms with sine,
(ω0bω0b) sin(ω0t+φ) + mb2/4m2ω02b2/2m+kcos(ω0t+φ) = 0.
The coefficient for the sine term is identically zero; furthermore, because the cosine term must then
vanish regardless of the value of t, the coefficient for the sine term must also vanish. Then
mb2/4m202b2/2m+k= 0,
or
ω02=k
mb2
4m2.
If this condition is met, then Eq. 17-39 is indeed a solution of Eq. 17-38.
E17-46 (a) Four complete cycles requires a time t4= 8π0. The amplitude decays to 3/4 the
original value in this time, so 0.75 = ebt4/2m, or
ln(4/3) = 8πb
20.
It is probably reasonable at this time to assume that b/2mis small compared to ωso that ω0ω.
We’ll do it the hard way anyway. Then
ω02=8π
ln(4/3)2b
2m2
,
k
mb
2m2
=8π
ln(4/3)2b
2m2
,
k
m= (7630) b
2m2
Numerically, then,
b=s4(1.91 kg)(12.6 N/m)
(7630) = 0.112 kg/s.
(b)
E17-47 (a) Use Eqs. 17-43 and 17-44. At resonance ω00 =ω, so
G=b2ω2=,
and then xm=Fm/bω.
(b) vm=ωxm=Fm/b.
214
E17-48 We need the first two derivatives of
x=Fm
Gcos(ω00tβ)
The derivatives are easy enough to find,
dx
dt =Fm
G(ω00) sin(ω00tβ),
and d2x
dt2=Fm
G(ω00)2cos(ω00tβ),
We’ll substitute this into Eq. 17-42,
mFm
G(ω00)2cos(ω00tβ)
,
+bFm
G(ω00) sin(ω00tβ)+kFm
Gcos(ω00tβ) = Fmcos ω00t.
Then we’ll cancel out as much as we can and collect the sine and cosine terms,
km(ω00)2cos(ω00tβ)(00) sin(ω00tβ) = Gcos ω00t.
We can write the left hand side of this equation in the form
Acos α1cos α2Asin α1sin α2,
if we let α2=ω00tβand choose Aand α1correctly. The best choice is
Acos α1=km(ω00)2,
Asin α1=00,
and then taking advantage of the fact that sin2+ cos2= 1,
A2=km(ω00)22+ (00)2,
which looks like Eq. 17-44! But then we can apply the cosine angle addition formula, and
Acos(α1+ω00tβ) = Gcos ω00t.
This expression needs to be true for all time. This means that A=Gand α1=β.
E17-49 The derivatives are easy enough to find,
dx
dt =Fm
G(ω000) sin(ω000tβ),
and d2x
dt2=Fm
G(ω000)2cos(ω000tβ),
We’ll substitute this into Eq. 17-42,
mFm
G(ω000)2cos(ω000tβ)
,
+bFm
G(ω000) sin(ω000tβ)+kFm
Gcos(ω000tβ) = Fmcos ω00t.
215
Then we’ll cancel out as much as we can and collect the sine and cosine terms,
km(ω000)2cos(ω000tβ)(000) sin(ω000tβ) = Gcos ω00t.
We can write the left hand side of this equation in the form
Acos α1cos α2Asin α1sin α2,
if we let α2=ω000tβand choose Aand α1correctly. The best choice is
Acos α1=km(ω000)2,
Asin α1=000,
and then taking advantage of the fact that sin2+ cos2= 1,
A2=km(ω000)22+ (000)2,
which looks like Eq. 17-44! But then we can apply the cosine angle addition formula, and
Acos(α1+ω000tβ) = Gcos ω00t.
This expression needs to be true for all time. This means that A=Gand α1+ω000tβ=ω00tand
α1=βand ω000 =ω00.
E17-50 Actually, Eq. 17-39 is not a solution to Eq. 17-42 by itself, this is a wording mistake in
the exercise. Instead, Eq. 17-39 can be added to any solution of Eq. 17-42 and the result will still
be a solution.
Let xnbe any solution to Eq. 17-42 (such as Eq. 17-43.) Let xhbe given by Eq. 17-39. Then
x=xn+xh.
Take the first two time derivatives of this expression.
dx
dt =dxn
dt +dxh
dt ,
d2x
dt2=d2xn
dt2+d2xh
dt2
Substitute these three expressions into Eq. 17-42.
md2xn
dt2+d2xh
dt2+bdxn
dt +dxh
dt +k(xn+xh) = Fmcos ω00t.
Rearrange and regroup.
md2xn
dt2+bdxn
dt +kxn+md2xh
dt2+bdxh
dt +kxh=Fmcos ω00t.
Consider the second term on the left. The parenthetical expression is just Eq. 17-38, the damped
harmonic oscillator equation. It is given in the text (and proved in Ex. 17-45) the xhis a solution,
so this term is identically zero. What remains is Eq. 17-42; and we took as a given that xnwas a
solution.
(b) The “add-on” solution of xhrepresents the transient motion that will die away with time.
216
E17-51 The time between “bumps” is the solution to
vt =x,
t=(13 ft)
(10 mi/hr) 1 mi
5280 ft3600 s
1 hr = 0.886 s
The angular frequency is
ω=2π
T= 7.09 rad/s
This is the driving frequency, and the problem states that at this frequency the up-down bounce
oscillation is at a maximum. This occurs when the driving frequency is approximately equal to the
natural frequency of oscillation. The force constant for the car is k, and this is related to the natural
angular frequency by
k=2=W
gω2,
where W= (2200 + 4 ×180) lb= 2920 lb is the weight of the car and occupants. Then
k=(2920 lb)
(32 ft/s2)(7.09 rad/s)2= 4590 lb/ft
When the four people get out of the car there is less downward force on the car springs. The
important relationship is
F=kx.
In this case ∆F= 720 lb, the weight of the four people who got out of the car. xis the distance
the car will rise when the people get out. So
x=F
k=(720 lb)
4590 lb/ft = 0.157 ft 2 in.
E17-52 The derivative is easy enough to find,
dx
dt =Fm
G(ω00) sin(ω00tβ),
The velocity amplitude is
vm=Fm
Gω00,
=Fm
1
ω00 pm2(ω002ω2)2+b2ω002,
=Fm
p(00 k/ω00)2+b2.
Note that this is exactly a maximum when ω00 =ω.
E17-53 The reduced mass is
m= (1.13 kg)(3.24 kg)/(1.12 kg + 3.24 kg) = 0.840 kg.
The period of oscillation is
T= 2πp(0.840 kg)/(252 N/m) = 0.363 s
217
E17-54
E17-55 Start by multiplying the kinetic energy expression by (m1+m2)/(m1+m2).
K=(m1+m2)
2(m1+m2)m1v2
1+m2v2
2,
=1
2(m1+m2)m2
1v2
1+m1m2(v2
1+v2
2) + m2
2v2
2,
and then add 2m1m2v1v22m1m2v1v2,
K=1
2(m1+m2)m2
1v2
1+ 2m1m2v1v2+m2
2v2
2+m1m2(v2
1+v2
22v1v2),
=1
2(m1+m2)(m1v1+m2v2)2+m1m2(v1v2)2.
But m1v2+m2v2= 0 by conservation of momentum, so
K=(m1m2)
2(m1+m2)(v1v2)2,
=m
2(v1v2)2.
P17-1 The mass of one silver atom is (0.108 kg)/(6.02×1023) = 1.79×1025kg.The effective spring
constant is
k= (1.79×1025kg)4π2(10.0×1012/s)2= 7.07×102N/m.
P17-2 (a) Rearrange Eq. 17-8 except replace mwith the total mass, or m+M. Then (M+m)/k =
T2/(4π2),or
M= (k/4π2)T2m.
(b) When M= 0 we have
m= [(605.6 N/m)/(4π2)](0.90149 s)2= 12.467 kg.
(c) M= [(605.6 N/m)/(4π2)](2.08832 s)2(12.467 kg) = 54.432 kg.
P17-3 The maximum static friction is FfµsN. Then
Ff=µsN=µsW=µsmg
is the maximum available force to accelerate the upper block. So the maximum acceleration is
am=Ff
m=µsg
The maximum possible amplitude of the oscillation is then given by
xm=am
ω2=µsg
k/(m+M),
where in the last part we substituted the total mass of the two blocks because both blocks are
oscillating. Now we put in numbers, and find
xm=(0.42)(1.22 kg + 8.73 kg)(9.81 m/s2)
(344 N/m) = 0.119 m.
218
P17-4 (a) Equilibrium occurs when F= 0, or b/r3=a/r2. This happens when r=b/a.
(b) dF/dr = 2a/r33b/r4. At r=b/a this becomes
dF/dr = 2a4/b33a4/b3=a4/b3,
which corresponds to a force constant of a4/b3.
(c) T= 2πpm/k = 2πmb3/a2, where mis the reduced mass.
P17-5 Each spring helps to restore the block. The net force on the block is then of magnitude
F1+F2=k1x+k2x= (k1+k2)x=kx. We can then write the frequency as
f=1
2πrk
m=1
2πrk1+k2
m.
With a little algebra,
f=1
2πrk1+k2
m,
=r1
4π2
k1
m+1
4π2
k2
m,
=qf2
1+f2
2.
P17-6 The tension in the two spring is the same, so k1x1=k2x2, where xiis the extension of the
ith spring. The total extension is x1+x2, so the effective spring constant of the combination is
F
x=F
x1+x2
=1
x1/F +x2/F =1
1/k1+ 1/k2
=k1k2
k1+k2
.
The period is then
T=1
2πrk
m=1
2πsk1k2
(k1+k2)m
With a little algebra,
f=1
2πsk1k2
(k1+k2)m,
=1
2πs1
m/k1+m/k2
,
=1
2πs1
12
1+ 12
2
,
=1
2πsω2
1ω2
2
ω2
1+ω2
2
,
=f1f2
pf2
1+f2
2
.
219
P17-7 (a) When a spring is stretched the tension is the same everywhere in the spring. The
stretching, however, is distributed over the entire length of the spring, so that the relative amount
of stretch is proportional to the length of the spring under consideration. Half a spring, half the
extension. But k=F/x, so half the extension means twice the spring constant.
In short, cutting the spring in half will create two stiffer springs with twice the spring constant,
so k= 7.20 N/cm for each spring.
(b) The two spring halves now support a mass M. We can view this as each spring is holding
one-half of the total mass, so in effect
f=1
2πsk
M/2
or, solving for M,
M=2k
4π2f2=2(720 N/m)
4π2(2.87 s1)2= 4.43 kg.
P17-8 Treat the spring a being composed of Nmassless springlets each with a point mass ms/N
at the end. The spring constant for each springlet will be kN . An expression for the conservation
of energy is then
m
2v2+ms
2N
N
X
1
vn2+Nk
2
N
X
1
xn2=E.
Since the spring stretches proportionally along the length then we conclude that each springlet
compresses the same amount, and then xn=A/N sin ωt could describe the change in length of each
springlet. The energy conservation expression becomes
m
2v2+ms
2N
N
X
1
vn2+k
2A2sin2ωt =E.
v=cos ωt. The hard part to sort out is the vn, since the displacements for all springlets to one
side of the nth must be added to get the net displacement. Then
vn=nA
Nωcos ωt,
and the energy expression becomes
m
2+ms
2N3
N
X
1
n2!A2ω2cos2ωt +k
2A2sin2ωt =E.
Replace the sum with an integral, then
1
N3ZN
0
n2dn =1
3,
and the energy expression becomes
1
2m+ms
3A2ω2cos2ωt +k
2A2sin2ωt =E.
This will only be constant if
ω2=m+ms
3/k,
or T= 2πp(m+ms/3)/k.
220
P17-9 (a) Apply conservation of energy. When x=xmv= 0, so
1
2kxm2=1
2m[v(0)]2+1
2k[x(0)]2,
xm2=m
k[v(0)]2+ [x(0)]2,
xm=p[v(0)]2+ [x(0)]2.
(b) When t= 0 x(0) = xmcos φand v(0) = ωxmsin φ, so
v(0)
ωx(0) =sin φ
cos φ= tan φ.
P17-10
P17-11 Conservation of momentum for the bullet block collision gives mv = (m+M)vfor
vf=m
m+Mv.
This vfwill be equal to the maximum oscillation speed vm. The angular frequency for the oscillation
is given by
ω=rk
m+M.
Then the amplitude for the oscillation is
xm=vm
ω=vm
m+Mrm+M
k=mv
pk(m+M).
P17-12 (a) W=Fs, or mg =kx, so x=mg/k.
(b) F=ma, but F=WFs=mg kx, and since ma =m d2x/dt2,
md2x
dt2+kx =mg.
The solution can be verified by direct substitution.
(c) Just look at the answer!
(d) dE/dt is
mv dv
dt +kxdx
dt mg dx
dt = 0,
mdv
dt +kx =mg.
P17-13 The initial energy stored in the spring is kxm2/2. When the cylinder passes through the
equilibrium point it has a translational velocity vmand a rotational velocity ωr=vm/R, where R
is the radius of the cylinder. The total kinetic energy at the equilibrium point is
1
2mvm2+1
2Iωr2=1
2m+1
2mvm2.
Then the kinetic energy is 2/3 translational and 1/3 rotational. The total energy of the system is
E=1
2(294 N/m)(0.239 m)2= 8.40 J.
221
(a) Kt= (2/3)(8.40 J) = 5.60 J.
(b) Kr= (1/3)(8.40 J) = 2.80 J.
(c) The energy expression is
1
23m
2v2+1
2kx2=E,
which leads to a standard expression for the period with 3M/2 replacing m. Then T= 2πp3M/2k.
P17-14 (a) Integrate the potential energy expression over one complete period and then divide by
the time for one period:
ω
2πZ2π
0
1
2kx2dt =
4πZ2π
0
xm2cos2ωt dt,
=kω
4πxm2π
ω,
=1
4kxm2.
This is half the total energy; since the average total energy is E, then the average kinetic energy
must be the other half of the average total energy, or (1/4)kxm2.
(b) Integrate over half a cycle and divide by twice the amplitude.
1
2xmZxm
xm
1
2kx2dx =1
2xm
1
3kxm3,
=1
6kxm2.
This is one-third the total energy. The average kinetic energy must be two-thirds the total energy,
or (1/3)kxm2.
P17-15 The rotational inertia is
I=1
2MR2+Md2=M1
2(0.144 m)2+ (0.102 m)2= (2.08×102m2)M.
The period of oscillation is
T= 2πsI
Mgd = 2πs(2.08×102m2)M
M(9.81 m/s2)(0.102 m) = 0.906 s.
P17-16 (a) The rotational inertia of the pendulum about the pivot is
(0.488 kg) 1
2(0.103 m)2+ (0.103 m + 0.524 m)2+1
3(0.272 kg)(0.524 m)2= 0.219 kg ·m2.
(b) The center of mass location is
d=(0.524 m)(0.272 kg)/2 + (0.103 m + 0.524 m)(0.488 kg)
(0.272 kg) + (0.488 kg) = 0.496 m.
(c) The period of oscillation is
T= 2πp(0.291 kg ·m2)/(0.272 kg + 0.488 kg)(9.81 m/s2)(0.496 m) = 1.76 s.
222
P17-17 (a) The rotational inertia of a stick about an axis through a point which is a distance d
from the center of mass is given by the parallel axis theorem,
I=Icm +md2=1
12mL2+md2.
The period of oscillation is given by Eq. 17-28,
T= 2πsI
mgd = 2πsL2+ 12d2
12gd
(b) We want to find the minimum period, so we need to take the derivative of Twith respect to
d. It’ll look weird, but
dT
dd =π12d2L2
p12gd3(L2+ 12d2).
This will vanish when 12d2=L2, or when d=L/12.
P17-18 The energy stored in the spring is given by kx2/2, the kinetic energy of the rotating wheel
is 1
2(MR2)v
r2,
where vis the tangential velocity of the point of attachment of the spring to the wheel. If x=
xmsin ωt, then v=xmωcos ωt, and the energy will only be constant if
ω2=k
M
r2
R2.
P17-19 The method of solution is identical to the approach for the simple pendulum on page 381
except replace the tension with the normal force of the bowl on the particle. The effective pendulum
with have a length R.
P17-20 Let xbe the distance from the center of mass to the first pivot point. Then the period is
given by
T= 2πsI+Mx2
Mgx .
Solve this for xby expressing the above equation as a quadratic:
MgT 2
4π2x=I+Mx2,
or
Mx2MgT 2
4π2x+I= 0
There are two solutions. One corresponds to the first location, the other the second location. Adding
the two solutions together will yield L; in this case the discriminant of the quadratic will drop out,
leaving
L=x1+x2=MgT 2
M4π2=gT 2
4π2.
Then g= 4π2L/T 2.
223
P17-21 In this problem
I= (2.50 kg) (0.210 m)2
2+ (0.760 m + 0.210 m)2= 2.41 kg/·m2.
The center of mass is at the center of the disk.
(a) T= 2πp(2.41 kg/·m2)/(2.50 kg)(9.81 m/s2)(0.760 m + 0.210 m) = 2.00 s.
(b) Replace Mgd with Mgd +κand 2.00 s with 1.50 s. Then
κ=4π2(2.41 kg/·m2)
(1.50 s)2(2.50 kg)(9.81 m/s2)(0.760 m + 0.210 m) = 18.5 N ·m/rad.
P17-22 The net force on the bob is toward the center of the circle, and has magnitude Fnet =
mv2/R. This net force comes from the horizontal component of the tension. There is also a vertical
component of the tension of magnitude mg. The tension then has magnitude
T=p(mg)2+ (mv2/R)2=mpg2+v4/R2.
It is this tension which is important in finding the restoring force in Eq. 17-22; in effect we want to
replace gwith pg2+v4/R2in Eq. 17-24. The frequency will then be
f=1
2πsL
pg2+v4/R2.
P17-23 (a) Consider an object of mass mat a point Pon the axis of the ring. It experiences a
gravitational force of attraction to all points on the ring; by symmetry, however, the net force is not
directed toward the circumference of the ring, but instead along the axis of the ring. There is then
a factor of cos θwhich will be thrown in to the mix.
The distance from Pto any point on the ring is r=R2+z2, and θis the angle between the
axis on the line which connects Pand any point on the circumference. Consequently,
cos θ=z/r,
and then the net force on the star of mass mat Pis
F=GMm
r2cos θ=GMmz
r3=GMmz
(R2+z2)3/2.
(b) If zRwe can apply the binomial expansion to the denominator, and
(R2+z2)3/2=R31 + z
R23/2
R313
2z
R2.
Keeping terms only linear in zwe have
F=GMm
R3z,
which corresponds to a spring constant k=GM m/R3. The frequency of oscillation is then
f=pk/m/(2π) = pGM/R3/(2π).
(c) Using some numbers from the Milky Way galaxy,
f=q(7×1011N·m2/kg2)(2×1043kg)/(6×1019m)3/(2π) = 1×1014 Hz.
224
P17-24 (a) The acceleration of the center of mass (point C) is a=F/M. The torque about an
axis through the center of mass is tau =F R/2, since Ois R/2 away from the center of mass. The
angular acceleration of the disk is then
α=τ/I = (F R/2)/(MR2/2) = F/(MR).
Note that the angular acceleration will tend to rotate the disk anti-clockwise. The tangential com-
ponent to the angular acceleration at Pis aT=αR =F/M; this is exactly the opposite of the
linear acceleration, so Pwill not (initially) accelerate.
(b) There is no net force at P.
P17-25 The value for kis closest to
k(2000 kg/4)(9.81 m/s2)/(0.10 m) = 4.9×104N/m.
One complete oscillation requires a time t1= 2π0. The amplitude decays to 1/2 the original
value in this time, so 0.5 = ebt1/2m, or
ln(2) = 2πb
20.
It is not reasonable at this time to assume that b/2mis small compared to ωso that ω0ω. Then
ω02=2π
ln(2)2b
2m2
,
k
mb
2m2
=2π
ln(2)2b
2m2
,
k
m= (81.2) b
2m2
Then the value for bis
b=s4(2000 kg/4)(4.9×104N/m)
(81.2) = 1100 kg/s.
P17-26 a=d2x/dt2=2cos ωt. Substituting into the non-linear equation,
mAω2cos ωt +kA3cos3ωt =Fcos ωdt.
Now let ωd= 3ω. Then
kA3cos3ωt mAω2cos ωt =Fcos 3ωt
Expand the right hand side as cos 3ωt = 4 cos3ωt 3 cos ωt, then
kA3cos3ωt mAω2cos ωt =F(4 cos3ωt 3 cos ωt)
This will only work if 4F=kA3and 3F=mAω2. Dividing one condition by the other means
4mAω2=kA3, so Aωand then Fω3ωd3.
225
P17-27 (a) Divide the top and the bottom by m2. Then
m1m2
m1+m2
=m1
(m1/m2)+1,
and in the limit as m2→ ∞ the value of (m1/m2)0, so
lim
m2→∞
m1m2
m1+m2
= lim
m2→∞
m1
(m1/m2)+1 =m1.
(b) mis called the reduced mass because it is always less than either m1or m2. Think about it
in terms of
m=m1
(m1/m2)+1 =m2
(m2/m1)+1.
Since mass is always positive, the denominator is always greater than or equal to 1. Equality only
occurs if one of the masses is infinite. Now ω=pk/m, and since mis always less than m1, so the
existence of a finite wall will cause ωto be larger, and the period to be smaller.
(c) If the bodies have equal mass then m=m1/2. This corresponds to a value of ω=p2k/m1.
In effect, the spring constant is doubled, which is what happens if a spring is cut in half.
226
E18-1 (a) f=v= (243 m/s)/(0.0327 m) = 7.43×103Hz.
(b) T= 1/f = 1.35×104s.
E18-2 (a) f= (12)/(30 s) = 0.40 Hz.
(b) v= (15 m)/(5.0 s) = 3.0 m/s.
(c) λ=v/f = (3.0 m/s)/(0.40 Hz) = 7.5 m.
E18-3 (a) The time for a particular point to move from maximum displacement to zero dis-
placement is one-quarter of a period; the point must then go to maximum negative displacement,
zero displacement, and finally maximum positive displacement to complete a cycle. So the period is
4(178 ms) = 712 ms.
(b) The frequency is f= 1/T = 1/(712×103s) = 1.40 Hz.
(c) The wave-speed is v=fλ = (1.40 Hz)(1.38 m) = 1.93 m/s.
E18-4 Use Eq. 18-9, except let f= 1/T :
y= (0.0213 m) sin 2πx
(0.114 m) (385 Hz)t= (0.0213 m) sin [(55.1 rad/m)x(2420 rad/s)t].
E18-5 The dimensions for tension are [F] = [M][L]/[T]2where M stands for mass, L for length, T
for time, and F stands for force. The dimensions for linear mass density are [M]/[L].The dimensions
for velocity are [L]/[T].
Inserting this into the expression v=Fab,
[L]
[T] =[M][L]
[T]2a
/[M]
[L] b
,
[L]
[T] =[M]a[L]a
[T]2a
[L]b
[M]b,
[L]
[T] =[M]ab[L]a+b
[T]2a
There are three equations here. One for time, 1 = 2a; one for length, 1 = a+b; and one for
mass, 0 = ab. We need to satisfy all three equations. The first is fairly quick; a= 1/2. Either of
the other equations can be used to show that b= 1/2.
E18-6 (a) ym= 2.30 mm.
(b) f= (588 rad/s)/(2πrad) = 93.6 Hz.
(c) v= (588 rad/s)/(1822 rad/m) = 0.323 m/s.
(d) λ= (2πrad)/(1822 rad/m) = 3.45 mm.
(e) uy=ymω= (2.30 mm)(588 rad/s) = 1.35 m/s.
E18-7 (a) ym= 0.060 m.
(b) λ= (2πrad)/(2.0πrad/m) = 1.0 m.
(c) f= (4.0πrad/s)/(2πrad) = 2.0 Hz.
(d) v= (4.0πrad/s)/(2.0πrad/m) = 2.0 m/s.
(e) Since the second term is positive the wave is moving in the xdirection.
(f) uy=ymω= (0.060 m)(4.0πrad/s) = 0.75 m/s.
E18-8 v=pF=p(487 N)/[(0.0625 kg)/(2.15 m)] = 129 m/s.
227
E18-9 We’ll first find the linear mass density by rearranging Eq. 18-19,
µ=F
v2
Since this is the same string, we expect that changing the tension will not significantly change the
linear mass density. Then for the two different instances,
F1
v2
1
=F2
v2
2
We want to know the new tension, so
F2=F1
v2
2
v2
1
= (123 N)(180 m/s)2
(172 m/s)2= 135 N
E18-10 First v= (317 rad/s)/(23.8 rad/m) = 13.32 m/s. Then
µ=F/v2= (16.3 N)/(13.32 m/s)2= 0.0919 kg/m.
E18-11 (a) ym= 0.05 m.
(b) λ= (0.55 m) (0.15 m) = 0.40 m.
(c) v=pF=p(3.6 N)/(0.025 kg/m) = 12 m/s.
(d) T= 1/f =λ/v = (0.40 m)/(12 m/s) = 3.33×102s.
(e) uy=ymω= 2πym/T = 2π(0.05 m)/(3.33×102s) = 9.4 m/s.
(f) k= (2πrad)/(0.40 m) = 5.0πrad/m; ω=kv = (5.0πrad/m)(12 m/s) = 60πrad/s. The
phase angle can be found from
(0.04 m) = (0.05 m) sin(φ),
or φ= 0.93 rad. Then
y= (0.05 m) sin[(5.0πrad/m)x+ (60πrad/s)t+ (0.93 rad)].
E18-12 (a) The tensions in the two strings are equal, so F= (0.511 kg)(9.81 m/s2)/2=2.506 N.
The wave speed in string 1 is
v=pF=p(2.506 N)/(3.31×103kg/m) = 27.5 m/s,
while the wave speed in string 2 is
v=pF=p(2.506 N)/(4.87×103kg/m) = 22.7 m/s.
(b) We have pF11=pF22, or F11=F22. But Fi=Mig, so M11=M22. Using
M=M1+M2,
M1
µ1
=MM1
µ2
,
M1
µ1
+M1
µ2
=M
µ2
,
M1=M
µ2 1
µ1
+1
µ2,
=(0.511 kg)
(4.87×103kg/m)  1
(3.31×103kg/m) +1
(4.87×103kg/m) ,
= 0.207 kg
and M2= (0.511 kg) (0.207 kg) = 0.304 kg.
228
E18-13 We need to know the wave speed before we do anything else. This is found from Eq.
18-19,
v=sF
µ=sF
m/L =s(248 N)
(0.0978 kg)/(10.3 m) = 162 m/s.
The two pulses travel in opposite directions on the wire; one travels as distance x1in a time t, the
other travels a distance x2in a time t+ 29.6 ms, and since the pulses meet, we have x1+x2= 10.3
m.
Our equations are then x1=vt = (162 m/s)t, and x2=v(t+29.6 ms) = (162 m/s)(t+29.6 ms) =
(162 m/s)t+ 4.80 m.We can add these two expressions together to solve for the time tat which the
pulses meet,
10.3 m = x1+x2= (162 m/s)t+ (162 m/s)t+ 4.80 m = (324 m/s)t+ 4.80 m.
which has solution t= 0.0170 s. The two pulses meet at x1= (162 m/s)(0.0170 s) = 2.75 m, or
x2= 7.55 m.
E18-14 (a) y/∂r = (A/r)kcos(kr ωt)(A/r2) sin(kr ωt).Multiply this by r2, and then find
r r2y
r =Ak cos(kr ωt)Ak2rsin(kr ωt)Ak cos(kr ωt).
Simplify, and then divide by r2to get
1
r2
r r2y
r =(Ak2/r) sin(kr ωt).
Now find 2y/∂t2=2sin(kr ωt).But since 1/v2=k22, the two sides are equal.
(b) [length]2.
E18-15 The liner mass density is µ= (0.263 kg)/(2.72 m) = 9.669×102kg/m. The wave speed is
v=p(36.1 N)/(9.669×102kg/m) = 19.32 m/s.
Pav =1
2µω2ym2v, so
ω=s2(85.5 W)
(9.669×102kg/m)(7.70×103m)2(19.32 m/s) = 1243 rad/s.
Then f= (1243 rad/s)/2π= 199 Hz.
E18-16 (a) If the medium absorbs no energy then the power flow through any closed surface
which contains the source must be constant. Since for a cylindrical surface the area grows as r, then
intensity must fall of as 1/r.
(b) Intensity is proportional to the amplitude squared, so the amplitude must fall off as 1/r.
E18-17 The intensity is the average power per unit area (Eq. 18-33); as you get farther from the
source the intensity falls off because the perpendicular area increases. At some distance rfrom the
source the total possible area is the area of a spherical shell of radius r, so intensity as a function of
distance from the source would be
I=Pav
4πr2
229
We are given two intensities: I1= 1.13 W/m2at a distance r1;I2= 2.41 W/m2at a distance
r2=r15.30 m. Since the average power of the source is the same in both cases we can equate
these two values as
4πr2
1I1= 4πr2
2I2,
4πr2
1I1= 4π(r1d)2I2,
where d= 5.30 m, and then solve for r1. Doing this we find a quadratic expression which is
r2
1I1= (r2
12dr1+d2)I2,
0 = 1I1
I2r2
12dr1+d2,
0 = 1(1.13 W/m2)
(2.41 W/m2)r2
12(5.30 m)r1+ (5.30 m)2,
0 = (0.531)r2
1(10.6 m)r1+ (28.1 m2).
The solutions to this are r1= 16.8 m and r1= 3.15 m; but since the person walked 5.3 m toward
the lamp we will assume they started at least that far away. Then the power output from the light
is
P= 4πr2
1I1= 4π(16.8 m)2(1.13 W/m2) = 4.01×103W.
E18-18 Energy density is energy per volume, or u=U/V . A wave front of cross sectional area A
sweeps out a volume of V=Al when it travels a distance l. The wave front travels that distance
lin a time t=l/v. The energy flow per time is the power, or P=U/t. Combine this with the
definition of intensity, I=P/A, and
I=P
A=U
At =uV
At =uAl
At =uv.
E18-19 Refer to Eq. 18-40, where the amplitude of the combined wave is
2ymcos(∆φ/2),
where ymis the amplitude of the combining waves. Then
cos(∆φ/2) = (1.65ym)/(2ym) = 0.825,
which has solution ∆φ= 68.8.
E18-20 Consider only the point x= 0. The displacement yat that point is given by
y=ym1 sin(ωt) + ym2 sin(ωt +π/2) = ym1 sin(ωt) + ym2 cos(ωt).
This can be written as
y=ym(A1sin ωt +A2cos ωt),
where Ai=ymi/ym. But if ymis judiciously chosen, A1= cos βand A2= sin β, so that
y=ymsin(ωt +β).
Since we then require A2
1+A2
2= 1, we must have
ym=p(3.20 cm)2+ (4.19 cm)2= 5.27 cm.
230
E18-21 The easiest approach is to use a phasor representation of the waves.
Write the phasor components as
x1=ym1 cos φ1,
y1=ym1 sin φ1,
x2=ym2 cos φ2,
y2=ym2 sin φ2,
and then use the cosine law to find the magnitude of the resultant.
The phase angle can be found from the arcsine of the opposite over the hypotenuse.
E18-22 (a) The diagrams for all times except t= 15 ms should show two distinct pulses, first
moving closer together, then moving farther apart. The pulses don not flip over when passing each
other. The t= 15 ms diagram, however, should simply be a flat line.
E18-23 Use a program such as Maple or Mathematica to plot this.
E18-24 Use a program such as Maple or Mathematica to plot this.
E18-25 (a) The wave speed can be found from Eq. 18-19; we need to know the linear mass
density, which is µ=m/L = (0.122 kg)/(8.36 m) = 0.0146 kg/m. The wave speed is then given by
v=sF
µ=s(96.7 N)
(0.0146 kg/m) = 81.4 m/s.
(b) The longest possible standing wave will be twice the length of the string; so λ= 2L= 16.7 m.
(c) The frequency of the wave is found from Eq. 18-13, v=fλ.
f=v
λ=(81.4 m/s)
(16.7 m) = 4.87 Hz
E18-26 (a) v=p(152 N)/(7.16×103kg/m) = 146 m/s.
(b) λ= (2/3)(0.894 m) = 0.596 m.
(c) f=v= (146 m/s)/(0.596 m) = 245 Hz.
E18-27 (a) y=3.9 cm.
(b) y= (0.15 m) sin[(0.79 rad/m)x+ (13 rad/s)t].
(c) y= 2(0.15 m) sin[(0.79 rad/m)(2.3 m)] cos[(13 rad/s)(0.16 s)] = 0.14 m.
E18-28 (a) The amplitude is half of 0.520 cm, or 2.60 mm. The speed is
v= (137 rad/s)/(1.14 rad/cm) = 1.20 m/s.
(b) The nodes are (πrad)/(1.14 rad/cm) = 2.76 cm apart.
(c) The velocity of a particle on the string at position xand time tis the derivative of the wave
equation with respect to time, or
uy=(0.520 cm)(137 rad/s) sin[(1.14 rad/cm)(1.47 cm)] sin[(137 rad/s)(1.36 s)] = 0.582 m/s.
231
E18-29 (a) We are given the wave frequency and the wave-speed, the wavelength is found from
Eq. 18-13,
λ=v
f=(388 m/s)
(622 Hz) = 0.624 m
The standing wave has four loops, so from Eq. 18-45
L=nλ
2= (4)(0.624 m)
2= 1.25 m
is the length of the string.
(b) We can just write it down,
y= (1.90 mm) sin[(2π/0.624 m)x] cos[(2π622 s1)t].
E18-30 (a) fn=nv/2L= (1)(250 m/s)/2(0.150 m) = 833 Hz.
(b) λ=v/f = (348 m/s)/(833 Hz) = 0.418 m.
E18-31 v=pF=pF L/m. Then fn=nv/2L=npF/4mL, so
f1= (1)p(236 N)/4(0.107 kg)(9.88 m)7.47 Hz,
and f2= 2f1= 14.9 Hz while f3= 3f1= 22.4 Hz.
E18-32 (a) v=pF=pF L/m =p(122 N)(1.48 m)/(8.62×103kg) = 145 m/s.
(b) λ1= 2(1.48 m) = 2.96 m; λ2= 1.48 m.
(c) f1= (145 m/s)/(2.96 m) = 49.0 Hz; f2= (145 m/s)/(1.48 m) = 98.0 Hz.
E18-33 Although the tied end of the string forces it to be a node, the fact that the other end
is loose means that it should be an anti-node. The discussion of Section 18-10 indicated that the
spacing between nodes is always λ/2. Since anti-nodes occur between nodes, we can expect that the
distance between a node and the nearest anti-node is λ/4.
The longest possible wavelength will have one node at the tied end, an anti-node at the loose
end, and no other nodes or anti-nodes. In this case λ/4 = 120 cm, or λ= 480 cm.
The next longest wavelength will have a node somewhere in the middle region of the string. But
this means that there must be an anti-node between this new node and the node at the tied end
of the string. Moving from left to right, we then have an anti-node at the loose end, a node, and
anti-node, and finally a node at the tied end. There are four points, each separated by λ/4, so the
wavelength would be given by 3λ/4 = 120 cm, or λ= 160 cm.
To progress to the next wavelength we will add another node, and another anti-node. This will
add another two lengths of λ/4 that need to be fit onto the string; hence 5λ/4 = 120 cm, or λ= 100
cm.
In the figure below we have sketched the first three standing waves.
232
E18-34 (a) Note that fn=nf1. Then fn+1 fn=f1. Since there is no resonant frequency
between these two then they must differ by 1, and consequently f1= (420 Hz) (315 Hz) = 105 Hz.
(b) v=fλ = (105 Hz)[2(0.756 m)] = 159 m/s.
P18-1 (a) λ=v/f and k= 360. Then
x= (55)λ/(360) = 55(353 m/s)/360(493 Hz) = 0.109 m.
(b) ω= 360f, so
φ=ωt = (360)(493 Hz)(1.12×103s) = 199.
P18-2 ω= (2πrad)(548 Hz) = 3440 rad/s; λ=v/f and then
k= (2πrad)/[(326 m/s)/(548 Hz)] = 10.6 rad/m.
Finally, y= (1.12×102m) sin[(10.6 rad/m)x+ (3440 rad/s)t].
P18-3 (a) This problem really isn’t as bad as it might look. The tensile stress Sis tension per
unit cross sectional area, so
S=F
Aor F=SA.
We already know that linear mass density is µ=m/L, where Lis the length of the wire. Substituting
into Eq. 18-19,
v=sF
µ=sSA
m/LsS
m/(AL).
But AL is the volume of the wire, so the denominator is just the mass density ρ.
(b) The maximum speed of the transverse wave will be
v=sS
ρ=s(720 ×106Pa)
(7800 kg/m3)= 300 m/s.
P18-4 (a) f=ω/2π= (4.08 rad/s)/(2πrad) = 0.649 Hz.
(b) λ=v/f = (0.826 m/s)/(0.649 Hz) = 1.27 m.
(c) k= (2πrad)/(1.27 m) = 4.95 rad/m, so
y= (5.12 cm) sin[(4.95 rad/mx(4.08 rad/s)t+φ],
where φis determined by (4.95 rad/m)(9.60×102m) + φ= (1.16 rad), or φ= 0.685 rad.
(d) F=µv2= (0.386 kg/m)(0.826 m/s)2= 0.263 m/s.
P18-5 We want to show that dy/dx =uy/v. The easy way, although not mathematically rigorous:
dy
dx =dy
dx
dt
dt =dy
dt
dt
dx =uy
1
v=uy
x.
P18-6 The maximum value for uyoccurs when the cosine function in Eq. 18-14 returns unity.
Consequently, um/ym=ω.
233
P18-7 (a) The linear mass density changes as the rubber band is stretched! In this case,
µ=m
L+ ∆L.
The tension in the rubber band is given by F=kL. Substituting this into Eq. 18-19,
v=sF
µ=rkL(L+ ∆L)
m.
(b) We want to know the time it will take to travel the length of the rubber band, so
v=L+ ∆L
tor t=L+ ∆L
v.
Into this we will substitute our expression for wave speed
t= (L+ ∆L)rm
kL(L+ ∆L)=rm(L+ ∆L)
kL
We have to possibilities to consider: either ∆LLor ∆LL. In either case we are only
interested in the part of the expression with L+ ∆L; whichever term is much larger than the other
will be the only significant part.
Then if ∆LLwe get L+ ∆LLand
t=rm(L+ ∆L)
kLrmL
kL,
so that tis proportional to 1/L.
But if ∆LLwe get L+ ∆LLand
t=rm(L+ ∆L)
kLrmL
kL=rm
k,
so that tis constant.
P18-8 (a) The tension in the rope at some point is a function of the weight of the cable beneath
it. If the bottom of the rope is y= 0, then the weight beneath some point yis W=y(m/L)g. The
speed of the wave at that point is v=pT/(m/L) = py(M/L)g/(m/L) = gy.
(b) dy/dt =gy, so
dt =dy
gy ,
t=ZL
0
dy
gy = 2pL/g.
(c) No.
P18-9 (a) M=Rµ dx, so
M=ZL
0
kx dx =1
2kL2.
Then k= 2M/L2.
(b) v=pF=pF/kx, then
dt =pkx/F dx,
t=ZL
0p2M/F L2x dx =2
3p2M/F L2L3/2=p8ML/9F .
234
P18-10 Take a cue from pressure and surface tension. In the rotating non-inertial reference frame
for which the hoop appears to be at rest there is an effective force per unit length acting to push on
each part of the loop directly away from the center. This force per unit length has magnitude
F
L= (∆m/L)v2
r=µv2
r.
There must be a tension Tin the string to hold the loop together. Imagine the loop to be replaced
with two semicircular loops. Each semicircular loop has a diameter part; the force tending to pull
off the diameter section is (∆F/L)2r= 2µv2. There are two connections to the diameter section,
so the tension in the string must be half the force on the diameter section, or T=µv2.
The wave speed is vw=pT=v.
Note that the wave on the string can travel in either direction relative to an inertial observer.
One wave will appear to be fixed in space; the other will move around the string with twice the
speed of the string.
P18-11 If we assume that Handel wanted his violins to play in tune with the other instruments
then all we need to do is find an instrument from Handel’s time that will accurately keep pitch over
a period of several hundred years. Most instruments won’t keep pitch for even a few days because of
temperature and humidity changes; some (like the piccolo?) can’t even play in tune for more than
a few notes! But if someone found a tuning fork...
Since the length of the string doesn’t change, and we are using a string with the same mass
density, the only choice is to change the tension. But fvT, so the percentage change in the
tension of the string is
TfTi
Ti
=ff2fi2
fi2=(440 Hz)2(422.5 Hz)2
(422.5 Hz)2= 8.46 %.
P18-12
P18-13 (a) The point sources emit spherical waves; the solution to the appropriate wave equation
is found in Ex. 18-14:
yi=A
ri
sin(kriωt).
If riis sufficiently large compared to A, and r1r2, then let r1=rδr and r2=rδr;
A
r1
+A
r22A
r,
with an error of order (δr/r)2. So ignore it.
Then
y1+y2A
r[sin(kr1ωt) + sin(kr2ωt)] ,
=2A
rsin(kr ωt) cos k
2(r1r2),
ym=2A
rcos k
2(r1r2).
(b) A maximum (minimum) occurs when the operand of the cosine, k(r1r2)/2 is an integer
multiple of π(a half odd-integer multiple of π)
235
P18-14 The direct wave travels a distance dfrom Sto D. The wave which reflects off the original
layer travels a distance d2+ 4H2between Sand D. The wave which reflects off the layer after
it has risen a distance htravels a distance pd2+ 4(H+h)2. Waves will interfere constructively if
there is a difference of an integer number of wavelengths between the two path lengths. In other
words originally we have
pd2+ 4H2d=nλ,
and later we have destructive interference so
pd2+ 4(H+h)2d= (n+ 1/2)λ.
We don’t know n, but we can subtract the top equation from the bottom and get
pd2+ 4(H+h)2pd2+ 4H2=λ/2
P18-15 The wavelength is
λ=v/f = (3.00×108m/s)/(13.0×106Hz) = 23.1 m.
The direct wave travels a distance dfrom Sto D. The wave which reflects off the original layer
travels a distance d2+ 4H2between Sand D. The wave which reflects off the layer one minute
later travels a distance pd2+ 4(H+h)2. Waves will interfere constructively if there is a difference
of an integer number of wavelengths between the two path lengths. In other words originally we
have pd2+ 4H2d=n1λ,
and then one minute later we have
pd2+ 4(H+h)2d=n2λ.
We don’t know either n1or n2, but we do know the difference is 6, so we can subtract the top
equation from the bottom and get
pd2+ 4(H+h)2pd2+ 4H2= 6λ
We could use that expression as written, do some really obnoxious algebra, and then get the
answer. But we don’t want to; we want to take advantage of the fact that his small compared to d
and H. Then the first term can be written as
pd2+ 4(H+h)2=pd2+ 4H2+ 8Hh + 4h2,
pd2+ 4H2+ 8Hh,
pd2+ 4H2r1 + 8H
d2+ 4H2h,
pd2+ 4H21 + 1
2
8H
d2+ 4H2h.
Between the second and the third lines we factored out d2+ 4H2; that last line is from the binomial
expansion theorem. We put this into the previous expression, and
pd2+ 4(H+h)2pd2+ 4H2= 6λ,
pd2+ 4H21 + 4H
d2+ 4H2hpd2+ 4H2= 6λ,
4H
d2+ 4H2h= 6λ.
236
Now what were we doing? We were trying to find the speed at which the layer is moving. We know
H,d, and λ; we can then find h,
h=6(23.1 m)
4(510×103m) p(230×103m)2+ 4(510×103m)2= 71.0 m.
The layer is then moving at v= (71.0 m)/(60 s) = 1.18 m/s.
P18-16 The equation of the standing wave is
y= 2ymsin kx cos ωt.
The transverse speed of a point on the string is the derivative of this, or
uy=2ymωsin kx sin ωt,
this has a maximum value when ωt π/2 is a integer multiple of π. The maximum value is
um= 2ymωsin kx.
Each mass element on the string dm then has a maximum kinetic energy
dKm= (dm/2)um2=ym2ω2sin2kx dm.
Using dm =µdx, and integrating over one loop from kx = 0 to kx =π, we get
Km=ym2ω2µ/2k= 2π2ym2fµv.
P18-17 (a) For 100% reflection the amplitudes of the incident and reflected wave are equal, or
Ai=Ar, which puts a zero in the denominator of the equation for SWR. If there is no reflection,
Ar= 0 leaving the expression for SWR to reduce to Ai/Ai= 1.
(b) Pr/Pi=A2
r/A2
i. Do the algebra:
Ai+Ar
AiAr
= SWR,
Ai+Ar= SWR(AiAr),
Ar(SWR + 1) = Ai(SWR 1),
Ar/Ai= (SWR 1)/(SWR + 1).
Square this, and multiply by 100.
P18-18 Measure with a ruler; I get 2Amax = 1.1 cm and 2Amin = 0.5 cm.
(a) SWR = (1.1/0.5) = 2.2
(b) (2.21)2/(2.2 + 1)2= 0.14 %.
P18-19 (a) Call the three waves
yi=Asin k1(xv1t),
yt=Bsin k2(xv2t),
yr=Csin k1(x+v1t),
where the subscripts i, t, and r refer to the incident, transmitted, and reflected waves respectively.
237
Apply the principle of superposition. Just to the left of the knot the wave has amplitude yi+yr
while just to the right of the knot the wave has amplitude yt. These two amplitudes must line up
at the knot for all times t, or the knot will come undone. Remember the knot is at x= 0, so
yi+yr=yt,
Asin k1(v1t) + Csin k1(+v1t) = Bsin k2(v2t),
Asin k1v1t+Csin k1v1t=Bsin k2v2t
We know that k1v1=k2v2=ω, so the three sin functions are all equivalent, and can be canceled.
This leaves A=B+C.
(b) We need to match more than the displacement, we need to match the slope just on either
side of the knot. In that case we need to take the derivative of
yi+yr=yt
with respect to x, and then set x= 0. First we take the derivative,
d
dx (yi+yr) = d
dx (yt),
k1Acos k1(xv1t) + k1Ccos k1(x+v1t) = k2Bcos k2(xv2t),
and then we set x= 0 and simplify,
k1Acos k1(v1t) + k1Ccos k1(+v1t) = k2Bcos k2(v2t),
k1Acos k1v1t+k1Ccos k1v1t=k2Bcos k2v2t.
This last expression simplifies like the one in part (a) to give
k1(A+C) = k2B
We can combine this with A=B+Cto solve for C,
k1(A+C) = k2(AC),
C(k1+k2) = A(k2k1),
C=Ak2k1
k1+k2
.
If k2< k1Cwill be negative; this means the reflected wave will be inverted.
P18-20
P18-21 Find the wavelength from
λ= 2(0.924 m)/4 = 0.462 m.
Find the wavespeed from
v=fλ = (60.0 Hz)(0.462 m) = 27.7 m/s.
Find the tension from
F=µv2= (0.0442 kg)(27.7 m/s)2/(0.924 m) = 36.7 N.
238
P18-22 (a) The frequency of vibration fis the same for both the aluminum and steel wires; they
don not, however, need to vibrate in the same mode. The speed of waves in the aluminum is v1,
that in the steel is v2. The aluminum vibrates in a mode given by n1= 2L1f/v1, the steel vibrates
in a mode given by n2= 2L2f/v2. Both n1and n2need be integers, so the ratio must be a rational
fraction. Note that the ratio is independent of f, so that L1and L2must be chosen correctly for
this problem to work at all!
This ratio is
n2
n1
=L2
L1rµ2
µ1
=(0.866 m)
(0.600 m)s(7800 kg/m3)
(2600 kg/m3)= 2.50 5
2
Note that since the wires have the same tension and the same cross sectional area it is acceptable
to use the volume density instead of the linear density in the problem.
The smallest integer solution is then n1= 2 and n2= 5. The frequency of vibration is then
f=n1v
2L1
=n1
2L1sT
ρ1A=(2)
2(0.600 m)s(10.0 kg)(9.81 m/s2)
(2600 kg/m3)(1.00×106m2)= 323 Hz.
(b) There are three nodes in the aluminum and six in the steel. But one of those nodes is shared,
and two are on the ends of the wire. The answer is then six.
239
E19-1 (a) v=fλ = (25 Hz)(0.24 m) = 6.0 m/s.
(b) k= (2πrad)/(0.24 m) = 26 rad/m; ω= (2πrad)(25 Hz) = 160 rad/s. The wave equation is
s= (3.0×103m) sin[(26 rad/m)x+ (160 rad/s)t]
E19-2 (a) [∆P]m= 1.48 Pa.
(b) f= (334πrad/s)/(2πrad) = 167 Hz.
(c) λ= (2πrad)/(1.07πrad/m) = 1.87 m.
(d) v= (167 Hz)(1.87 m) = 312 m/s.
E19-3 (a) The wavelength is given by λ=v/f = (343 m/s)/(4.50×106Hz = 7.62×105m.
(b) The wavelength is given by λ=v/f = (1500 m/s)/(4.50×106Hz = 3.33×104m.
E19-4 Note: There is a typo; the mean free path should have been measured in “µm” instead of
“pm”.
λmin = 1.0×106m; fmax = (343 m/s)/(1.0×106m) = 3.4×108Hz.
E19-5 (a) λ= (240 m/s)/(4.2×109Hz) = 5.7×108m.
E19-6 (a) The speed of sound is
v= (331 m/s)(6.21×104mi/m) = 0.206 mi/s.
In five seconds the sound travels (0.206 mi/s)(5.0 s) = 1.03 mi, which is 3% too large.
(b) Count seconds and divide by 3.
E19-7 Marching at 120 paces per minute means that you move a foot every half a second. The
soldiers in the back are moving the wrong foot, which means they are moving the correct foot half
a second later than they should. If the speed of sound is 343 m/s, then the column of soldiers must
be (343 m/s)(0.5 s) = 172 m long.
E19-8 It takes (300 m)/(343 m/s) = 0.87 s for the concert goer to hear the music after it has passed
the microphone. It takes (5.0×106m)/(3.0×108m/s) = 0.017 s for the radio listener to hear the music
after it has passed the microphone. The radio listener hears the music first, 0.85 s before the concert
goer.
E19-9 x/vP=tPand x/vS=tS; subtracting and rearranging,
x= ∆t/[1/vS1/vP] = (180 s)/[1/(4.5 km/s) 1/(8.2 km/s)] = 1800 km.
E19-10 Use Eq. 19-8, sm= [∆p]m/kB, and Eq. 19-14, v=pB0. Then
[∆p]m=kBsm=kv2ρ0sm= 2πfvρ0sm.
Insert into Eq. 19-18, and
I= 2π2ρvf2sm2.
E19-11 If the source emits equally in all directions the intensity at a distance ris given by the
average power divided by the surface area of a sphere of radius rcentered on the source.
The power output of the source can then be found from
P=IA =I(4πr2) = (197×106W/m2)4π(42.5 m)2= 4.47 W.
240
E19-12 Use the results of Exercise 19-10.
sm=s(1.13×106W/m2)
2π2(1.21 kg/m3)(343 m/s)(313 Hz)2= 3.75×108m.
E19-13 U=IAt = (1.60×102W/m2)(4.70×104m2)(3600 s) = 2.71×102W.
E19-14 Invert Eq. 19-21:
I1/I2= 10(1.00dB)/10 = 1.26.
E19-15 (a) Relative sound level is given by Eq. 19-21,
SL1SL2= 10 log I1
I2
or I1
I2
= 10(SL1SL2)/10,
so if ∆SL = 30 then I1/I2= 1030/10 = 1000.
(b) Intensity is proportional to pressure amplitude squared according to Eq. 19-19; so
pm,1/pm,2=pI1/I2=1000 = 32.
E19-16 We know where her ears hurt, so we know the intensity at that point. The power output
is then
P= 4π(1.3 m)2(1.0 W/m2) = 21 W.
This is less than the advertised power.
E19-17 Use the results of Exercise 18-18, I=uv. The intensity is
I= (5200 W)/4π(4820 m)2= 1.78×105W/m2,
so the energy density is
u=I/v = (1.78×105W/m2)/(343 m/s) = 5.19×108J/m3.
E19-18 I2= 2I1, since I1/r2then r2
1= 2r2
2. Then
D=2(D51.4 m),
D(21) = 2(51.4 m),
D= 176 m.
E19-19 The sound level is given by Eq. 19-20,
SL = 10 log I
I0
where I0is the threshold intensity of 1012 W/m2. Intensity is given by Eq. 19-19,
I=(∆pm)2
2ρv
If we assume the maximum possible pressure amplitude is equal to one atmosphere, then
I=(∆pm)2
2ρv =(1.01×105Pa)2
2(1.21 kg/m3)(343 m/s) = 1.22×107W/m2.
241
The sound level would then be
SL = 10 log I
I0
= 10 log 1.22×107W/m2
(1012 W/m2)= 191 dB
E19-20 Let one person speak with an intensity I1.Npeople would have an intensity NI1. The
ratio is N, so by inverting Eq. 19-21,
N= 10(15dB)/10 = 31.6,
so 32 people would be required.
E19-21 Let one leaf rustle with an intensity I1.Nleaves would have an intensity N I1. The ratio
is N, so by Eq. 19-21,
SLN= (8.4 dB) + 10 log(2.71×105) = 63 dB.
E19-22 Ignoring the finite time means that we can assume the sound waves travels vertically,
which considerably simplifies the algebra.
The intensity ratio can be found by inverting Eq. 19-21,
I1/I2= 10(30dB)/10 = 1000.
But intensity is proportional to the inverse distance squared, so I1/I2= (r2/r1)2, or
r2= (115 m)p(1000) = 3640 m.
E19-23 A minimum will be heard at the detector if the path length difference between the
straight path and the path through the curved tube is half of a wavelength. Both paths involve a
straight section from the source to the start of the curved tube, and then from the end of the curved
tube to the detector. Since it is the path difference that matters, we’ll only focus on the part of
the path between the start of the curved tube and the end of the curved tube. The length of the
straight path is one diameter, or 2r. The length of the curved tube is half a circumference, or πr.
The difference is (π2)r. This difference is equal to half a wavelength, so
(π2)r=λ/2,
r=λ
2π4=(42.0 cm)
2π4= 18.4 cm.
E19-24 The path length difference here is
p(3.75 m)2+ (2.12 m)2(3.75 m) = 0.5578 m.
(a) A minimum will occur if this is equal to a half integer number of wavelengths, or (n1/2)λ=
0.5578 m. This will occur when
f= (n1/2) (343 m/s)
(0.5578 m) = (n1/2)(615 Hz).
(b) A maximum will occur if this is equal to an integer number of wavelengths, or = 0.5578 m.
This will occur when
f=n(343 m/s)
(0.5578 m) =n(615 Hz).
242
E19-25 The path length difference here is
p(24.4 m + 6.10 m)2+ (15.2 m)2p(24.4 m)2+ (15.2 m)2= 5.33 m.
A maximum will occur if this is equal to an integer number of wavelengths, or = 5.33 m.This
will occur when
f=n(343 m/s)/(5.33 m) = n(64.4 Hz)
The two lowest frequencies are then 64.4 Hz and 129 Hz.
E19-26 The wavelength is λ= (343 m/s)/(300 Hz) = 1.143 m. This means that the sound maxima
will be half of this, or 0.572 m apart. Directly in the center the path length difference is zero, but
since the waves are out of phase, this will be a minimum. The maxima should be located on either
side of this, a distance (0.572 m)/2=0.286 m from the center. There will then be maxima located
each 0.572 m farther along.
E19-27 (a) f1=v/2Land f2=v/2(LL). Then
1
r=f1
f2
=LL
L= 1 L
L,
or ∆L=L(1 1/r).
(b) The answers are ∆L= (0.80 m)(1 5/6) = 0.133 m; L= (0.80 m)(1 4/5) = 0.160 m;
L= (0.80 m)(1 3/4) = 0.200 m; and L= (0.80 m)(1 2/3) = 0.267 m.
E19-28 The wavelength is twice the distance between the nodes in this case, so λ= 7.68 cm. The
frequency is
f= (1520 m/s)/(7.68×102m) = 1.98×104Hz.
E19-29 The well is a tube open at one end and closed at the other; Eq. 19-28 describes the
allowed frequencies of the resonant modes. The lowest frequency is when n= 1, so f1=v/4L. We
know f1; to find the depth of the well, L, we need to know the speed of sound.
We should use the information provided, instead of looking up the speed of sound, because maybe
the well is filled with some kind of strange gas.
Then, from Eq. 19-14,
v=sB
ρ=s(1.41 ×105Pa)
(1.21 kg/m3)= 341 m/s.
The depth of the well is then
L=v/(4f1) = (341 m/s)/[4(7.20 Hz)] = 11.8 m.
E19-30 (a) The resonant frequencies of the pipe are given by fn=nv/2L, or
fn=n(343 m/s)/2(0.457 m) = n(375 Hz).
The lowest frequency in the specified range is f3= 1130 Hz; the other allowed frequencies in the
specified range are f4= 1500 Hz, and f5= 1880 Hz.
243
E19-31 The maximum reflected frequencies will be the ones that undergo constructive inter-
ference, which means the path length difference will be an integer multiple of a wavelength. A
wavefront will strike a terrace wall and part will reflect, the other part will travel on to the next
terrace, and then reflect. Since part of the wave had to travel to the next terrace and back, the path
length difference will be 2 ×0.914 m = 1.83 m.
If the speed of sound is v= 343 m/s, the lowest frequency wave which undergoes constructive
interference will be
f=v
λ=(343 m/s)
(1.83 m) = 187 Hz
Any integer multiple of this frequency will also undergo constructive interference, and will also be
heard. The ear and brain, however, will most likely interpret the complex mix of frequencies as a
single tone of frequency 187 Hz.
E19-32 Assume there is no frequency between these two that is amplified. Then one of these
frequencies is fn=nv/2L, and the other is fn+1 = (n+ 1)v/2L. Subtracting the larger from the
smaller, ∆f=v/2L, or
L=v/2∆f= (343 m/s)/2(138 Hz 135 Hz) = 57.2 m.
E19-33 (a) v= 2Lf = 2(0.22 m)(920 Hz) = 405 m/s.
(b) F=v2µ= (405 m/s)2(820×106kg)/(0.220 m) = 611 N.
E19-34 fv, and vF, so fF. Doubling frequires Fincrease by a factor of 4.
E19-35 The speed of a wave on the string is the same, regardless of where you put your finger,
so fλ is a constant. The string will vibrate (mostly) in the lowest harmonic, so that λ= 2L, where
Lis the length of the part of the string that is allowed to vibrate. Then
f2λ2=f1λ1,
2f2L2= 2f1L1,
L2=L1
f1
f2
= (30 cm) (440 Hz)
(528 Hz) = 25 cm.
So you need to place your finger 5 cm from the end.
E19-36 The open organ pipe has a length
Lo=v/2f1= (343 m/s)/2(291 Hz) = 0.589 m.
The second harmonic of the open pipe has frequency 2f1; this is the first overtone of the closed pipe,
so the closed pipe has a length
Lc= (3)v/4(2f1) = (3)(343 m/s)/4(2)(291 Hz) = 0.442 m.
E19-37 The unknown frequency is either 3 Hz higher or lower than the standard fork. A small
piece of wax placed on the fork of this unknown frequency tuning fork will result in a lower frequency
because fpk/m. If the beat frequency decreases then the two tuning forks are getting closer
in frequency, so the frequency of the first tuning fork must be above the frequency of the standard
fork. Hence, 387 Hz.
244
E19-38 If the string is too taut then the frequency is too high, or f= (440 + 4)Hz. Then
T= 1/f = 1/(444 Hz) = 2.25×103s.
E19-39 One of the tuning forks need to have a frequency 1 Hz different from another. Assume
then one is at 501 Hz. The next fork can be played against the first or the second, so it could have
a frequency of 503 Hz to pick up the 2 and 3 Hz beats. The next one needs to pick up the 5, 7, and
8 Hz beats, and 508 Hz will do the trick. There are other choices.
E19-40 f=v= (5.5 m/s)/(2.3 m) = 2.39 Hz. Then
f0=f(v+vO)/v = (2.39 Hz)(5.5 m/s + 3.3 m/s)/(5.5 m/s) = 3.8 Hz.
E19-41 We’ll use Eq. 19-44, since both the observer and the source are in motion. Then
f0=fv±vO
vvS
= (15.8 kHz)(343 m/s) + (246 m/s)
(343 m/s) + (193 m/s) = 17.4 kHzr
E19-42 Solve Eq. 19-44 for vS;
vS= (v+vO)f/f0v= (343 m/s + 2.63 m/s)(1602 Hz)/(1590 Hz) (343 m/s) = 5.24 m/s.
E19-43 vS= (14.7 Rad/s)(0.712 m) = 10.5 m/s.
(a) The low frequency heard is
f0= (538 Hz)(343 m/s)/(343 m/s + 10.5 m/s) = 522 Hz.
(a) The high frequency heard is
f0= (538 Hz)(343 m/s)/(343 m/s10.5 m/s) = 555 Hz.
E19-44 Solve Eq. 19-44 for vS;
vS=vvf/f0= (343 m/s) (343 m/s)(440 Hz)/(444 Hz) = 3.1 m/s.
E19-45
E19-46 The approaching car “hears”
f0=fv+vO
vvS
= (148 Hz) (343 m/s) + (44.7 m/s)
(343 m/s) (0) = 167 Hz
This sound is reflected back at the same frequency, so the police car “hears”
f0=fv+vO
vvS
= (167 Hz) (343 m/s) + (0)
(343 m/s) (44.7 m/s) = 192 Hz
E19-47 The departing intruder “hears”
f0=fvvO
v+vS
= (28.3 kHz)(343 m/s) (0.95 m/s)
(343 m/s) (0) = 28.22 kHz
This sound is reflected back at the same frequency, so the alarm “hears”
f0=fvvO
v+vS
= (28.22 kHz) (343 m/s) + (0)
(343 m/s) + (0.95 m/s) = 28.14 kHz
The beat frequency is 28.3 kHz 28.14 kHz = 160 Hz.
245
E19-48 (a) f0= (1000 Hz)(330 m/s)(330 m/s + 10.0 m/s) = 971 Hz.
(b) f0= (1000 Hz)(330 m/s)(330 m/s10.0 m/s) = 1030 Hz.
(c) 1030 Hz 971 Hz = 59 Hz.
E19-49 (a) The frequency “heard” by the wall is
f0=fv+vO
vvS
= (438 Hz) (343 m/s) + (0)
(343 m/s) (19.3 m/s) = 464 Hz
(b) The wall then reflects a frequency of 464 Hz back to the trumpet player. Sticking with Eq.
19-44, the source is now at rest while the observer moving,
f0=fv+vO
vvS
= (464 Hz) (343 m/s) + (19.3 m/s)
(343 m/s) (0) = 490 Hz
E19-50 The body part “hears”
f0=fv+vb
v.
This sound is reflected back to the detector which then “hears”
f00 =f0v
vvb
=fv+vb
vvb
.
Rearranging,
vb/v =f00 f
f00 +f1
2
f
f,
so v2(1×103m/s)/(1.3×106)1500 m/s.
E19-51 The wall “hears”
f0=fv+vO
vvS
= (39.2 kHz) (343 m/s) + (0)
(343 m/s) (8.58 m/s) = 40.21 kHz
This sound is reflected back at the same frequency, so the bat “hears”
f0=fv+vO
vvS
= (40.21 kHz)(343 m/s) + (8.58 m/s)
(343 m/s) (0) = 41.2 kHz.
P19-1 (a) tair =L/vair and tm=L/v, so the difference is
t=L(1/vair 1/v)
(b) Rearrange the above result, and
L= (0.120 s)/[1/(343 m/s) 1/(6420 m/s)] = 43.5 m.
P19-2 The stone falls for a time t1where y=gt2
1/2 is the depth of the well. Note yis positive in
this equation. The sound travels back in a time t2where v=y/t2is the speed of sound in the well.
t1+t2= 3.00 s, so
2y=g(3.00 s t2)2=g[(9.00 s2)(6.00 s)y/v +y2/v2],
or, using g= 9.81 m/s2and v= 343 m/s,
y2(2.555×105m)y+ (1.039×107m2) = 0,
which has a positive solution y= 40.7 m.
246
P19-3 (a) The intensity at 28.5 m is found from the 1/r2dependence;
I2=I1(r1/r2)2= (962 µW/m2)(6.11 m/28.5 m)2= 44.2µW/m2.
(c) We’ll do this part first. The pressure amplitude is found from Eq. 19-19,
pm=p2ρvI =p2(1.21 kg/m3)(343 m/s)(962×106W/m2) = 0.894 Pa.
(b) The displacement amplitude is found from Eq. 19-8,
sm= ∆pm/(kB),
where k= 2πf/v is the wave number. From Eq. 19-14 w know that B=ρv2, so
sm=pm
2πfρv =(0.894 Pa)
2π(2090 Hz)(1.21 kg/m3)(343 m/s) = 1.64×107m.
P19-4 (a) If the intensities are equal, then ∆pmρv, so
[∆pm]water
[∆pm]air
=s(998 kg/m3)(1482 m/s)
(1.2 kg/m3)(343 m/s) = 59.9.
(b) If the pressure amplitudes are equal, then I∝∝ 1/ρv, so
Iwater
Iair
=(1.2 kg/m3)(343 m/s)
(998 kg/m3)(1482 m/s) = 2.78×104.
P19-5 The energy is dissipated on a cylindrical surface which grows in area as r, so the intensity is
proportional to 1/r. The amplitude is proportional to the square root of the intensity, so sm1/r.
P19-6 (a) The first position corresponds to maximum destructive interference, so the waves are half
a wavelength out of phase; the second position corresponds to maximum constructive interference, so
the waves are in phase. Shifting the tube has in effect added half a wavelength to the path through
B. But each segment is added, so
λ= (2)(2)(1.65 cm) = 6.60 cm,
and f= (343 m/s)/(6.60 cm) = 5200 Hz.
(b) Imin (s1s2)2,Imax (s1+s2)2, then dividing one expression by the other and rearranging
we find
s1
s2
=Imax +Imin
Imax Imin
=90 + 10
90 10 = 2
P19-7 (a) I=P/4πr2= (31.6 W)/4π(194 m)2= 6.68×105W/m2.
(b) P=IA = (6.68×105W/m2)(75.2×106m2) = 5.02×109W.
(c) U=P t = (5.02×109W)(25.0 min)(60.0 s/min) = 7.53 µJ.
P19-8 Note that the reverberation time is logarithmically related to the intensity, but linearly
related to the sound level. As such, the reverberation time is the amount of time for the sound level
to decrease by
SL = 10 log(106) = 60 dB.
Then
t= (87 dB)(2.6s)/(60 dB) = 3.8 s
247
P19-9 What the device is doing is taking all of the energy which strikes a large surface area and
concentrating it into a small surface area. It doesn’t succeed; only 12% of the energy is concentrated.
We can think, however, in terms of power: 12% of the average power which strikes the parabolic
reflector is transmitted into the tube.
If the sound intensity on the reflector is I1, then the average power is P1=I1A1=I1πr2
1, where
r1is the radius of the reflector. The average power in the tube will be P2= 0.12P1, so the intensity
in the tube will be
I2=P2
A2
=0.12I1πr2
1
πr2
2
= 0.12I1
r2
1
r2
2
Since the lowest audible sound has an intensity of I0= 1012 W/m2, we can set I2=I0as the
condition for “hearing” the whisperer through the apparatus. The minimum sound intensity at the
parabolic reflector is
I1=I0
0.12
r2
2
r2
1
.
Now for the whisperers. Intensity falls off as 1/d2, where dis the distance from the source. We
are told that when d= 1.0 m the sound level is 20 dB; this sound level has an intensity of
I=I01020/10 = 100I0
Then at a distance dfrom the source the intensity must be
I1= 100I0
(1 m)2
d2.
This would be the intensity “picked-up” by the parabolic reflector. Combining this with the condition
for being able to hear the whisperers through the apparatus, we have
I0
0.12
r2
2
r2
1
= 100I0
(1 m)2
d2
or, upon some rearranging,
d= (12 m)r1
r2
= (12 m) (0.50 m)
(0.005 m) = 346 m.
P19-10 (a) A displacement node; at the center the particles have nowhere to go.
(b) This systems acts like a pipe which is closed at one end.
(c) vpB, so
T= 4(0.009)(6.96×108m)p(1.0×1010kg/m3)/(1.33×1022Pa) = 22 s.
P19-11 The cork filing collect at pressure antinodes when standing waves are present, and the
antinodes are each half a wavelength apart. Then v=fλ =f(2d).
P19-12 (a) f=v/4L= (343 m/s)/4(1.18 m) = 72.7 Hz.
(b) F=µv2=µf2λ2, or
F= (9.57×103kg/0.332 m)(72.7 Hz)2[2(0.332 m)]2= 67.1N.
248
P19-13 In this problem the string is observed to resonate at 880 Hz and then again at 1320 Hz,
so the two corresponding values of nmust differ by 1. We can then write two equations
(880 Hz) = nv
2Land (1320 Hz) = (n+ 1)v
2L
and solve these for v. It is somewhat easier to first solve for n. Rearranging both equations, we get
(880 Hz)
n=v
2Land (1320 Hz)
n+ 1 =v
2L.
Combining these two equations we get
(880 Hz)
n=(1320 Hz)
n+ 1 ,
(n+ 1)(880 Hz) = n(1320 Hz),
n=(880 Hz)
(1320 Hz) (880 Hz) = 2.
Now that we know nwe can find v,
v= 2(0.300 m)(880 Hz)
2= 264 m/s
And, finally, we are in a position to find the tension, since
F=µv2= (0.652×103kg/m)(264 m/s)2= 45.4 N.
P19-14 (a) There are five choices for the first fork, and four for the second. That gives 20 pairs.
But order doesn’t matter, so we need divide that by two to get a maximum of 10 possible beat
frequencies.
(b) If the forks are ordered to have equal differences (say, 400 Hz, 410 Hz, 420 Hz, 430 Hz, and
440 Hz) then there will actually be only 4 beat frequencies.
P19-15 v= (2.25×108m/s)/sin(58.0) = 2.65×108m/s.
P19-16 (a) f1= (442 Hz)(343 m/s)/(343 m/s31.3 m/s) = 486 Hz, while
f2= (442 Hz)(343 m/s)/(343 m/s + 31.3 m/s) = 405 Hz,
so ∆f= 81 Hz.
(b) f1= (442 Hz)(343 m/s31.3 m/s)/(343 m/s) = 402 Hz, while
f2= (442 Hz)(343 m/s + 31.3 m/s)/(343 m/s) = 482 Hz,
so ∆f= 80 Hz.
P19-17 The sonic boom that you hear is not from the sound given off by the plane when it is
overhead, it is from the sound given off before the plane was overhead. So this problem isn’t as
simple as distance equals velocity ×time. It is very useful to sketch a picture.
249
θ
x2 x1H
O
S
We can find the angle θfrom the figure, we’ll get Eq. 19-45, so
sin θ=v
vs
=(330 m/s)
(396 m/s) = 0.833 or θ= 56.4
Note that vsis the speed of the source, not the speed of sound!
Unfortunately t= 12 s is not the time between when the sonic boom leaves the plane and when
it arrives at the observer. It is the time between when the plane is overhead and when the sonic
boom arrives at the observer. That’s why there are so many marks and variables on the figure. x1
is the distance from where the sonic boom which is heard by the observer is emitted to the point
directly overhead; x2is the distance from the point which is directly overhead to the point where
the plane is when the sonic boom is heard by the observer. We do have x2=vs(12.0 s). This length
forms one side of a right triangle HSO, the opposite side of this triangle is the side HO, which is
the height of the plane above the ground, so
h=x2tan θ= (343 m/s)(12.0 s) tan(56.4) = 7150 m.
P19-18 (a) The target “hears”
f0=fs
v+V
v.
This sound is reflected back to the detector which then “hears”
fr=f0v
vV=fs
v+V
vV.
(b) Rearranging,
V/v =frfs
fr+fs1
2
frfs
fs
,
where we have assumed that the source frequency and the reflected frequency are almost identical,
so that when added fr+fs2fs.
P19-19 (a) We apply Eq. 19-44
f0=fv+vO
vvS
= (1030 Hz) (5470 km/h) + (94.6 km/h)
(5470 km/h) (20.2 km/h) = 1050 Hz
250
(b) The reflected signal has a frequency equal to that of the signal received by the second sub
originally. Applying Eq. 19-44 again,
f0=fv+vO
vvS
= (1050 Hz) (5470 km/h) + (20.2 km/h)
(5470 km/h) (94.6 km/h) = 1070 Hz
P19-20 In this case vS= 75.2 km/h 30.5 km/h = 12.4 m/s. Then
f0= (989 Hz)(1482 m/s)(1482 m/s12.4 m/s) = 997 Hz.
P19-21 There is no relative motion between the source and observer, so there is no frequency shift
regardless of the wind direction.
P19-22 (a) vS= 34.2 m/s and vO= 34.2 m/s, so
f0= (525 Hz)(343 m/s + 34.2 m/s)/(343 m/s34.2 m/s) = 641 Hz.
(b) vS= 34.2 m/s + 15.3 m/s = 49.5 m/s and vO= 34.2 m/s15.3 m/s = 18.9 m/s, so
f0= (525 Hz)(343 m/s + 18.9 m/s)/(343 m/s49.5 m/s) = 647 Hz.
(c) vS= 34.2 m/s15.3 m/s = 18.9 m/s and vO= 34.2 m/s + 15.3 m/s = 49.5 m/s, so
f0= (525 Hz)(343 m/s + 49.5 m/s)/(343 m/s18.9 m/s) = 636 Hz.
251
E20-1 (a) t=x/v = (0.20 m)/(0.941)(3.00×108m/s) = 7.1×1010s.
(b) y=gt2/2 = (9.81 m/s2)(7.1×1010s)2/2 = 2.5×1018m.
E20-2 L=L0p1u2/c2= (2.86 m)p1(0.999987)2= 1.46 cm.
E20-3 L=L0p1u2/c2= (1.68 m)p1(0.632)2= 1.30 m.
E20-4 Solve ∆t= ∆t0/p1u2/c2for u:
u=cs1t0
t2
= (3.00×108m/s)s1(2.20 µs)
(16.0µs)2
= 2.97×108m/s.
E20-5 We can apply ∆x=vtto find the time the particle existed before it decayed. Then
t=x
v
(1.05 ×103m)
(0.992)(3.00 ×108m/s) = 3.53 ×1012 s.
The proper lifetime of the particle is
t0= ∆tp1u2/c2= (3.53 ×1012 s)p1(0.992)2= 4.46 ×1013 s.
E20-6 Apply Eq. 20-12:
v=(0.43c) + (0.587c)
1 + (0.43c)(0.587c)/c2= 0.812c.
E20-7 (a) L=L0p1u2/c2= (130 m)p1(0.740)2= 87.4 m
(b) ∆t=L/v = (87.4 m)/(0.740)(3.00×108m/s) = 3.94×107s.
E20-8 t= ∆t0/p1u2/c2= (26 ns)p1(0.99)2= 184 ns.Then
L=vt= (0.99)(3.00×108m/s)(184×109s) = 55 m.
E20-9 (a) vg= 2v= (7.91 + 7.91) km/s = 15.82 km/s.
(b) A relativistic treatment yields vr= 2v/(1 + v2/c2). The fractional error is
vg
vr1 = 1 + v2
c21 = v2
c2=(7.91×103m/s)2
(3.00×108m/s)2= 6.95×1010.
E20-10 Invert Eq. 20-15 to get β=p112.
(a) β=p11/(1.01)2= 0.140.
(b) β=p11/(10.0)2= 0.995.
(c) β=p11/(100)2= 0.99995.
(d) β=p11/(1000)2= 0.9999995.
252
E20-11 The distance traveled by the particle is (6.0 y)c; the time required for the particle to
travel this distance is 8.0 years. Then the speed of the particle is
v=x
t=(6.0 y)c
(8.0 y) =3
4c.
The speed parameter βis given by
β=v
c=
3
4c
c=3
4.
E20-12 γ= 1/p1(0.950)2= 3.20. Then
x0= (3.20)[(1.00×105m) (0.950)(3.00×108m/s)(2.00×104s)] = 1.38×105m,
t0= (3.20)[(2.00×104s) (1.00×105m)(0.950)/(3.00×108m/s)] = 3.73×104s.
E20-13 (a) γ= 1/p1(0.380)2= 1.081. Then
x0= (1.081)[(3.20×108m) (0.380)(3.00×108m/s)(2.50 s)] = 3.78×107m,
t0= (1.081)[(2.50 s) (3.20×108m)(0.380)/(3.00×108m/s)] = 2.26 s.
(b) γ= 1/p1(0.380)2= 1.081. Then
x0= (1.081)[(3.20×108m) (0.380)(3.00×108m/s)(2.50 s)] = 6.54×108m,
t0= (1.081)[(2.50 s) (3.20×108m)(0.380)/(3.00×108m/s)] = 3.14 s.
E20-14
E20-15 (a) v0
x= (u)/(1 0) and v0
y=cp1u2/c2.
(b) (v0
x)2+ (v0
y)2=u2+c2u2=c2.
E20-16 v0= (0.787c+ 0.612c)/[1 + (0.787)(0.612)] = 0.944c.
E20-17 (a) The first part is easy; we appear to be moving away from Aat the same speed as A
appears to be moving away from us: 0.347c.
(b) Using the velocity transformation formula, Eq. 20-18,
v0
x=vxu
1uvx/c2=(0.347c)(0.347c)
1(0.347c)(0.347c)/c2= 0.619c.
The negative sign reflects the fact that these two velocities are in opposite directions.
E20-18 v0= (0.788c0.413c)/[1 + (0.788)(0.413)] = 0.556c.
E20-19 (a) γ= 1/p1(0.8)2= 5/3.
v0
x=vx
γ(1 uvy/c2)=3(0.8c)
5[1 (0)] =12
25c,
v0
y=vyu
1uvy/c2=(0) (0.8c)
1(0) =4
5c.
Then v0=cp(4/5)2+ (12/25)2= 0.933cdirected θ= arctan(12/20) = 31East of South.
253
(b) γ= 1/p1(0.8)2= 5/3.
v0
x=vxu
1uvx/c2=(0) (0.8c)
1(0) = +4
5c,
v0
y=vy
γ(1 uvx/c2)=3(0.8c)
5[1 (0)] =12
25c.
Then v0=cp(4/5)2+ (12/25)2= 0.933cdirected θ= arctan(20/12) = 59West of North.
E20-20 This exercise should occur in Section 20-9.
(a) v= 2π(6.37×106m)c/(1 s)(3.00×108m/s) = 0.133c.
(b) K= (γ1)mc2= (1/p1(0.133)21)(511 keV) = 4.58 keV.
(c) Kc=mv2/2 = mc2(v2/c2)/2 = (511 keV)(0.133)2/2 = 4.52 keV. The percent error is
(4.52 4.58)/(4.58) = 1.31%.
E20-21 L=L0L0so
L= 2(6.370×106m)(1 p1(29.8×103m/s)2/(3.00×108m/s)2) = 6.29×102m.
E20-22 (a) ∆L/L0= 1 L0/L0so
L= (1 p1(522 m/s)2/(3.00×108m/s)2) = 1.51×1012.
(b) We want to solve ∆tt0= 1 µs, or
1µs=∆t(1 1/p1(522 m/s)2/(3.00×108m/s)2),
which has solution ∆t= 6.61×105s. That’s 7.64 days.
E20-23 The length of the ship as measured in the “certain” reference frame is
L=L0p1v2/c2= (358 m)p1(0.728)2= 245 m.
In a time ∆tthe ship will move a distance x1=v1twhile the micrometeorite will move a distance
x2=v2t; since they are moving toward each other then the micrometeorite will pass then ship
when x1+x2=L. Then
t=L/(v1+v2) = (245 m)/[(0.728 + 0.817)(3.00×108m/s)] = 5.29×107s.
This answer is the time measured in the “certain” reference frame. We can use Eq. 20-21 to find
the time as measured on the ship,
t=t0+ux0/c2
p1u2/c2=(5.29×107s) + (0.728c)(116 m)/c2
p1(0.728)2= 1.23×106s.
E20-24 (a) γ= 1/p1(0.622)2= 1.28.
(b) ∆t= (183 m)/(0.622)(3.00×108m/s) = 9.81×107s.On the clock, however,
t0= ∆t/γ = (9.81×107s)/(1.28) = 7.66×107s.
254
E20-25 (a) ∆t= (26.0 ly)/(0.988)(1.00 ly/y) = 26.3 y.
(b) The signal takes 26 years to return, so 26 + 26.3 = 52.3 years.
(c) ∆t0= (26.3 y)p1(0.988)2= 4.06 y.
E20-26 (a) γ= (1000 y)(1 y) = 1000;
v=cp112c(1 1/2γ2) = 0.9999995c
(b) No.
E20-27 (5.61×1029 MeV/c2)c/(3.00×108m/s) = 1.87×1021 MeV/c.
E20-28 p2=m2c2=m2v2/(1 v2/c2), so 2v2/c2= 1, or v=2c.
E20-29 The magnitude of the momentum of a relativistic particle in terms of the magnitude of
the velocity is given by Eq. 20-23,
p=mv
p1v2/c2.
The speed parameter, β, is what we are looking for, so we need to rearrange the above expression
for the quantity v/c.
p/c =mv/c
p1v2/c2,
p
c=
p1β2,
mc
p=p1β2
β,
mc
p=p121.
Rearranging,
mc2
pc =p121,
mc2
pc 2
=1
β21,
smc2
pc 2
+ 1 = 1
β,
pc
pm2c4+p2c2=β
(a) For the electron,
β=(12.5 MeV/c)c
p(0.511 MeV/c2)2c4+ (12.5 MeV/c)2c2= 0.999.
(b) For the proton,
β=(12.5 MeV/c)c
p(938 MeV/c2)2c4+ (12.5 MeV/c)2c2= 0.0133.
255
E20-30 K=mc2(γ1), so γ= 1 + K/mc2.β=p112.
(a) γ= 1 + (1.0 keV)/(511 keV) = 1.00196. β= 0.0625c.
(b) γ= 1 + (1.0 MeV)/(0.511 MeV) = 2.96. β= 0.941c.
(c) γ= 1 + (1.0 GeV)/(0.511 MeV) = 1960. β= 0.99999987c.
E20-31 The kinetic energy is given by Eq. 20-27,
K=mc2
p1v2/c2mc2.
We rearrange this to solve for β=v/c,
β=s1mc2
K+mc22
.
It is actually much easier to find γ, since
γ=1
p1v2/c2,
so K=γmc2mc2implies
γ=K+mc2
mc2
(a) For the electron,
β=s1(0.511 MeV/c2)c2
(10 MeV) + (0.511 MeV/c2)c22
= 0.9988,
and
γ=(10 MeV) + (0.511 MeV/c2)c2
(0.511 MeV/c2)c2= 20.6.
(b) For the proton,
β=s1(938 MeV/c2)c2
(10 MeV) + (938 MeV/c2)c22
= 0.0145,
and
γ=(10 MeV) + (938 MeV/c2)c2
(938 MeV/c2)c2= 1.01.
(b) For the alpha particle,
β=s14(938 MeV/c2)c2
(10 MeV) + 4(938 MeV/c2)c22
= 0.73,
and
γ=(10 MeV) + 4(938 MeV/c2)c2
4(938 MeV/c2)c2= 1.0027.
E20-32 γ= 1/p1(0.99)2= 7.089.
(a) E=γmc2= (7.089)(938.3 MeV) = 6650 MeV. K=Emc2= 5710 MeV. p=mvγ =
(938.3 MeV/c2)(0.99c)(7.089) = 6580 MeV/c.
(b) E=γmc2= (7.089)(0.511 MeV) = 3.62 MeV. K=Emc2= 3.11 MeV. p=mvγ =
(0.511 MeV/c2)(0.99c)(7.089) = 3.59 MeV/c.
256
E20-33 m/t= (1.2×1041W)/(3.0×108m/s)2= 1.33×1024kg/s, which is
m
t=(1.33×1024kg/s)(3.16×107s/y)
(1.99×1030kg/sun) = 21.1
E20-34 (a) If K=Emc2= 2mc2, then E= 3mc2, so γ= 3, and
v=cp112=cp11/(3)2= 0.943c.
(b) If E= 2mc2, then γ= 2, and
v=cp112=cp11/(2)2= 0.866c.
E20-35 (a) The kinetic energy is given by Eq. 20-27,
K=mc2
p1v2/c2mc2=mc21β21/21.
We want to expand the 1 β2part for small β,
1β21/2= 1 + 1
2β2+3
8β4+···
Inserting this into the kinetic energy expression,
K=1
2mc2β2+3
8mc2β4+···
But β=v/c, so
K=1
2mv2+3
8mv4
c2+···
(b) We want to know when the error because of neglecting the second (and higher) terms is 1%;
or
0.01 = (3
8mv4
c2)/(1
2mv2) = 3
4v
c2.
This will happen when v/c =p(0.01)4/3) = 0.115.
E20-36 Kc= (1000 kg)(20 m/s)2/2 = 2.0×105J. The relativistic calculation is slightly harder:
Kr= (1000 kg)(3×108m/s)2(1/p1(20 m/s)2/(3×108m/s)21),
(1000 kg) 1
2(20 m/s)2+3
8(20 m/s)4/(3×108m/s)2+...,
= 2.0×105J+6.7×1010J.
E20-37 Start with Eq. 20-34 in the form
E2= (pc)2+ (mc2)2
The rest energy is mc2, and if the total energy is three times this then E= 3mc2, so
(3mc2)2= (pc)2+ (mc2)2,
8(mc2)2= (pc)2,
8mc =p.
257
E20-38 The initial kinetic energy is
Ki=1
2m2v
1 + v2/c2=2mv2
(1 + v2/c2)2.
The final kinetic energy is
Kf= 21
2mvp2v2/c22=mv2(2 v2/c2).
E20-39 This exercise is much more involved than the previous one!
The initial kinetic energy is
Ki=mc2
r12v
1+v2/c22/c2mc2,
=mc2(1 + v2/c2)
p(1 + v2/c2)24v2/c2mc2,
=m(c2+v2)
1v2/c2m(c2v2)
1v2/c2,
=2mv2
1v2/c2.
The final kinetic energy is
Kf= 2 mc2
r1vp2v2/c22/c22mc2,
= 2 mc2
p1(v2/c2)(2 v2/c2)2mc2,
= 2 mc2
1v2/c22mc2,
= 2 mc2
1v2/c22m(c2v2)
1v2/c2,
=2mv2
1v2/c2.
E20-40 For a particle with mass, γ=K/mc2+ 1. For the electron, γ= (0.40)/(0.511) + 1 = 1.78.
For the proton, γ= (10)/(938) + 1 = 1.066.
For the photon, pc =E. For a particle with mass, pc =p(K+mc2)2m2c4. For the electron,
pc =p[(0.40 MeV) + (0.511 MeV)]2(0.511 MeV)2= 0.754 MeV.
For the proton,
pc =p[(10 MeV) + (938 MeV)]2(938 MeV)2= 137 MeV.
(a) Only photons move at the speed of light, so it is moving the fastest.
(b) The proton, since it has smallest value for γ.
(c) The proton has the greatest momentum.
(d) The photon has the least.
258
E20-41 Work is change in energy, so
W=mc2/p1(vf/c)2mc2/p1(vi/c)2.
(a) Plug in the numbers,
W= (0.511 MeV)(1/p1(0.19)21/p1(0.18)2) = 0.996 keV.
(b) Plug in the numbers,
W= (0.511 MeV)(1/p1(0.99)21/p1(0.98)2) = 1.05 MeV.
E20-42 E= 2γm0c2=mc2, so
m= 2γm0= 2(1.30 mg)/p1(0.580)2= 3.19 mg.
E20-43 (a) Energy conservation requires Ek= 2Eπ, or mkc2= 2γmπc2. Then
γ= (498 MeV)/2(140 MeV) = 1.78
This corresponds to a speed of v=cp11/(1.78)2= 0.827c.
(b) γ= (498 MeV + 325 MeV)/(498 MeV) = 1.65, so v=cp11/(1.65)2= 0.795c.
(c) The lab frame velocities are then
v0
1=(0.795) + (0.827)
1 + (0.795)(0.827)c=0.0934c,
and
v0
2=(0.795) + (0.827)
1 + (0.795)(0.827)c= 0.979c,
The corresponding kinetic energies are
K1= (140 MeV)(1/p1(0.0934)21) = 0.614 MeV
and
K1= (140 MeV)(1/p1(0.979)21) = 548 MeV
E20-44
P20-1 (a) γ= 2, so v=p11/(2)2= 0.866c.
(b) γ= 2.
P20-2 (a) Classically, v0= (0.620c) + (0.470c) = 1.09c. Relativistically,
v0=(0.620c) + (0.470c)
1 + (0.620)(0.470) = 0.844c.
(b) Classically, v0= (0.620c)+(0.470c) = 0.150c. Relativistically,
v0=(0.620c)+(0.470c)
1 + (0.620)(0.470) = 0.211c.
259
P20-3 (a) γ= 1/p1(0.247)2= 1.032.Use the equations from Table 20-2.
t= (1.032)[(0) (0.247)(30.4×103m)/(3.00×108m/s)] = 2.58×105s.
(b) The red flash appears to go first.
P20-4 Once again, the “pico” should have been a µ.
γ= 1/p1(0.60)2= 1.25.Use the equations from Table 20-2.
t= (1.25)[(4.0×106s) (0.60)(3.0×103m)/(3.00×108m/s)] = 2.5×106s.
P20-5 We can choose our coordinate system so that uis directed along the xaxis without any
loss of generality. Then, according to Table 20-2,
x0=γ(∆xut),
y0= ∆y,
z0= ∆z,
ct0=γ(ctux/c).
Square these expressions,
(∆x0)2=γ2(∆xut)2=γ2(∆x)22u(∆x)(∆t) + (∆t)2,
(∆y0)2= (∆y)2,
(∆z0)2= (∆z)2,
c2(∆t0)2=γ2(ctux/c)2=γ2c2(∆t)22u(∆t)(∆x) + u2(∆x)2/c2.
We’ll add the first three equations and then subtract the fourth. The left hand side is the equal to
(∆x0)2+ (∆y0)2+ (∆z0)2c2(∆t0)2,
while the right hand side will equal
γ2(∆x)2+u2(∆t)2c2(∆t)2u2/c2(∆x)2+ (∆y)2+ (∆z)2,
which can be rearranged as
γ21u2/c2(∆x)2+γ2u2c2(∆t)2+ (∆y)2+ (∆z)2,
γ21u2/c2(∆x)2+ (∆y)2+ (∆z)2c2γ21u2/c2(∆t)2.
But
γ2=1
1u2/c2,
so the previous expression will simplify to
(∆x)2+ (∆y)2+ (∆z)2c2(∆t)2.
P20-6 (a) vx= [(0.780c) + (0.240c)]/[1 + (0.240)(0.780)] = 0.859c.
(b) vx= [(0) + (0.240c)]/[1 + (0)] = 0.240c, while
vy= (0.780c)p1(0.240)2/[1 + (0)] = 0.757c.
Then v=p(0.240c)2+ (0.757c)2= 0.794c.
(b) v0
x= [(0) (0.240c)]/[1 + (0)] = 0.240c, while
v0
y= (0.780c)p1(0.240)2/[1 + (0)] = 0.757c.
Then v0=p(0.240c)2+ (0.757c)2= 0.794c.
260
P20-7 If we look back at the boost equation we might notice that it looks very similar to the
rule for the tangent of the sum of two angles. It is exactly the same as the rule for the hyperbolic
tangent,
tanh(α1+α2) = tanh α1+ tanh α2
1 + tanh α1tanh α2
.
This means that each boost of β= 0.5 is the same as a “hyperbolic” rotation of αrwhere tanh αr=
0.5. We need only add these rotations together until we get to αf, where tanh αf= 0.999.
αf= 3.800, and αR= 0.5493. We can fit (3.800)/(0.5493) = 6.92 boosts, but we need an integral
number, so there are seven boosts required. The final speed after these seven boosts will be 0.9991c.
P20-8 (a) If ∆x0= 0, then x=ut, or
u= (730 m)/(4.96×106s) = 1.472×108m/s=0.491c.
(b) γ= 1/p1(0.491)2= 1.148,
t0= (1.148)[(4.96×106s) (0.491)(730 m)/(3×108)] = 4.32×106s.
P20-9 Since the maximum value for uis c, then the minimum ∆tis
t(730 m)/(3.00×108m/s) = 2.43×106s.
P20-10 (a) Yes.
(b) The speed will be very close to the speed of light, consequently γ(23,000)/(30) = 766.7.
Then
v=p11211/2γ2= 1 1/2(766.7)2= 0.99999915c.
P20-11 (a) ∆t0= (5.00 µs)p1(0.6)2= 4.00 µs.
(b) Note: it takes time for the reading on the S0clock to be seen by the Sclock. In this case,
t1+ ∆t2= 5.00 µs, where ∆t1=x/u and ∆t2=x/c. Solving for ∆t1,
t1=(5.00 µs)/(0.6c)
1/(0.6c)+1/c = 3.125 s,
and
t0
1= (3.125 µs)p1(0.6)2= 2.50 µs.
P20-12 The only change in the components of ∆roccur parallel to the boost. Then we can choose
the boost to be parallel to ∆rand then
r0=γ[∆ru(0)] = γrr,
since γ1.
P20-13 (a) Start with Eq. 20-34,
E2= (pc)2+ (mc2)2,
and substitute into this E=K+mc2,
K2+ 2Kmc2+ (mc2)2= (pc)2+ (mc2)2.
261
We can rearrange this, and then
K2+ 2Kmc2= (pc)2,
m=(pc)2K2
2Kc2
(b) As v/c 0 we have K1
2mv2and pmv, the classical limits. Then the above expression
becomes
m=m2v2c21
4m2v4
mv2c2,
=mv2c21
4v4
v2c2,
=m11
4
v2
c2
But v/c 0, so this expression reduces to m=min the classical limit, which is a good thing.
(c) We get
m=(121 MeV)2(55.0 MeV)2
2(55.0 MeV)c2= 1.06 MeV/c2,
which is (1.06 MeV/c2)/(0.511 MeV/c2) = 207me. A muon.
P20-14 Since Emc2the particle is ultra-relativistic and vc.γ= (135)/(0.1396) = 967.
Then the particle has a lab-life of ∆t0= (967)(35.0×109s) = 3.385×105s. The distance traveled
is
x= (3.00×108m/s)(3.385×105s) = 1.016×104m,
so the pion decays 110 km above the Earth.
P20-15 (a) A completely inelastic collision means the two particles, each of mass m1, stick
together after the collision, in effect becoming a new particle of mass m2. We’ll use the subscript 1
for moving particle of mass m1, the subscript 0 for the particle which is originally at rest, and the
subscript 2 for the new particle after the collision. We need to conserve momentum,
p1+p0=p2,
γ1m1u1+ (0) = γ2m2u2,
and we need to conserve total energy,
E1+E0=E2,
γ1m1c2+m1c2=γ2m2c2,
Divide the momentum equation by the energy equation and then
γ1u1
γ1+ 1 =u2.
But u1=cp112
1, so
u2=cγ1p112
1
γ1+ 1 ,
=cpγ2
11
γ1+ 1 ,
262
=cp(γ1+ 1)(γ11)
γ1+ 1 ,
=crγ11
γ1+ 1.
(b) Using the momentum equation,
m2=m1
γ1u1
γ2u2
,
=m1
1p112
1
u2/p1(u2/c)2,
=m1pγ2
11
1/p(c/u2)21,
=m1pγ2
11
1/p(γ1+ 1)/(γ11) 1,
=m1p(γ1+ 1)(γ11)
p(γ11)/2,
=m1p2(γ1+ 1).
P20-16 (a) K=W=RF dx =R(dp/dt)dx =R(dx/dt)dp =Rv dp.
(b) dp =mγ dv +mv(/dv)dv. Now use Maple or Mathematica to save time, and get
dp =m dv
(1 v2/c2)1/2+mv2dv
c2(1 v2/c2)3/2.
Now integrate:
K=Zvm
(1 v2/c2)1/2+mv2
c2(1 v2/c2)3/2dv,
=mv2
p1v2/c2.
P20-17 (a) Since E=K+mc2, then
Enew = 2E= 2mc2+ 2K= 2mc2(1 + K/mc2).
(b) Enew = 2(0.938 GeV) + 2(100 GeV) = 202 GeV.
(c) K= (100 GeV)/2(0.938 GeV) = 49.1 GeV.
P20-18 (a) Assume only one particle is formed. That particle can later decay, but it sets the
standard on energy and momentum conservation. The momentum of this one particle must equal
that of the incident proton, or
p2c2= [(mc2+K)2m2c4].
The initial energy was K+ 2mc2, so the mass of the “one” particle is given by
M2c4= [(K+ 2mc2)2p2c2] = 2Kmc2+ 4m2c4.
This is a measure of the available energy; the remaining energy is required to conserve momentum.
Then
Enew =M2c4= 2mc2p1 + K/2mc2.
263
P20-19 The initial momentum is ivi. The final momentum is (Mm)γfvf. Manipulating the
momentum conservation equation,
ivi= (Mm)γfvf,
1
iβi
=p1βf2
(Mm)βf
,
Mm
iβi
=1
βf21,
Mm
iβi
+ 1 = 1
βf2,
264
E21-1 (a) We’ll assume that the new temperature scale is related to the Celsius scale by a linear
transformation; then TS=mTC+b, where mand bare constants to be determined, TSis the
temperature measurement in the “new” scale, and TCis the temperature measurement in Celsius
degrees.
One of our known points is absolute zero;
TS=mTC+b,
(0) = m(273.15C) + b.
We have two other points, the melting and boiling points for water,
(TS)bp =m(100C) + b,
(TS)mp =m(0C) + b;
we can subtract the top equation from the bottom equation to get
(TS)bp (Ts)Smp = 100 Cm.
We are told this is 180 S, so m= 1.8 S/C. Put this into the first equation and then find b,
b= 273.15Cm= 491.67S.The conversion is then
TS= (1.8 S/C)TC+ (491.67S).
(b) The melting point for water is 491.67S; the boiling point for water is 180 Sabove this, or
671.67S.
E21-2 TF= 9(273.15 deg C)/5 + 32F = 459.67F.
E21-3 (a) We’ll assume that the new temperature scale is related to the Celsius scale by a linear
transformation; then TS=mTC+b, where mand bare constants to be determined, TSis the
temperature measurement in the “new” scale, and TCis the temperature measurement in Celsius
degrees.
One of our known points is absolute zero;
TS=mTC+b,
(0) = m(273.15C) + b.
We have two other points, the melting and boiling points for water,
(TS)bp =m(100C) + b,
(TS)mp =m(0C) + b;
we can subtract the top equation from the bottom equation to get
(TS)bp (Ts)Smp = 100 Cm.
We are told this is 100 Q, so m= 1.0 Q/C. Put this into the first equation and then find b,
b= 273.15C = 273.15Q.The conversion is then
TS=TC+ (273.15S).
(b) The melting point for water is 273.15Q; the boiling point for water is 100 Qabove this, or
373.15Q.
(c) Kelvin Scale.
265
E21-4 (a) T= (9/5)(6000 K 273.15) + 32 = 10000F.
(b) T= (5/9)(98.6F32) = 37.0C.
(c) T= (5/9)(70F32) = 57C.
(d) T= (9/5)(183C) + 32 = 297F.
(e) It depends on what you think is hot. My mom thinks 79F is too warm; that’s T=
(5/9)(79F32) = 26C.
E21-5 T= (9/5)(310 K 273.15) + 32 = 98.3F, which is fine.
E21-6 (a) T= 2(5/9)(T32), so T/10 = 32, or T= 320F.
(b) 2T= (5/9)(T32), so 13T/5 = 32, or T=12.3F.
E21-7 If the temperature (in Kelvin) is directly proportional to the resistance then T=kR,
where kis a constant of proportionality. We are given one point, T= 273.16 K when R= 90.35 Ω,
but that is okay; we only have one unknown, k. Then (273.16 K) = k(90.35 Ω) or k= 3.023 K/Ω.
If the resistance is measured to be R= 96.28 Ω, we have a temperature of
T=kR = (3.023 K/Ω)(96.28 Ω) = 291.1 K.
E21-8 T= (510C)/(0.028 V)V, so T= (1.82×104C/V)(0.0102 V) = 186C.
E21-9 We must first find the equation which relates gain to temperature, and then find the gain
at the specified temperature. If we let Gbe the gain we can write this linear relationship as
G=mT +b,
where mand bare constants to be determined. We have two known points:
(30.0) = m(20.0C) + b,
(35.2) = m(55.0C) + b.
If we subtract the top equation from the bottom we get 5.2 = m(35.0C),or m= 1.49 C1. Put
this into either of the first two equations and
(30.0) = (0.149 C1)(20.0C) + b,
which has a solution b= 27.0
Now to find the gain when T= 28.0C:
G=mT +b= (0.149 C1)(28.0C) + (27.0) = 31.2
E21-10 p/ptr = (373.15 K)/(273.16 K) = 1.366.
E21-11 100 cm Hg is 1000 torr. PHe = (100 cm Hg)(373 K)/(273.16 K) = 136.550 cm Hg. Ni-
trogen records a temperature which is 0.2 K higher, so PN= (100 cm Hg)(373.2 K)/(273.16 K) =
136.623 cm Hg. The difference is 0.073 cm Hg.
E21-12 L= (23×106/C)(33 m)(15C) = 1.1×102m.
E21-13 L= (3.2×106/C)(200 in)(60C) = 3.8×102in.
266
E21-14 L0= (2.725cm)[1 + (23×106/C)(128C)] = 2.733 cm.
E21-15 We want to focus on the temperature change, not the absolute temperature. In this
case, ∆T=TfTi= (42C) (5.0C) = 47 C.
Then
L= (11 ×106C1)(12.0 m)(47 C) = 6.2×103m.
E21-16 A= 2αAT, so
A= 2(9×106/C)(2.0 m)(3.0 m)(30C) = 3.2×103m2.
E21-17 (a) We’ll apply Eq. 21-10. The surface area of a cube is six times the area of one face,
which is the edge length squared. So A= 6(0.332 m)2= 0.661 m2.The temperature change is
T= (75.0C) (20.0C) = 55.0 C. Then the increase in surface area is
A= 2αAT= 2(19 ×106C1)(0.661 m2)(55.0 C) = 1.38 ×103m2
(b) We’ll now apply Eq. 21-11. The volume of the cube is the edge length cubed, so
V= (0.332 m)3= 0.0366 m3.
and then from Eq. 21-11,
V= 2αV T= 3(19 ×106C1)(0.0366 m3)(55.0 C) = 1.15 ×104m3,
is the change in volume of the cube.
E21-18 V0=V(1 + 3αT), so
V0= (530 cm3)[1 + 3(29×106/C)(172 C)] = 522 cm3.
E21-19 (a) The slope is approximately 1.6×104/C.
(b) The slope is zero.
E21-20 r= (β/3)rT, so
r= [(3.2×105/K)/3](6.37×106m)(2700 K) = 1.8×105m.
E21-21 We’ll assume that the steel ruler measures length correctly at room temperature. Then
the 20.05 cm measurement of the rod is correct. But both the rod and the ruler will expand in the
oven, so the 20.11 cm measurement of the rod is not the actual length of the rod in the oven. What
is the actual length of the rod in the oven? We can only answer that after figuring out how the 20.11
cm mark on the ruler moves when the ruler expands.
Let L= 20.11 cm correspond to the ruler mark at room temperature. Then
L=αsteelLT= (11 ×106C1)(20.11 cm)(250 C) = 5.5×102cm
is the shift in position of the mark as the ruler is raised to the higher temperature. Then the change
in length of the rod is not (20.11 cm) (20.05 cm) = 0.06 cm, because the 20.11 cm mark is shifted
out. We need to add 0.055 cm to this; the rod changed length by 0.115 cm.
The coefficient of thermal expansion for the rod is
α=L
LT=(0.115 cm)
(20.05 cm)(250 C)= 23 ×106C1.
267
E21-22 A=ab,A0= (a+ ∆a)(b+ ∆b) = ab +ab+ba+ ∆ab, so
A=ab+ba+ ∆ab,
=A(∆b/b + ∆a/a + ∆ab/ab),
A(αT+αT),
= 2αAT.
E21-23 Solve this problem by assuming the solid is in the form of a cube.
If the length of one side of a cube is originally L0, then the volume is originally V0=L3
0. After
heating, the volume of the cube will be V=L3, where L=L0+ ∆L.
Then
V=L3,
= (L0+ ∆L)3,
= (L0+αL0T)3,
=L3
0(1 + αT)3.
As long as the quantity αTis much less than one we can expand the last line in a binomial
expansion as
VV0(1 + 3αT+···),
so the change in volume is ∆V3αV0T.
E21-24 (a) ∆A/A = 2(0.18%) = (0.36%).
(b) ∆L/L = 0.18%.
(c) ∆V/V = 3(0.18%) = (0.54%).
(d) Zero.
(e) α= (0.0018)/(100 C) = 1.8×105/C.
E21-25 ρ0ρ=m/V 0m/V =m/(V+ ∆V)m/V ≈ −mV/V 2.Then
ρ=(m/V )(∆V/V ) = ρβT.
E21-26 Use the results of Exercise 21-25.
(a) ∆V/V = 3∆L/L = 3(0.092%) = 0.276%. The change in density is
ρ/ρ =V/V =(0.276%) = 0.28
(b) α=β/3 = (0.28%)/3(40 C) = 2.3×105/C.Must be aluminum.
E21-27 The diameter of the rod as a function of temperature is
ds=ds,0(1 + αsT),
The diameter of the ring as a function of temperature is
db=db,0(1 + αbT).
268
We are interested in the temperature when the diameters are equal,
ds,0(1 + αsT) = db,0(1 + αbT),
αsds,0Tαbdb,0T=db,0ds,0,
T=db,0ds,0
αsds,0αbdb,0
,
T=(2.992 cm) (3.000 cm)
(11×106/C)(3.000 cm) (19×106/C)(2.992 cm) ,
= 335 C.
The final temperature is then Tf= (25) + 335 C= 360.
E21-28 (a) ∆L= ∆L1+ ∆L2= (L1α1+L2α2)∆T. The effective value for αis then
α=L
LT=α1L1+α2L2
L.
(b) Since L2=LL1we can write
α1L1+α2(LL1) = αL,
L1=Lαα2
α1α2
,
= (0.524 m)(13×106)(11×106)
(19×106)(11×106)= 0.131 m.
The brass length is then 13.1 cm and the steel is 39.3 cm.
E21-29 At 100C the glass and mercury each have a volume V0. After cooling, the difference in
volume changes is given by
V=V0(3αgβm)∆T.
Since m=ρV , the mass of mercury that needs to be added can be found by multiplying though by
the density of mercury. Then
m= (0.891 kg)[3(9.0×106/C)(1.8×104/C)](135C) = 0.0184 kg.
This is the additional amount required, so the total is now 909 g.
E21-30 (a) The rotational inertia is given by I=Rr2dm; changing the temperature requires
rr0=r+ ∆r=r(1 + αT). Then
I0=Z(1 + αT)2r2dm (1 + 2αT)Zr2dm,
so ∆I= 2αIT.
(b) Since L=Iω, then 0 = ωI+Iω. Rearranging, ∆ω=I/I =2αT. Then
ω=2(19×106/C)(230 rev/s)(170 C) = 1.5 rev/s.
269
E21-31 This problem is related to objects which expand when heated, but we never actually need
to calculate any temperature changes. We will, however, be interested in the change in rotational
inertia. Rotational inertia is directly proportional to the square of the (appropriate) linear dimension,
so
If/Ii= (rf/ri)2.
(a) If the bearings are frictionless then there are no external torques, so the angular momentum
is constant.
(b) If the angular momentum is constant, then
Li=Lf,
Iiωi=Ifωf.
We are interested in the percent change in the angular velocity, which is
ωfωi
ωi
=ωf
ωi1 = Ii
If1 = ri
rf2
1 = 1
1.00182
1 = 0.36%.
(c) The rotational kinetic energy is proportional to Iω2= (Iω)ω=, but Lis constant, so
KfKi
Ki
=ωfωi
ωi
=0.36%.
E21-32 (a) The period of a physical pendulum is given by Eq. 17-28. There are two variables
in the equation that depend on length. I, which is proportional to a length squared, and d, which
is proportional to a length. This means that the period have an overall dependence on length
proportional to r. Taking the derivative,
PdP =1
2
P
rdr 1
2P αT.
(b) ∆P/P = (0.7×106C)(10C)/2 = 3.5×106.After 30 days the clock will be slow by
t= (30 ×24 ×60 ×60 s)(3.5×106) = 9.07 s.
E21-33 Refer to the Exercise 21-32.
P= (3600 s)(19×106C)(20C)/2 = 0.68 s.
E21-34 At 22C the aluminum cup and glycerin each have a volume V0. After heating, the
difference in volume changes is given by
V=V0(3αaβg)∆T.
The amount that spills out is then
V= (110 cm3)[3(23×106/C)(5.1×104/C)](6C) = 0.29 cm3.
E21-35 At 20.0C the glass tube is filled with liquid to a volume V0. After heating, the difference
in volume changes is given by
V=V0(3αgβl)∆T.
The cross sectional area of the tube changes according to
A=A02αgT.
270
Consequently, the height of the liquid changes according to
V= (h0+ ∆h)(A0+ ∆A)h0A,
h0A+A0h,
V/V0= ∆A/A0+ ∆h/h0.
Then
h= (1.28 m/2)[(1.1×105/C)(4.2×105/C)](13 C) = 2.6×104m.
E21-36 (a) β= (dV/dT )/V . If pV =nRT , then p dV =nR dT , so
β= (nR/p)/V =nR/pV = 1/T.
(b) Kelvins.
(c) β1/(300/K) = 3.3×103/K.
E21-37 (a) V= (1 mol)(8.31 J/mol ·K)(273 K)/(1.01×105Pa) = 2.25×102m3.
(b) (6.02×1023mol1)/(2.25×104/cm3) = 2.68×1019.
E21-38 n/V =p/kT , so
n/V = (1.01×1013Pa)/(1.38×1023J/K)(295 K) = 25 part/cm3.
E21-39 (a) Using Eq. 21-17,
n=pV
RT =(108×103Pa)(2.47 m3)
(8.31 J/mol·K)([12 + 273] K) = 113 mol.
(b) Use the same expression again,
V=nRT
p=(113 mol)(8.31 J/mol·K)([31 + 273] K)
(316×103Pa) = 0.903 m3.
E21-40 (a) n=pV/RT = (1.01×105Pa)(1.13×103m3)/(8.31 J/mol·K)(315 K) = 4.36×102mol.
(b) Tf=TipfVf/piVi, so
Tf=(315 K)(1.06×105Pa)(1.530×103m3)
(1.01×105Pa)(1.130×103m3)= 448 K.
E21-41 pi= (14.7 + 24.2) lb/in2= 38.9 lb/in2.pf=piTfVi/T iVf, so
pf=(38.9 lb/in2)(299K)(988 in3)
(270 K)(1020 in3)= 41.7 lb/in2.
The gauge pressure is then (41.714.7) lb/in2= 27.0 lb/in2.
E21-42 Since p=F/A and F=mg, a reasonable estimate for the mass of the atmosphere is
m=pA/g = (1.01×105Pa)4π(6.37×106m)2/(9.81 m/s2) = 5.25×1018kg.
271
E21-43 p=p0+ρgh, where his the depth. Then Pf= 1.01×105Pa and
pi= (1.01×105Pa) + (998 kg/m3)(9.81 m/s2)(41.5 m) = 5.07×105Pa.
Vf=VipiTf/pfTi, so
Vf=(19.4 cm3)(5.07×105Pa.)(296 K)
(1.01×105Pa)(277 K) = 104 cm3.
E21-44 The new pressure in the pipe is
pf=piVi/V f= (1.01×105Pa)(2) = 2.02×105Pa.
The water pressure at some depth yis given by p=p0+ρgy, so
y=(2.02×105Pa) (1.01×105Pa)
(998 kg/m3)(9.81 m/s2)= 10.3 m.
Then the water/air interface inside the tube is at a depth of 10.3 m; so h= (10.3 m) + (25.0 m)/2 =
22.8 m.
P21-1 (a) The dimensions of Amust be [time]1, as can be seen with a quick inspection of the
equation. We would expect that Awould depend on the surface area at the very least; however,
that means that it must also depend on some other factor to fix the dimensionality of A.
(b) Rearrange and integrate,
ZT
T0
dT
T=Zt
0
A dt,
ln(∆T/T0) = At,
T= ∆T0eAt.
P21-2 First find A.
A=ln(∆T0/T)
t=ln[(29 C)/(25 C)]
(45 min) = 3.30×103/min.
Then find time to new temperature difference.
t=ln(∆T0/T)
t=ln[(29 C)/(21 C)]
(3.30×103/min) = 97.8min
This happens 97.845 = 53 minutes later.
P21-3 If we neglect the expansion of the tube then we can assume the cross sectional area of the
tube is constant. Since V=Ah, we can assume that ∆V=Ah. Then since ∆V=βV0T, we
can write ∆h=βh0T.
P21-4 For either container we can write piVi=niRTi.We are told that Viand niare constants.
Then ∆p=AT1BT2,where Aand Bare constants. When T1=T2p= 0, so A=B. When
T1=Ttr and T2=Tbwe have
(120 mm Hg) = A(373 K 273.16 K),
so A= 1.202 mm Hg/K. Then
T=(90 mm Hg) + (1.202 mm Hg/K)(273.16 K)
(1.202 mm Hg/K) = 348 K.
Actually, we could have assumed Awas negative, and then the answer would be 198 K.
272
P21-5 Start with a differential form for Eq. 21-8, dL/dT =αL0, rearrange, and integrate:
ZL
L0
dL =ZT
T0
αL0dT,
LL0=L0ZT
T0
α dT,
L=L0 1 + ZT
T0
α dT !.
P21-6 L=αLT, so
T
t=1
αL
L
t=(96×109m/s)
(23×106/C)(1.8×102m) = 0.23C/s.
P21-7 (a) Consider the work that was done for Ex. 21-27. The length of rod ais
La=La,0(1 + αaT),
while the length of rod bis
Lb=Lb,0(1 + αbT).
The difference is
LaLb=La,0(1 + αaT)Lb,0(1 + αbT),
=La,0Lb,0+ (La,0αaLb,0αb)∆T,
which will be a constant is La,0αa=Lb,0αbor
Li,01i.
(b) We want La,0Lb,0= 0.30 m so
kak/αb= 0.30 m,
where kis a constant of proportionality;
k= (0.30 m)/1/(11×106/C)1/(19×106/C)= 7.84×106m/C.
The two lengths are
La= (7.84×106m/C)/(11×106/C) = 0.713 m
for steel and
Lb= (7.84×106m/C)/(19×106/C) = 0.413 m
for brass.
P21-8 The fractional increase in length of the bar is ∆L/L0=αT. The right triangle on the left
has base L0/2, height x, and hypotenuse (L0+ ∆L)/2. Then
x=1
2q(L0+ ∆L)2L2
0=L0
2r2L
L0
.
With numbers,
x=(3.77 m)
2p2(25×106/C)(32 C) = 7.54×102m.
273
P21-9 We want to evaluate V=V0(1 + Rβ dT ); the integral is the area under the graph; the
graph looks like a triangle, so the result is
V=V0[1 + (16 C)(0.0002/C)/2] = (1.0016)V0.
The density is then
ρ=ρ0(V0/V ) = (1000 kg/m3)/(1.0016) = 0.9984 kg/m3.
P21-10 At 0.00C the glass bulb is filled with mercury to a volume V0. After heating, the difference
in volume changes is given by
V=V0(β3α)∆T.
Since T0= 0.0C, then ∆T=T, if it is measured in C. The amount of mercury in the capillary is
V, and since the cross sectional area is fixed at A, then the length is L= ∆V/A, or
L=V
A(β3α)∆T.
P21-11 Let a,b, and ccorrespond to aluminum, steel, and invar, respectively. Then
cos C=a2+b2c2
2ab .
We can replace awith a0(1 + αaT), and write similar expressions for band c. Since a0=b0=c0,
this can be simplified to
cos C=(1 + αaT)2+ (1 + αbT)2(1 + αcT)2
2(1 + αaT)(1 + αbT).
Expand this as a Taylor series in terms of ∆T, and we find
cos C1
2+1
2(αa+αb2αc) ∆T.
Now solve:
T=2 cos(59.95)1
(23×106/C) + (11×106/C)2(0.7×106/C)= 46.4C.
The final temperature is then 66.4C.
P21-12 The bottom of the iron bar moves downward according to ∆L=αLT. The center of
mass of the iron bar is located in the center; it moves downward half the distance. The mercury
expands in the glass upwards; subtracting off the distance the iron moves we get
h=βhTL= (βh αL)∆T.
The center of mass in the mercury is located in the center. If the center of mass of the system is to
remain constant we require
miL/2 = mm(∆hL)/2;
or, since ρ=mV =mAy,
ρiαL =ρm(βh 2αL).
Solving for h,
h=(12×106/C)(1.00 m)[(7.87×103kg/m3) + 2(13.6×103kg/m3)]
(13.6×103kg/m3)(18×105/C)= 0.17 m.
274
P21-13 The volume of the block which is beneath the surface of the mercury displaces a mass
of mercury equal to the mass of the block. The mass of the block is independent of the temperature
but the volume of the displaced mercury changes according to
Vm=Vm,0(1 + βmT).
This volume is equal to the depth which the block sinks times the cross sectional area of the block
(which does change with temperature). Then
hshb2=hs,0hb,02(1 + βmT),
where hsis the depth to which the block sinks and hb,0= 20 cm is the length of the side of the
block. But
hb=hb,0(1 + αbT),
so
hs=hs,0
1 + βmT
(1 + αbT)2.
Since the changes are small we can expand the right hand side using the binomial expansion; keeping
terms only in ∆Twe get
hshs,0(1 + (βm2αb)∆T),
which means the block will sink a distance hshs,0given by
hs,0(βm2αb)∆T=hs,0(1.8×104/C)2(23×106/C)(50 C) = (6.7×103)hs,0.
In order to finish we need to know how much of the block was submerged in the first place. Since
the fraction submerged is equal to the ratio of the densities, we have
hs,0/hb,0=ρbm= (2.7×103kg/m3)/(1.36×104kg/m3),
so hs,0= 3.97 cm, and the change in depth is 0.27 mm.
P21-14 The area of glass expands according to ∆Ag= 2αgAgT. The are of Dumet wire expands
according to
Ac+ ∆Ai = 2(αcAc+αiAi)∆T.
We need these to be equal, so
αgAg=αcAc+αiAi,
αgrg2=αc(rc2ri2) + αiri2,
αg(rc2+ri2) = αc(rc2ri2) + αiri2,
ri2
rc2=αcαg
αcαi
.
P21-15
P21-16 V2=V1(p1/p2)(T1/T2),so
V2= (3.47 m3)[(76 cm Hg)/(36 cm Hg)][(225 K)/(295 K)] = 5.59 m3.
275
P21-17 Call the containers one and two so that V1= 1.22 L and V2= 3.18 L. Then the initial
number of moles in the two containers are
n1,i=piV1
RT i
and n2,i=piV2
RT i
.
The total is
n=pi(V1+V2)/(RT i).
Later the temperatures are changed and then the number of moles of gas in each container is
n1,f=pfV1
RT 1,f
and n2,f=pfV2
RT 2,f
.
The total is still n, so
pf
RV1
T1,f
+V2
T2,f=pi(V1+V2)
RT i
.
We can solve this for the final pressure, so long as we remember to convert all temperatures to
Kelvins,
pf=pi(V1+V2)
TiV1
T1,f
+V2
T2,f1
,
or
pf=(1.44 atm)(1.22L + 3.18 L)
(289 K) (1.22 L)
(289 K) +(3.18 L)
(381 K) 1
= 1.74 atm.
P21-18 Originally nA=pAVA/RTAand nB=pBVB/RTB;VB= 4VA. Label the final state of
Aas Cand the final state of Bas D. After mixing, nC=pCVA/RTAand nD=pDVB/RTB, but
PC=PDand nA+nB=nC+nD. Then
pA/TA+ 4pB/TB=pC(1/TA+ 4/TB),
or
pC=(5×105Pa)/(300 K) + 4(1×105Pa)/(400 K)
1/(300 K) + 4/(400 K)= 2.00×105Pa.
P21-19 If the temperature is uniform then all that is necessary is to substitute p0=nRT/V and
p=nRT/V ; cancel RT from both sides, and then equate n/V with nV.
P21-20 Use the results of Problem 15-19. The initial pressure inside the bubble is pi=p0+ 4γ/ri.
The final pressure inside the bell jar is zero, sopf= 4γ/rf. The initial and final pressure inside the
bubble are related by piri3=pfrf3= 4γrf2. Now for numbers:
pi= (1.01×105Pa) + 4(2.5×102N/m)/(2.0×103m) = 1.0105×105Pa.
and
rf=s(1.0105×105Pa)(2.0×103m)3
4(2.5×102N/m) = 8.99×102m.
P21-21
P21-22
276
E22-1 (a) n= (2.56 g)/(197 g/mol) = 1.30×102mol.
(b) N= (6.02×1023mol1)(1.30×102mol) = 7.83×1021.
E22-2 (a) N=pV/kT = (1.01×105Pa)(1.00 m3)/(1.38×1023J/K)(293 K) = 2.50×1025.
(b) n= (2.50×1025)/(6.02×1023mol1) = 41.5 mol.Then
m= (41.5 mol)[75%(28 g/mol) + 25%(32 g/mol)] = 1.20 kg.
E22-3 (a) We first need to calculate the molar mass of ammonia. This is
M=M(N) + 3M(H) = (14.0 g/mol) + 3(1.01 g/mol) = 17.0 g/mol
The number of moles of nitrogen present is
n=m/M r= (315 g)/(17.0 g/mol) = 18.5 mol.
The volume of the tank is
V=nRT/p = (18.5 mol)(8.31 J/mol ·K)(350 K)/(1.35×106Pa) = 3.99×102m3.
(b) After the tank is checked the number of moles of gas in the tank is
n=pV/(RT ) = (8.68×105Pa)(3.99×102m3)/[(8.31 J/mol ·K)(295 K)] = 14.1 mol.
In that case, 4.4 mol must have escaped; that corresponds to a mass of
m=nMr= (4.4 mol)(17.0 g/mol) = 74.8 g.
E22-4 (a) The volume per particle is V/N =kT/P , so
V/N = (1.38×1023J/K)(285 K)/(1.01×105Pa) = 3.89×1026m3.
The edge length is the cube root of this, or 3.39×109m. The ratio is 11.3.
(b) The volume per particle is V/NA, so
V/NA= (18×106m3)/(6.02×1023) = 2.99×1029m3.
The edge length is the cube root of this, or 3.10×1010m. The ratio is 1.03.
E22-5 The volume per particle is V/N =kT /P , so
V/N = (1.38×1023J/K)(308 K)/(1.22)(1.01×105Pa) = 3.45×1026m3.
The fraction actually occupied by the particle is
4π(0.710×1010m)3/3)
(3.45×1026m3)= 4.34×105.
E22-6 The component of the momentum normal to the wall is
py= (3.3×1027kg)(1.0×103m/s) cos(55) = 1.89×1024kg ·m/s.
The pressure exerted on the wall is
p=F
A=(1.6×1023/s)2(1.89×1024kg ·m/s)
(2.0×104m2)= 3.0×103Pa.
277
E22-7 (a) From Eq. 22-9,
vrms =r3p
ρ.
Then
p= 1.23 ×103atm 1.01 ×105Pa
1 atm = 124 Pa
and
ρ= 1.32 ×105g/cm31kg
1000 g100 cm
1 m 3
= 1.32 ×102kg/m3.
Finally,
vrms =s3(1240 Pa)
(1.32 ×102kg/m3)= 531 m/s.
(b) The molar density of the gas is just n/V ; but this can be found quickly from the ideal gas
law as n
V=p
RT =(1240 Pa)
(8.31 J/mol ·K)(317 K) = 4.71 ×101mol/m3.
(c) We were given the density, which is mass per volume, so we could find the molar mass from
ρ
n/V =(1.32 ×102kg/m3)
(4.71 ×101mol/m3)= 28.0 g/mol.
But what gas is it? It could contain any atom lighter than silicon; trial and error is the way to go.
Some of my guesses include C2H4(ethene), CO (carbon monoxide), and N2. There’s no way to tell
which is correct at this point, in fact, the gas could be a mixture of all three.
E22-8 The density is ρ=m/V =nMr/V , or
ρ= (0.350 mol)(0.0280 kg/mol)(0.125 m/2)2(0.560 m) = 1.43 kg/m3.
The rms speed is
vrms =s3(2.05)(1.01×105Pa)
(1.43 kg/m3)= 659 m/s.
E22-9 (a) N/V =p/kT = (1.01×105Pa)/(1.38×1023J/K)(273 K) = 2.68×1025/m3.
(b) Note that Eq. 22-11 is wrong; for the explanation read the last two paragraphs in the first
column on page 502. We need an extra factor of 2, so πd2=V/2Nλ, so
d=q1/2π(2.68×1025/m3)(285×109m) = 1.72×1010m.
E22-10 (a) λ=V/2Nπd2, so
λ=1
2(1.0×106/m3)π(2.0×1010m)2= 5.6×1012m.
(b) Particles effectively follow ballistic trajectories.
278
E22-11 We have v=fλ, where λis the wavelength (which we will set equal to the mean free
path), and vis the speed of sound. The mean free path is, from Eq. 22-13,
λ=kT
2πd2p
so
f=2πd2pv
kT =2π(315×1012m)2(1.02 ×1.01×105Pa)(343 m/s)
(1.38×1023J/K)(291 K) = 3.88×109Hz.
E22-12 (a) p= (1.10×106mm Hg)(133 Pa/mm Hg) = 1.46×104Pa. The particle density is
N/V = (1.46×104Pa)/(1.38×1023J/K)(295 K) = 3.59×1016/m3.
(b) The mean free path is
λ= 1/p(2)(3.59×1016/m3)π(2.20×1010m)2= 130 m.
E22-13 Note that vav T, while λT. Then the collision rate is proportional to 1/T. Then
T= (300 K)(5.1×109/s)2
(6.0×109/s)2= 216 K.
E22-14 (a) vav = (65 km/s)/(10) = 6.5 km/s.
(b) vrms =p(505 km/s)/(10) = 7.1 km/s.
E22-15 (a) The average is
4(200 s) + 2(500 m/s) + 4(600 m/s)
4+2+4 = 420 m/s.
The mean-square value is
4(200 s)2+ 2(500 m/s)2+ 4(600 m/s)2
4+2+4 = 2.1×105m2/s2.
The root-mean-square value is the square root of this, or 458 m/s.
(b) I’ll be lazy. Nine particles are not moving, and the tenth has a speed of 10 m/s. Then the
average speed is 1 m/s, and the root-mean-square speed is 3.16 m/s. Look, vrms is larger than vav!
(c) Can vrms=vav? Assume that the speeds are not all the same. Transform to a frame of
reference where vav = 0, then some of the individual speeds must be greater than zero, and some
will be less than zero. Squaring these speeds will result in positive, non-zero, numbers; the mean
square will necessarily be greater than zero, so vrms >0.
Only if all of the particles have the same speed will vrms=vav.
E22-16 Use Eq. 22-20:
vrms =s3(1.38×1023J/K)(329 K)
(2.33×1026kg + 3 ×1.67×1027kg) = 694 m/s.
E22-17 Use Eq. 22-20:
vrms =s3(1.38×1023J/K)(2.7 K)
(2 ×1.67×1027kg) = 180 m/s.
279
E22-18 Eq. 22-14 is in the form N=Av2eBv2. Taking the derivative,
dN
dv = 2AveBv22ABv3eBv2,
and setting this equal to zero,
v2= 1/B = 2kT/m.
E22-19 We want to integrate
vav =1
NZ
0
N(v)v dv,
=1
NZ
0
4πN m
2πkT 3/2v2emv2/2kT v dv,
= 4πm
2πkT 3/2Z
0
v2emv2/2kT v dv.
The easiest way to attack this is first with a change of variables— let x=mv2/2kT , then kT dx =
mv dv. The limits of integration don’t change, since =. Then
vav = 4πm
2πkT 3/2Z
0
2kT
mxexkT
mdx,
= 2 2kT
πm 1/2Z
0
xeαxdx
The factor of αthat was introduced in the last line is a Feynman trick; we’ll set it equal to one when
we are finished, so it won’t change the result.
Feynman’s trick looks like
d
Zeαxdx =Z
α eαxdx =Z(x)eαxdx.
Applying this to our original problem,
vav = 2 2kT
πm 1/2Z
0
xeαxdx,
=d
22kT
πm 1/2Z
0
eαxdx,
=22kT
πm 1/2d
1
αeαx
0,
=22kT
πm 1/2d
1
α,
=22kT
πm 1/21
α2.
We promised, however, that we would set α= 1 in the end, so this last line is
vav = 2 2kT
πm 1/2
,
=r8kT
πm .
280
E22-20 We want to integrate
(v2)av =1
NZ
0
N(v)v2dv,
=1
NZ
0
4πN m
2πkT 3/2v2emv2/2kT v2dv,
= 4πm
2πkT 3/2Z
0
v2emv2/2kT v2dv.
The easiest way to attack this is first with a change of variables— let x2=mv2/2kT , then
p2kT/mdx =dv. The limits of integration don’t change. Then
(v2)av = 4πm
2πkT 3/2Z
02kT
m5/2
x4ex2dx,
=8kT
π m Z
0
x4ex2dx
Look up the integral; although you can solve it by first applying a Feynman trick (see solution to
Exercise 22-21) and then squaring the integral and changing to polar coordinates. I looked it up. I
found 3π/8, so
(v2)av =8kT
π m3π/8 = 3kT/m.
E22-21 Apply Eq. 22-20:
vrms =p3(1.38×1023J/K)(287 K)/(5.2×1017kg) = 1.5×102m/s.
E22-22 Since vrms pT/m, we have
T= (299 K)(4/2) = 598 K,
or 325C.
E22-23 (a) The escape speed is found on page 310; v= 11.2×103m/s. For hydrogen,
T= (2)(1.67×1027kg)(11.2×103m/s)2/3(1.38×1023J/K) = 1.0×104K.
For oxygen,
T= (32)(1.67×1027kg)(11.2×103m/s)2/3(1.38×1023J/K) = 1.6×105K.
(b) The escape speed is found on page 310; v= 2.38×103m/s. For hydrogen,
T= (2)(1.67×1027kg)(2.38×103m/s)2/3(1.38×1023J/K) = 460K.
For oxygen,
T= (32)(1.67×1027kg)(2.38×103m/s)2/3(1.38×1023J/K) = 7300K.
(c) There should be more oxygen than hydrogen.
E22-24 (a) vav = (70 km/s)/(22) = 3.18 km/s.
(b) vrms =q(250 km2/s2)/(22) = 3.37 km/s.
(c) 3.0 km/s.
281
E22-25 According to the equation directly beneath Fig. 22-8,
ω=vφ/L = (212 m/s)(0.0841 rad)/(0.204 m) = 87.3 rad/s.
E22-26 If vp=vrms then 2T2= 3T1, or T2/T1= 3/2.
E22-27 Read the last paragraph on the first column of page 505. The distribution of speeds is
proportional to
v3emv2/2kT =v3eBv2,
taking the derivative dN/dv and setting equal to zero yields
dN
dv = 3v2eBv22Bv4eBv2,
and setting this equal to zero,
v2= 3/2B= 3kT/m.
E22-28 (a) v=p3(8.31 J/mol ·K)(4220 K)/(0.07261 kg/mol) = 1200 m/s.
(b) Half of the sum of the diameters, or 273 pm.
(c) The mean free path of the germanium in the argon is
λ= 1/2(4.13×1025/m3)π(273×1012m)2= 7.31×108m.
The collision rate is
(1200 m/s)/(7.31×108m) = 1.64×1010/s.
E22-29 The fraction of particles that interests us is
2
π
1
(kT )3/2Z0.03kT
0.01kT
E1/2eE/kT dE.
Change variables according to E/kT =x, so that dE =kT dx. The integral is then
2
πZ0.03
0.01
x1/2exdx.
Since the value of xis so small compared to 1 throughout the range of integration, we can expand
according to
ex1xfor x1.
The integral then simplifies to
2
πZ0.03
0.01
x1/2(1 x)dx =2
π2
3x3/22
5x5/20.03
0.01
= 3.09×103.
E22-30 Write N(E) = N(Ep+). Then
N(Ep+)N(Ep) + dN(E)
dE Ep
+...
But the very definition of Epimplies that the first derivative is zero. Then the fraction of [particles
with energies in the range Ep±0.02kT is
2
π
1
(kT )3/2(kT /2)1/2e1/2(0.02kT ),
or 0.04p1/2= 9.68×103.
282
E22-31 The volume correction is on page 508; we need first to find d. If we assume that the
particles in water are arranged in a cubic lattice (a bad guess, but we’ll use it anyway), then 18
grams of water has a volume of 18×106m3, and
d3=(18×106m3)
(6.02×1023)= 3.0×1029m3
is the volume allocated to each water molecule. In this case d= 3.1×1010m. Then
b=1
2(6.02×1023)( 4
3π(3.1×1010m)3) = 3.8 m3/mol.
E22-32 d3= 3b/2πNA, or
d=3
s3(32×106m3/mol)
2π(6.02×1023/mol) = 2.9×1010m.
E22-33 ahas units of energy volume per square mole, which is the same as energy per mole times
volume per mole.
P22-1 Solve (1 x)(1.429) + x(1.250) = 1.293 for x. The result is x= 0.7598.
P22-2
P22-3 The only thing that matters is the total number of moles of gas (2.5) and the number of
moles of the second gas (0.5). Since 1/5 of the total number of moles of gas is associated with the
second gas, then 1/5 of the total pressure is associated with the second gas.
P22-4 Use Eq. 22-11 with the appropriate 2 inserted.
λ=(1.0×103m3)
2(35)π(1.0×102m)2= 6.4×102m.
P22-5 (a) Since λ1/d2, we have
da
dn
=rλn
λa
=s(27.5×108m)
(9.90×108m) = 1.67.
(b) Since λ1/p, we have
λ2=λ1
p1
p2
= (9.90×108m) (75.0 cm Hg)
(15.0 cm Hg) = 49.5×108m.
(c) Since λT, we have
λ2=λ1
T2
T1
= (9.90×108m) (233 K)
(293 K) = 7.87×108m.
283
P22-6 We can assume the molecule will collide with something. Then
1 = Z
0
Aecrdr =A/c,
so A=c. If the molecule has a mean free path of λ, then
λ=Z
0
rcecrdr = 1/c,
so A=c= 1.
P22-7 What is important here is the temperature; since the temperatures are the same then the
average kinetic energies per particle are the same. Then
1
2m1(vrms,1)2=1
2m2(vrms,2)2.
We are given in the problem that vav,2= 2vrms,1. According to Eqs. 22-18 and 22-20 we have
vrms =r3RT
M=r3π
8r8RT
πM =r3π
8vav.
Combining this with the kinetic energy expression above,
m1
m2
=vrms,2
vrms,12
= 2r3π
8!2
= 4.71.
P22-8 (a) Assume that the speeds are not all the same. Transform to a frame of reference where
vav = 0, then some of the individual speeds must be greater than zero, and some will be less
than zero. Squaring these speeds will result in positive, non-zero, numbers; the mean square will
necessarily be greater than zero, so vrms >0.
(b) Only if all of the particles have the same speed will vrms=vav.
P22-9 (a) We need to first find the number of particles by integrating
N=Z
0
N(v)dv,
=Zv0
0
Cv2dv +Z
v0
(0) dv =CZv0
0
v2dv =C
3v3
0.
Invert, then C= 3N/v3
0.
(b) The average velocity is found from
vav =1
NZ
0
N(v)v dv.
Using our result from above,
vav =1
NZv0
03N
v3
0
v2v dv,
=3
v3
0Zv0
0
v3dv =3
v3
0
v4
0
4=3
4v0.
284
As expected, the average speed is less than the maximum speed. We can make a prediction about
the root mean square speed; it will be larger than the average speed (see Exercise 22-15 above) but
smaller than the maximum speed.
(c) The root-mean-square velocity is found from
v2
rms =1
NZ
0
N(v)v2dv.
Using our results from above,
v2
rms =1
NZv0
03N
v3
0
v2v2dv,
=3
v3
0Zv0
0
v4dv =3
v3
0
v5
0
5=3
5v2
0.
Then, taking the square root,
v2
rms =r3
5v0
Is p3/5>3/4? It had better be.
P22-10
P22-11
P22-12
P22-13
P22-14
P22-15 The mass of air displaced by 2180 m3is m= (1.22 kg/m3)(2180 m3) = 2660 kg.The mass
of the balloon and basket is 249 kg and we want to lift 272 kg; this leaves a remainder of 2140 kg for
the mass of the air inside the balloon. This corresponds to (2140 kg)/(0.0289 kg/mol) = 7.4×104mol.
The temperature of the gas inside the balloon is then
T= (pV )/(nR) = [(1.01×105Pa)(2180 m3)]/[(7.4×104mol)(8.31 J/mol ·K) = 358 K.
That’s 85C.
P22-16
P22-17
285
E23-1 We apply Eq. 23-1,
H=kAT
x
The rate at which heat flows out is given as a power per area (mW/m2), so the quantity given is
really H/A. Then the temperature difference is
T=H
A
x
k= (0.054 W/m2)(33,000 m)
(2.5 W/m·K) = 710 K
The heat flow is out, so that the temperature is higher at the base of the crust. The temperature
there is then
710 + 10 = 720 C.
E23-2 We apply Eq. 23-1,
H=kAT
x= (0.74 W/m·K)(6.2 m)(3.8 m) (44 C)
(0.32 m) = 2400 W.
E23-3 (a) ∆T/x= (136 C)/(0.249 m) = 546 C/m.
(b) H=kAT/x= (401 W/m·K)(1.80 m2)(546 C/m) = 3.94×105W.
(c) TH= (12C + 136 C) = 124C. Then
T= (124C) (546 C/m)(0.11 m) = 63.9C.
E23-4 (a) H= (0.040 W/m·K)(1.8 m2)(32 C)/(0.012 m) = 190 W.
(b) Since khas increased by a factor of (0.60)/(0.04) = 15 then Hshould also increase by a
factor of 15.
E23-5 There are three possible arrangements: a sheet of type 1 with a sheet of type 1; a sheet
of type 2 with a sheet of type 2; and a sheet of type 1 with a sheet of type 2. We can look back on
Sample Problem 23-1 to see how to start the problem; the heat flow will be
H12 =AT
(L/k1)+(L/k2)
for substances of different types; and
H11 =AT/L
(L/k1)+(L/k1)=1
2
AT k1
L
for a double layer if substance 1. There is a similar expression for a double layer of substance 2.
For configuration (a) we then have
H11 +H22 =1
2
AT k1
L+1
2
AT k2
L=AT
2L(k1+k2),
while for configuration (b) we have
H12 +H21 = 2 AT
(L/k1)+(L/k2)=2AT
L((1/k1) + (1/k2))1.
We want to compare these, so expanding the relevant part of the second configuration
((1/k1) + (1/k2))1= ((k1+k2)/(k2k2))1=k1k2
k1+k2
.
286
Then which is larger
(k1+k2)/2 or 2k1k2
k1+k2
?
If k1k2then the expression become
k1/2 and 2k2,
so the first expression is larger, and therefore configuration (b) has the lower heat flow. Notice that
we get the same result if k1k2!
E23-6 There’s a typo in the exercise.
H=AT/R; since the heat flows through one slab and then through the other, we can write
(T1Tx)/R1= (TxT2)/R2. Rearranging,
Tx= (T1R2+T2R1)/(R1+R2).
E23-7 Use the results of Exercise 23-6. At the interface between ice and water Tx= 0C. Then
R1T2+R2T1= 0, or k1T1/L1+k2T2/L2= 0. Not only that, L1+L2=L, so
k1T1L2+ (LL2)k2T2= 0,
so
L2=(1.42 m)(1.67 W/m·K)(5.20C)
(1.67 W/m·K)(5.20C) (0.502 W/m·K)(3.98C) = 1.15 m.
E23-8 Tis the same in both cases. So is k. The top configuration has Ht=kAT/(2L). The
bottom configuration has Hb=k(2A)∆T/L. The ratio of Hb/Ht= 4, so heat flows through the
bottom configuration at 4 times the rate of the top. For the top configuration Ht= (10 J)/(2 min) =
5 J/min. Then Hb= 20 J/min. It will take
t= (30 J)/(20 J/min) = 1.5 min.
E23-9 (a) This exercise has a distraction: it asks about the heat flow through the window, but
what you need to find first is the heat flow through the air near the window. We are given the
temperature gradient both inside and outside the window. Inside,
T
x=(20C) (5C)
(0.08 m) = 190 C/m;
a similar expression exists for outside.
From Eq. 23-1 we find the heat flow through the air;
H=kAT
x= (0.026 W/m·K)(0.6 m)2(190 C/m) = 1.8 W.
The value that we arrived at is the rate that heat flows through the air across an area the size of
the window on either side of the window. This heat flow had to occur through the window as well,
so
H= 1.8 W
answers the window question.
(b) Now that we know the rate that heat flows through the window, we are in a position to find
the temperature difference across the window. Rearranging Eq. 32-1,
T=Hx
kA =(1.8 W)(0.005 m)
(1.0 W/m·K)(0.6 m)2= 0.025 C,
so we were well justified in our approximation that the temperature drop across the glass is very
small.
287
E23-10 (a) W= +214 J, done on means positive.
(b) Q=293 J, extracted from means negative.
(c) ∆Eint =Q+W= (293 J) + (+214 J) = 79.0 J.
E23-11 (a) ∆Eint along any path between these two points is
Eint =Q+W= (50 J) + (20 J) = 30 J.
Then along ibf W = (30 J) (36 J) = 6 J.
(b) Q= (30 J) (+13 J) = 43 J.
(c) Eint,f =Eint,i + ∆Eint = (10 J) + (30 J) = 40 J.
(d) ∆Eintib = (22 J) (10 J) = 12 J; while Eintbf = (40 J) (22 J) = 18 J. There is no work
done on the path bf, so
Qbf = ∆Eintbf Wbf = (18 J) (0) = 18 J,
and Qib =Qibf Qbf = (36 J) (18 J) = 18 J.
E23-12 Q=mL = (0.10)(2.1×108kg)(333×103J/kg) = 7.0×1012J.
E23-13 We don’t need to know the outside temperature because the amount of heat energy
required is explicitly stated: 5.22 GJ. We just need to know how much water is required to transfer
this amount of heat energy. Use Eq. 23-11, and then
m=Q
cT=(5.22 ×109J)
(4190 J/kg ·K)(50.0C22.0C) = 4.45 ×104kg.
This is the mass of the water, we want to know the volume, so we’ll use the density, and then
V=m
ρ=(4.45 ×104kg)
(998 kg/m3)= 44.5 m3.
E23-14 The heat energy required is Q=mcT. The time required is t=Q/P . Then
t=(0.136 kg)(4190 J/kg ·K)(100C23.5C)
(220 W) = 198 s.
E23-15 Q=mL, so m= (50.4×103J)/(333 ×103J/kg) = 0.151 kg is the mount which freezes.
Then (0.258 kg) (0.151 kg) = 0.107 kg is the amount which does not freeze.
E23-16 (a) W=mgy; if |Q|=|W|, then
T=mgy
mc =(9.81 m/s2)(49.4 m)
(4190 J/kg ·K) = 0.112 C.
E23-17 There are three “things” in this problem: the copper bowl (b), the water (w), and the
copper cylinder (c). The total internal energy changes must add up to zero, so
Eint,b+ ∆Eint,w+ ∆Eint,c= 0.
As in Sample Problem 23-3, no work is done on any object, so
Qb+Qw+Qc= 0.
288
The heat transfers for these three objects are
Qb=mbcb(Tf,bTi,b),
Qw=mwcw(Tf,wTi,w) + Lvm2,
Qc=mccc(Tf,cTi,c).
For the most part, this looks exactly like the presentation in Sample Problem 23-3; but there is an
extra term in the second line. This term reflects the extra heat required to vaporize m2= 4.70 g of
water at 100C into steam 100C.
Some of the initial temperatures are specified in the exercise: Ti,b=Ti,w= 21.0C and Tf,b=
Tf,w=Tf,c= 100C.
(a) The heat transferred to the water, then, is
Qw= (0.223 kg)(4190 J/kg·K) ((100C) (21.0C)) ,
+(2.26×106J/kg)(4.70×103kg),
= 8.44 ×104J.
This answer differs from the back of the book. I think that they (or was it me) used the latent heat
of fusion when they should have used the latent heat of vaporization!
(b) The heat transfered to the bowl, then, is
Qw= (0.146 kg)(387 J/kg ·K) ((100C) (21.0C)) = 4.46 ×103J.
(c) The heat transfered from the cylinder was transfered into the water and bowl, so
Qc=QbQw=(4.46 ×103J) (8.44 ×104J) = 8.89 ×104J.
The initial temperature of the cylinder is then given by
Ti,c=Tf,cQc
mccc
= (100C) (8.89 ×104J)
(0.314 kg)(387 J/kg ·K) = 832C.
E23-18 The temperature of the silver must be raised to the melting point and then the heated
silver needs to be melted. The heat required is
Q=mL +mcT= (0.130 kg)[(105×103J/kg) + (236 J/kg ·K)(1235 K 289 K)] = 4.27×104J.
E23-19 (a) Use Q=mcT,m=ρV , and t=Q/P . Then
t=[maca+ρwVwcw]∆T
P,
=[(0.56 kg)(900 J/kg·K) + (998 kg/m3)(0.64×103m3)(4190 J/kg·K)](100C12C)
(2400 W) = 117 s.
(b) Use Q=mL,m=ρV , and t=Q/P . Then
t=ρwVwLw
P=(998 kg/m3)(0.640×103m3)(2256×103J/kg)
(2400 W) = 600 s
is the additional time required.
289
E23-20 The heat given off by the steam will be
Qs=msLv+mscw(50 C).
The hear taken in by the ice will be
Qi=miLf+micw(50 C).
Equating,
ms=mrmi
Lf+cw(50 C)
Lv+cw(50 C),
= (0.150 kg) (333×103J/kg) + (4190 J/kg·K)(50 C)
(2256×103J/kg) + (4190 J/kg·K)(50 C)= 0.033 kg.
E23-21 The linear dimensions of the ring and sphere change with the temperature change ac-
cording to
dr=αrdr(Tf,rTi,r),
ds=αsds(Tf,sTi,s).
When the ring and sphere are at the same (final) temperature the ring and the sphere have the
same diameter. This means that
dr+ ∆dr=ds+ ∆ds
when Tf,s=Tf,r. We’ll solve these expansion equations first, and then go back to the heat equations.
dr+ ∆dr=ds+ ∆ds,
dr(1 + αr(Tf,rTi,r)) = ds(1 + αs(Tf,sTi,s)) ,
which can be rearranged to give
αrdrTf,rαsdsTf,s=ds(1 αsTi,s)dr(1 αrTi,r),
but since the final temperatures are the same,
Tf=ds(1 αsTi,s)dr(1 αrTi,r)
αrdrαsds
Putting in the numbers,
Tf=
(2.54533cm)[1(23×106/C)(100C)](2.54000cm)[1(17×106/C)(0C)]
(2.54000cm)(17×106/C)(2.54533cm)(23×106/C),
= 34.1C.
No work is done, so we only have the issue of heat flow, then
Qr+Qs= 0.
Where “r” refers to the copper ring and “s” refers to the aluminum sphere. The heat equations are
Qr=mrcr(TfTi,r),
Qs=mscs(TfTi,s).
Equating and rearranging,
ms=mrcr(Ti,rTf)
cs(TfTi,s)
or
ms=(21.6 g)(387 J/kg·K)(0C34.1C)
(900 J/kg·K)(34.1C100C) = 4.81 g.
290
E23-22 The problem is compounded because we don’t know if the final state is only water, only
ice, or a mixture of the two.
Consider first the water. Cooling it to 0C would require the removal of
Qw= (0.200 kg)(4190 J/kg ·K)(0C25C) = 2.095×104J.
Consider now the ice. Warming the ice to would require the addition of
Qi= (0.100 kg)(2220 J/kg ·K)(0C + 15C) = 3.33×103J.
The heat absorbed by the warming ice isn’t enough to cool the water to freezing. However, the ice
can melt; and if it does it will require the addition of
Qim = (0.100 kg)(333×103J/kg) = 3.33×104J.
This is far more than will be liberated by the cooling water, so the final temperature is 0C, and
consists of a mixture of ice and water.
(b) Consider now the ice. Warming the ice to would require the addition of
Qi= (0.050 kg)(2220 J/kg ·K)(0C + 15C) = 1.665×103J.
The heat absorbed by the warming ice isn’t enough to cool the water to freezing. However, the ice
can melt; and if it does it will require the addition of
Qim = (0.050 kg)(333×103J/kg) = 1.665×104J.
This is still not enough to cool the water to freezing. Hence, we need to solve
Qi+Qim +micw(T0C) + mwcw(T25C) = 0,
which has solution
T=(4190 J/kg ·K)(0.200 kg)(25C) (1.665×103J) + (1.665×104J)
(4190 J/kg ·K)(0.250 kg) = 2.5C.
E23-23 (a) c= (320 J)/(0.0371 kg)(42.0C26.1C) = 542 J/kg ·K.
(b) n=m/M = (37.1 g)/(51.4 g/mol) = 0.722 mol.
(c) c= (542 J/kg ·K)(51.4×103kg/mol) = 27.9 J/mol ·K.
E23-24 (1) W=pV= (15 Pa)(4 m3) = 60 J for the horizontal path; no work is done during
the vertical path; the net work done on the gas is 60 J.
(2) It is easiest to consider work as the (negative of) the area under the curve; then W=
(15 Pa + 5 Pa)(4 m3)/2 = 40 J.
(3) No work is done during the vertical path; W=pV= (5 Pa)(4 m3) = 20 J for the
horizontal path; the net work done on the gas is 20 J.
E23-25 Net work done on the gas is given by Eq. 23-15,
W=Zp dV.
But integrals are just the area under the curve; and that’s the easy way to solve this problem. In the
case of closed paths, it becomes the area inside the curve, with a clockwise sense giving a positive
value for the integral.
The magnitude of the area is the same for either path, since it is a rectangle divided in half by
a square. The area of the rectangle is
(15×103Pa)(6 m3) = 90×103J,
so the area of path 1 (counterclockwise) is -45 kJ; this means the work done on the gas is -(-45 kJ)
or 45 kJ. The work done on the gas for path 2 is the negative of this because the path is clockwise.
291
E23-26 During the isothermal expansion,
W1=nRT ln V2
V1
=p1V1ln p1
p2
.
During cooling at constant pressure,
W2=p2V=p2(V1V2) = p2V1(1 p1/p2) = V1(p1p2).
The work done is the sum, or
(204×103Pa)(0.142 m3) ln (204×103Pa)
(101×103Pa) + (0.142 m3)(103 Pa) = 5.74×103J.
E23-27 During the isothermal expansion,
W=nRT ln V2
V1
=p1V1ln V2
V1
,
so
W=(1.32)(1.01×105Pa)(0.0224 m3) ln (0.0153 m3)
(0.0224 m3)= 1.14×103J.
E23-28 (a) pV γis a constant, so
p2=p1(V1/V2)γ= (1.00 atm)[(1 l)/(0.5 l)]1.32 = 2.50 atm;
T2=T1(p2/p1)(V2/V1), so
T2= (273 K)(2.50 atm)
(1.00 atm)
(0.5 l)
(1 l) = 341 K.
(b) V3=V2(p2/p1)(T3/T2), so
V3= (0.5 l)(273 K)
(341 K) = 0.40 l.
(c) During the adiabatic process,
W12 =(1.01×105Pa/atm)(1×103m3/l)
(1.32) 1[(2.5 atm)(0.5 l) (1.0 atm)(1 l)] = 78.9 J.
During the cooling process,
W23 =pV=(1.01×105Pa/atm)(2.50 atm)(1×103m3/l)[(0.4 l) (0.5 l)] = 25.2 J.
The net work done is W123 = 78.9 J + 25.2 J = 104.1 J.
E23-29 (a) According to Eq. 23-20,
pf=piViγ
Vfγ=(1.17 atm)(4.33 L)(1.40)
(1.06 L)(1.40) = 8.39 atm.
(b) The final temperature can be found from the ideal gas law,
Tf=Ti
pfVf
piVi
= (310 K)(8.39 atm)(1.06 L)
(1.17 atm)(4.33 L) = 544 K.
(c) The work done (for an adiabatic process) is given by Eq. 23-22,
W=1
(1.40) 1(8.39 ×1.01×105Pa)(1.06×103m3)
(1.17 ×1.01×105Pa)(4.33×103m3),
= 966 J.
292
E23-30 Air is mostly diatomic (N2and O2), so use γ= 1.4.
(a) pV γis a constant, so
V2=V1γ
pp1/p2=V11.4
p(1.0 atm)/(2.3 atm) = 0.552V1.
T2=T1(p2/p1)(V2/V1), so
T2= (291 K)(2.3 atm)
(1.0 atm)
(0.552V1)
V1
= 369 K,
or 96C.
(b) The work required for delivering 1 liter of compressed air is
W12 =(1.01×105Pa/atm)(1×103m3/l)
(1.40) 1[(2.3 atm)(1.0 l) (1.0 atm)(1.0 l/0.552)] = 123 J.
The number of liters per second that can be delivered is then
V/t= (230 W)/(123 J/l) = 1.87 l.
E23-31 Eint,rot =nRT = (1 mol)(8.31 J/mol ·K)(298 K) = 2480 J.
E23-32 Eint,rot =3
2nRT = (1.5)(1 mol)(8.31 J/mol ·K)(523 K) = 6520 J.
E23-33 (a) Invert Eq. 32-20,
γ=ln(p1/p2)
ln(V2/V1)=ln(122 kPa/1450 kPa)
ln(1.36 m3/10.7 m3)= 1.20.
(b) The final temperature is found from the ideal gas law,
Tf=Ti
pfVf
piVi
= (250 K)(1450×103Pa)(1.36 m3)
(122×103Pa)(10.7 m3)= 378 K,
which is the same as 105C.
(c) Ideal gas law, again:
n= [pV ]/[RT ] = [(1450×103Pa)(1.36 m3)]/[(8.31 J/mol ·K)(378 K)] = 628 mol.
(d) From Eq. 23-24,
Eint =3
2nRT =3
2(628 mol)(8.31 J/mol ·K)(250 K) = 1.96×106J
before the compression and
Eint =3
2nRT =3
2(628 mol)(8.31 J/mol ·K)(378 K) = 2.96×106J
after the compression.
(e) The ratio of the rms speeds will be proportional to the square root of the ratio of the internal
energies,
p(1.96×106J)/(2.96×106J) = 0.813;
we can do this because the number of particles is the same before and after, hence the ratio of the
energies per particle is the same as the ratio of the total energies.
293
E23-34 We can assume neon is an ideal gas. Then ∆T= 2∆Eint/3nR, or
T=2(1.34×1012eV)(1.6×1019J/eV)
3(0.120 mol)(8.31 J/mol ·K) = 1.43×107J.
E23-35 At constant pressure, doubling the volume is the same as doubling the temperature. Then
Q=nCpT= (1.35 mol)7
2(8.31 J/mol ·K)(568 K 284 K) = 1.12×104J.
E23-36 (a) n=m/M = (12 g)/(28 g/mol) = 0.429 mol.
(b) This is a constant volume process, so
Q=nCVT= (0.429 mol)5
2(8.31 J/mol ·K)(125C25C) = 891J.
E23-37 (a) From Eq. 23-37,
Q=ncpT= (4.34 mol)(29.1 J/mol ·K)(62.4 K) = 7880 J.
(b) From Eq. 23-28,
Eint =5
2nRT=5
2(4.34 mol)(8.31 J/mol ·K)(62.4 K) = 5630 J.
(c) From Eq. 23-23,
Ktrans =3
2nRT=5
2(4.34 mol)(8.31 J/mol ·K)(62.4 K) = 3380 J.
E23-38 cV=3
2(8.31 J/mol ·K)/(4.00 g/mol) = 3120 J/kg ·K.
E23-39 Each species will experience the same temperature change, so
Q=Q1+Q2+Q3,
=n1C1T+n2C2T+n3C3T,
Dividing this by n=n1+n2+n3and ∆Twill return the specific heat capacity of the mixture, so
C=n1C1+n2C2+n3C3
n1+n2+n3
.
E23-40 WAB = 0, since it is a constant volume process, consequently, W=WAB +WABC =15 J.
But around a closed path Q=W, so Q= 15 J. Then
QCA =QQAB QBC = (15 J) (20 J) (0 J) = 5 J.
Note that this heat is removed from the system!
294
E23-41 According to Eq. 23-25 (which is specific to ideal gases),
Eint =3
2nRT,
and for an isothermal process ∆T= 0, so for an ideal gas ∆Eint = 0. Consequently, Q+W= 0 for
an ideal gas which undergoes an isothermal process.
But we know Wfor an isotherm, Eq. 23-18 shows
W=nRT ln Vf
Vi
Then finally
Q=W=nRT ln Vf
Vi
E23-42 Qis greatest for constant pressure processes and least for adiabatic. Wis greatest (in
magnitude, it is negative for increasing volume processes) for constant pressure processes and least
for adiabatic. Eint is greatest for constant pressure (for which it is positive), and least for adiabatic
(for which is is negative).
E23-43 (a) For a monatomic gas, γ= 1.667. Fast process are often adiabatic, so
T2=T1(V1/V2)γ1= (292 K)[(1)(1/10)]1.6671= 1360 K.
(b) For a diatomic gas, γ= 1.4. Fast process are often adiabatic, so
T2=T1(V1/V2)γ1= (292 K)[(1)(1/10)]1.41= 733 K.
E23-44 This problem cannot be solved without making some assumptions about the type of pro-
cess occurring on the two curved portions.
E23-45 If the pressure and volume are both doubled along a straight line then the process can
be described by
p=p1
V1
V
The final point involves the doubling of both the pressure and the volume, so according to the ideal
gas law, pV =nRT , the final temperature T2will be four times the initial temperature T1.
Now for the exercises.
(a) The work done on the gas is
W=Z2
1
p dV =Z2
1
p1
V1
V dV =p1
V1V2
2
2V2
1
2
We want to express our answer in terms of T1. First we take advantage of the fact that V2= 2V1,
then
W=p1
V14V2
1
2V2
1
2=3
2p1V1=3
2nRT1
(b) The nice thing about ∆Eint is that it is path independent, we care only of the initial and
final points. From Eq. 23-25,
Eint =3
2nRT=3
2nR (T2T1) = 9
2nRT1
295
(c) Finally we are in a position to find Qby applying the first law,
Q= ∆Eint W=9
2nRT1+3
2nRT1= 6nRT1.
(d) If we define specific heat as heat divided by temperature change, then
c=Q
nT=6RT1
4T1T1
= 2R.
E23-46 The work done is the area enclosed by the path. If the pressure is measured in units of
10MPa, then the shape is a semi-circle, and the area is
W= (π/2)(1.5)2(10MPa)(1×103m3) = 3.53×104J.
The heat is given by Q=W=3.53×104J.
E23-47 (a) Internal energy changes according to ∆Eint =Q+W, so
Eint = (20.9 J) (1.01×105Pa)(113×106m363×106m3) = 15.9 J.
(b) T1=p1V1/nR and T2=p2V2/nR, but pis constant, so ∆T=pV/nR. Then
CP=Q
nT=QR
pV=(20.9 J)(8.31 J/mol ·K)
(1.01×105Pa)(113×106m363×106m3)= 34.4 J/mol ·K.
(c) CV=CPR= (34.4 J/mol ·K) (8.31 J/mol ·K) = 26.1 J/mol ·K.
E23-48 Constant Volume
(a) Q= 3(3.15 mol)(8.31 J/mol ·K)(52.0 K) = 4080 J.
(b) W= 0.
(c) ∆Ermint = 3(3.15 mol)(8.31 J/mol ·K)(52.0 K) = 4080 J.
Constant Pressure
(a) Q= 4(3.15 mol)(8.31 J/mol ·K)(52.0 K) = 5450 J.
(b) W=pV=nRT=(3.15 mol)(8.31 J/mol ·K)(52.0 K) = 1360 J.
(c) ∆Ermint = 3(3.15 mol)(8.31 J/mol ·K)(52.0 K) = 4080 J.
Adiabatic
(a) Q= 0.
(b) W= (pfVfpiVi)/(γ1) = nRT/(γ1) = 3(3.15 mol)(8.31 J/mol ·K)(52.0 K) = 4080 J.
(c) ∆Ermint = 3(3.15 mol)(8.31 J/mol ·K)(52.0 K) = 4080 J.
P23-1 (a) The temperature difference is
(5 C/9 F)(72F− −20F) = 51.1 C.
The rate of heat loss is
H= (1.0 W/m·K)(1.4 m2)(51.1 C)/(3.0×103m) = 2.4×104W.
(b) Start by finding the Rvalues.
Rg= (3.0×103m)/(1.0 W/m·K) = 3.0×103m2·K/W,
Ra= (7.5×102m)/(0.026W/m·K) = 2.88m2·K/W.
Then use Eq. 23-5,
H=(1.4 m2)(51.1 C)
2(3.0×103m2·K/W) + (2.88m2·K/W) = 25 W.
Get double pane windows!
296
P23-2 (a) H= (428 W/m·K)(4.76×104m2)(100 C)/(1.17 m) = 17.4 W.
(b) ∆m/t=H/L = (17.4 W)/(333×103J/kg) = 5.23×105kg/s, which is the same as 188 g/h.
P23-3 Follow the example in Sample Problem 23-2. We start with Eq. 23-1:
H=kAdT
dr ,
H=k(4πr2)dT
dr ,
Zr2
r1
Hdr
4πr2=ZT2
T1
kdT,
H
4pi 1
r11
r2=k(T1T2),
Hr2r1
r1r2= 4πk(T1T2),
H=4πk(T1T2)r1r2
r2r1
.
P23-4 (a) H= (54×103W/m2)4π(6.37×106m)2= 2.8×1013W.
(b) Using the results of Problem 23-3,
T=(2.8×1013W)(6.37×106m3.47×106m)
4π(4.2 W/m·K)(6.37×106m)(3.47×106m) = 7.0×104C.
Since T2= 0C, we expect T1= 7.0×104C.
P23-5 Since H=kA dT/dx, then H dx =aT dT .His a constant, so integrate both side
according to
ZH dx =ZaT dT,
HL =a1
2(T2
2T2
1),
H=aA
2L(T2
1T2
2).
P23-6 Assume the water is all at 0C. The heat flow through the ice is then H=kAT/x; the
rate of ice formation is ∆m/t=H/L. But ∆m=ρAx, so
x
t=H
ρAL =kT
ρLx ,
(1.7 W/m·K)(10 C)
(920 kg/m3)(333×103J/kg)(0.05 m) = 1.11×106m/s.
That’s the same as 0.40 cm/h.
P23-7 (a) Start with the heat equation:
Qt+Qi+Qw= 0,
297
where Qtis the heat from the tea, Qiis the heat from the ice when it melts, and Qwis the heat
from the water (which used to be ice). Then
mtct(TfTt,i) + miLf+mwcw(TfTw,i) = 0,
which, since we have assumed all of the ice melts and the masses are all equal, can be solved for Tf
as
Tf=ctTt,i+cwTw,iLf
ct+cw
,
=(4190J/kg ·K)(90C) + (4190J/kg ·K)(0C) (333×103J/kg)
(4190J/kg ·K) + (4190J/kg ·K) ,
= 5.3C.
(b) Once again, assume all of the ice melted. Then we can do the same steps, and we get
Tf=ctTt,i+cwTw,iLf
ct+cw
,
=(4190J/kg ·K)(70C) + (4190J/kg ·K)(0C) (333×103J/kg)
(4190J/kg ·K) + (4190J/kg ·K) ,
=4.7C.
So we must have guessed wrong when we assumed that all of the ice melted. The heat equation
then simplifies to
mtct(TfTt,i) + miLf= 0,
and then
mi=mtct(Tt,iTf)
Lf
,
=(0.520 kg)(4190J/kg ·K)(90C0)
(333×103J/kg) ,
= 0.458 kg.
P23-8 c=Q/mT=H/(∆m/t)∆T. But ∆m/t=ρV/t. Combining,
c=H
(∆V/t)ρT=(250 W)
(8.2×106m3/s)(0.85×103kg/m3)(15 C)= 2.4×103J/kg ·K.
P23-9 (a) n=NA/M , so
=(2256×103J/kg)
(6.02×1023/mol)/(0.018 kg/mol) = 6.75×1020J.
(b) Eav =3
2kT , so
Eav
=2(6.75×1020J)
3(1.38×1023J/K)(305 K) = 10.7.
P23-10 Qw+Qt= 0, so
CtTt+mwcw(TfTi) = 0,
or
Ti=(0.3 kg)(4190 J/kg ·m)(44.4C) + (46.1 J/K)(44.4C15.0C)
(0.3 kg)(4190 J/kg ·m) = 45.5C.
298
P23-11 We can use Eq. 23-10, but we will need to approximate cfirst. If we assume that the
line is straight then we use c=mT +b. I approximate mfrom
m=(14 J/mol ·K) (3 J/mol ·K)
(500 K) (200 K) = 3.67×102J/mol.
Then I find bfrom those same data points,
b= (3 J/mol ·K) (3.67×102J/mol)(200 K) = 4.34 J/mol ·K.
Then from Eq. 23-10,
Q=nZTf
Ti
c dT,
=nZTf
Ti
(mT +b)dT,
=nhm
2T2+bT iTf
Ti
,
=nm
2(Tf2Ti2) + b(TfTi),
= (0.45mol) (3.67×102J/mol)
2((500 K)2(200 K)2)
+ (4.34 J/mol ·K)(500 K 200 K)) ,
= 1.15×103J.
P23-12 δQ =nCδT , so
Q=nZC dT,
=n(0.318 J/mol ·K2)T2/2(0.00109 J/mol ·K3)T3/3(0.628 J/mol ·K)T90 K
50 K ,
=n(645.8 J/mol).
Finally,
Q= (645.8 J/mol)(316 g)/(107.87 g/mol) = 189 J.
P23-13 T V γ1is a constant, so
T2= (292 K)(1/1.28)(1.40)1= 265 K
P23-14 W=Rp dV , so
W=ZnRT
Vnb an2
V2dV,
=nRT ln(Vnb)an2
V
f
i
,
=nRT ln Vfnb
Vinb an21
Vf1
Vi.
299
P23-15 When the tube is horizontal there are two regions filled with gas, one at p1,i,V1,i; the
other at p2,i,V2,i. Originally p1,i=p2,i= 1.01×105Pa and V1,i=V2,i= (0.45 m)A, where Ais the
cross sectional area of the tube.
When the tube is held so that region 1 is on top then the mercury has three forces on it: the
force of gravity, mg; the force from the gas above pushing down p2,fA; and the force from the gas
below pushing up p1,fA. The balanced force expression is
p1,fA=p2,fA+mg.
If we write m=ρlmAwhere lm= 0.10 m, then
p1,f=p2,f+ρglm.
Finally, since the tube has uniform cross section, we can write V=Al everywhere.
(a) For an isothermal process pili=pflf, where we have used V=Al, and then
p1,i
l1,i
l1,fp2,i
l2,i
l2,f
=ρglm.
But we can factor out p1,i=p2,iand l1,i=l2,i, and we can apply l1,f+l2,f= 0.90 m. Then
1
l1,f1
0.90 m l1,f
=ρglm
pili
.
Put in some numbers and rearrange,
0.90 m 2l1,f= (0.294 m1)l1,f(0.90 m l1,f),
which can be written as an ordinary quadratic,
(0.294 m1)l1,f2(2.265)l1,f+ (0.90 m) = 0
The solutions are l1,f= 7.284 m and 0.421 m. Only one of these solutions is reasonable, so the
mercury shifted down 0.450 0.421 = 0.029 m.
(b) The math is a wee bit uglier here, but we can start with piliγ=pflfγ, and this means that
everywhere we had a l1,fin the previous derivation we need to replace it with l1,fγ. Then we have
1
l1,fγ1
(0.90 m l1,f)γ=ρglm
piliγ.
This can be written as
(0.90 m l1,f)γl1,fγ= (0.404 mγ)l1,fγ(0.90 m l(0.1,f)γ,
which looks nasty to me! I’ll use Maple to get the answer, and find l1,f= 0.429, so the mercury
shifted down 0.450 0.429 = 0.021 m.
Which is more likely? Turn the tube fast, and the adiabatic approximation works. Eventually
the system will return to room temperature, and then the isothermal approximation is valid.
P23-16 Internal energy for an ideal diatomic gas can be written as
Eint =5
2nRT =5
2pV,
simply by applying the ideal gas law. The room, however, has a fixed pressure and volume, so the
internal energy is independent of the temperature. As such, any energy supplied by the furnace
leaves the room, either as heat or as expanding gas doing work on the outside.
300
P23-17 The speed of sound in the iodine gas is
v=fλ = (1000 Hz)(2 ×0.0677 m) = 135 m/s.
Then
γ=Mv2
RT =n(0.127 kg/mol)(135 m/s)2
(8.31 J/mol ·K)(400 K) =n(0.696).
Since γis greater than one, n2. If n= 2 then γ= 1.39, which is consistent; if n= 3 then
γ= 2.08, which is not consistent.
Consequently, iodine gas is diatomic.
P23-18 W=Q=mL = (333×103J/kg)(0.122 kg) = 4.06×104J.
P23-19 (a)
Process AB
Q=3
2(1.0 mol)(8.31 J/mol ·K)(300 K) = 3740 J.
W= 0.
Eint =Q+W= 3740 J.
Process BC
Q= 0.
W= (pfVfpiVi)/(γ1) = nRT/(γ1) = (1.0 mol)(8.31 J/mol ·K)(145 K)/(1.67 1) =
1800 J
Eint =Q+W=1800 J.
Process AB
Q=5
2(1.0 mol)(8.31 J/mol ·K)(155 K) = 3220 J.
W=pV=nRT=(1.0 mol)(8.31 J/mol ·K)(155 K) = 1290 J.
Eint =Q+W= 1930 J.
Cycle
Q= 520 J; W=510 J (rounding error!); Eint = 10 J (rounding error!)
P23-20
P23-21 pf= (16.0 atm)(50/250)1.40 = 1.68 atm.The work done by the gas during the expansion
is
W= [piVipfVf]/(γ1),
=(16.0 atm)(50×106m3)(1.68 atm)(250×106m3)
(1.40) 1(1.01×105Pa/atm),
= 96.0 J.
This process happens 4000 times per minute, but the actual time to complete the process is half of
the cycle, or 1/8000 of a minute. Then P= (96 J)(8000)/(60 s) = 12.8×103W.
301
E24-1 For isothermal processes the entropy expression is almost trivial, ∆S=Q/T, where if Q
is positive (heat flow into system) the entropy increases.
Then Q=TS= (405 K)(46.2 J/K) = 1.87×104J.
E24-2 Entropy is a state variable and is path independent, so
(a) ∆Sab,2= ∆Sab,1= +0.60 J/K,
(b) ∆Sba,2=Sab,2=0.60 J/K,
E24-3 (a) Heat only enters along the top path, so
Qin =TS= (400 K)(0.6 J/K0.1 J/K) = 200 J.
(b) Heat leaves only bottom path, so
Qout =TS= (250 K)(0.1 J/K0.6 J/K) = 125 J.
Since Q+W= 0 for a cyclic path,
W=Q=[(200 J) + (125 J)] = 75 J.
E24-4 (a) The work done for isothermal expansion is given by Eq. 23-18,
W=(4.00 mol)(8.31 J/mol ·K)(410 K) ln 3.45V1
V1
=1.69×104J.
(b) For isothermal process, Q=W, then
S=Q/T = (1.69×104J)/(410 K) = 41.2 J/K.
(c) Entropy change is zero for reversible adiabatic processes.
E24-5 (a) We want to find the heat absorbed, so
Q=mcT= (1.22 kg)(387 J/mol ·K) ((105C) (25.0C)) = 3.77×104J.
(b) We want to find the entropy change, so, according to Eq. 24-1,
S=ZTf
Ti
dQ
T,
=ZTf
Ti
mc dT
T,
=mc ln Tf
Ti
.
The entropy change of the copper block is then
S=mc ln Tf
Ti
= (1.22 kg)(387 J/mol ·K) ln (378 K)
(298 K) = 112 J/K.
E24-6 S=Q/T =mL/T , so
S= (0.001 kg)(333×103J/kg)/(263 K) = 1.27J/K.
302
E24-7 Use the first equation on page 551.
n=S
Rln(Vf/V i)=(24 J/K)
(8.31 J/mol ·K) ln(3.4/1.3) = 3.00 mol.
E24-8 S=Q/T cQ/T h.
(a) ∆S= (260 J)(1/100 K 1/400 K) = 1.95 J/K.
(b) ∆S= (260 J)(1/200 K 1/400 K) = 0.65 J/K.
(c) ∆S= (260 J)(1/300 K 1/400 K) = 0.217 J/K.
(d) ∆S= (260 J)(1/360 K 1/400 K) = 0.0722 J/K.
E24-9 (a) If the rod is in a steady state we wouldn’t expect the entropy of the rod to change.
Heat energy is flowing out of the hot reservoir into the rod, but this process happens at a fixed
temperature, so the entropy change in the hot reservoir is
SH=QH
TH
=(1200 J)
(403 K) =2.98 J/K.
The heat energy flows into the cold reservoir, so
SC=QH
TH
=(1200 J)
(297 K) = 4.04 J/K.
The total change in entropy of the system is the sum of these two terms
S= ∆SH+ ∆SC= 1.06 J/K.
(b) Since the rod is in a steady state, nothing is changing, not even the entropy.
E24-10 (a) Qc+Ql= 0, so
mccc(TTc) + mlcl(TTl) = 0,
which can be solved for Tto give
T=(0.05 kg)(387 J/kg ·K)(400 K) + (0.10 kg)(129 J/kg ·K)(200 K)
(0.05 kg)(387 J/kg ·K) + (0.10 kg)(129 J/kg ·K) = 320 K.
(b) Zero.
(c) ∆S=mc ln Tf/T i, so
S= (0.05 kg)(387 J/kg ·K) ln (320 K)
(400 K) + (0.10 kg)(129 J/kg ·K) ln (320 K)
(200 K) = 1.75 J/K.
E24-11 The total mass of ice and water is 2.04 kg. If eventually the ice and water have the same
mass, then the final state will have 1.02 kg of each. This means that 1.78 kg 1.02 kg = 0.76 kg of
water changed into ice.
(a) The change of water at 0C to ice at 0C is isothermal, so the entropy change is
S=Q
T=mL
T=(0.76 kg)(333×103J/kg)
(273 K) =927 J/K.
(b) The entropy change is now +927 J/K.
303
E24-12 (a) Qa+Qw= 0, so
maca(TTa) + mwcw(TTw) = 0,
which can be solved for Tto give
T=(0.196 kg)(900 J/kg ·K)(380 K) + (0.0523 kg)(4190 J/kg ·K)(292 K)
(0.196 kg)(900 J/kg ·K) + (0.0523 kg)(4190 J/kg ·K) = 331 K.
That’s the same as 58C.
(b) ∆S=mc ln Tf/T i, so
Sa= (0.196 kg)(900 J/kg ·K) ln (331 K)
(380 K) =24.4 J/K.
(c) For the water,
(0.0523 kg)(4190 J/kg ·K) ln (331 K)
(292 K) = 27.5 J/K.
(d) ∆S= (27.5 J/K) + (24.4 J/K) = 3.1 J/K.
E24-13 (a) e= 1 (36.2 J/52.4 J) = 0.309.
(b) W=QhQc= (52.4 J) (36.2 J) = 16.2 J.
E24-14 (a) Qh= (8.18 kJ)/(0.25) = 32.7 kJ, Qc=QhW= (32.7 kJ) (8.18 kJ) = 24.5 kJ.
(b) Qh= (8.18 kJ)/(0.31) = 26.4 kJ, Qc=QhW= (26.4 kJ) (8.18 kJ) = 18.2 kJ.
E24-15 One hour’s worth of coal, when burned, will provide energy equal to
(382×103kg)(28.0×106J/kg) = 1.07×1013J.
In this hour, however, the plant only generates
(755×106W)(3600 s) = 2.72×1012J.
The efficiency is then
e= (2.72×1012J)/(1.07×1013J) = 25.4%.
E24-16 We use the convention that all quantities are positive, regardless of direction. WA= 5WB;
Qi,A= 3Qi,B; and Qo,A= 2Qo,B. But WA=Qi,AQo,A, so
5WB= 3Qi,B2Qo,B,
or, applying WB=Qi,BQo,B,
5WB= 3Qi,B2(Qi,BWB),
3WB=Qi,B,
WB/Qi,B= 1/3 = eB.
Then
eA=WA
Qi,A
=5WB
3Qi,B
=5
3
1
3=5
9.
304
E24-17 (a) During an isothermal process W=Q=2090 J. The negative indicates that the
gas did work on the environment.
(b) The efficiency is e= 1 (297 K)/(412 K) = 0.279.Then
Qo=Qi(1 e) = (2090 J)[1 (0.279)] = 1510 J.
Since this is rejected heat it should actually be negative.
(c) During an isothermal process W=Q= 1510 J. Positive indicates that the gas did work on
the environment.
E24-18 1e=Tc/T h, or Tc=Th(1 e). The difference is
T=ThTc=The,
so Th= (75 C)/(0.22) = 341 K, and
Tc= (341 K)[(1 (0.22)] = 266 K.
E24-19 The BC and DA processes are both adiabatic; so if we could find an expression for work
done during an adiabatic process we might be almost done. But what is an adiabatic process? It is
a process for which Q= 0, so according to the first law
Eint =W.
But for an ideal gas
Eint =nCVT,
as was pointed out in Table 23-5. So we have
|W|=nCV|T|
and since the adiabatic paths BC and DA operate between the same two isotherms, we can conclude
that the magnitude of the work is the same for both paths.
E24-20 (a) To save typing, assume that all quantities are positive. Then
e1= 1 T2/T1,
W1=e1Q1, and Q2=Q1W1. Not only that, but
e2= 1 T3/T2,
and W2=e2Q2. Combining,
e=W1+W2
Q1
=e1Q1+e2(Q1W1)
Q1
=e1+e2(1 e1) = e1+e2e1e2,
or
e= 1 T2
T1
+ 1 T3
T21 + T2
T1
+T3
T2T3
T1
= 1 − −T3
T1
.
(b) e= 1 (311 K)/(742 K) = 0.581.
E24-21 (a) p2= (16.0 atm)(1/5.6)(1.33) = 1.62 atm.
(b) T2=T1(1/5.6)(1.33)1= (0.567)T1, so
e= 1 (0.567) = 0.433.
305
E24-22 (a) The area of the cycle is ∆Vp=p0V0, so the work done by the gas is
W= (1.01×105Pa)(0.0225 m3) = 2270 J.
(b) Let the temperature at abe Ta. Then
Tb=Ta(Vb/Va)(pb/pa) = 2Ta.
Let the temperature at cbe Tc. Then
Tc=Ta(Vc/Va)(pc/pa) = 4Ta.
Consequently, ∆Tab =Taand ∆Tbc = 2Ta. Putting this information into the constant volume and
constant pressure heat expressions,
Qab =3
2nRTab =3
2nRTa=3
2paVa,
and
Qbc =5
2nRTbc =5
2nR2Ta= 5paVa,
so that Qac =13
2p0V0, or
Qac =13
2(1.01×105Pa)(0.0225 m3) = 1.48×104J.
(c) e= (2270 J)/(14800 J) = 0.153.
(d) ec= 1 (Ta/4Ta) = 0.75.
E24-23 According to Eq. 24-15,
K=TL
THTL
=(261 K)
(299 K) (261 K) = 6.87
Now we solve the question out of order.
(b) The work required to run the freezer is
|W|=|QL|/K = (185 kJ)/(5.70) = 32.5 kJ.
(a) The freezer will discharge heat into the room equal to
|QL|+|W|= (185 kJ) + (32.5 kJ) = 218 kJ.
E24-24 (a) K=|QL|/|W|= (568 J)/(153 J) = 3.71.
(b) |QH|=|QL|+|W|= (568 J) + (153 J) = 721 J.
E24-25 K=TL/(THTL); |W|=|QL|/K =|QL|(TH/TL1).
(a) |W|= (10.0 J)(300 K/280K 1) = 0.714 J.
(b) |W|= (10.0 J)(300 K/200K 1) = 5.00 J.
(c) |W|= (10.0 J)(300 K/100K 1) = 20.0 J.
(d) |W|= (10.0 J)(300 K/50K 1) = 50.0 J.
E24-26 K=TL/(THTL); |W|=|QL|/K =|QL|(TH/TL1).Then
|QH|=|QL|+|W|=|QL|(TH/TL) = (0.150 J)(296 K/4.0 K) = 11 J.
306
E24-27 We will start with the assumption that the air conditioner is a Carnot refrigerator.
K=TL/(THTL); |W|=|QL|/K =|QL|(TH/TL1).For fun, I’ll convert temperature to the
absolute Rankine scale! Then
|QL|= (1.0 J)/(555R/530R1) = 21 J.
E24-28 The best coefficient of performance is
Kc= (276 K)/(308 K 276 K) = 8.62.
The inventor claims they have a machine with
K= (20 kW 1.9kW)/(1.9 kW) = 9.53.
Can’t be done.
E24-29 (a) e= 1 (258 K/322 K) = 0.199. |W|= (568 J)(0.199) = 113 J.
(b) K= (258 K)/(322 K 258 K) = 4.03. |W|= (1230 J)/(4.03) = 305 J.
E24-30 The temperatures are distractors!
|W|=|QH|−|QL|=|QH| − K|W|,
so
|W|=|QH|/(1 + K) = (7.6 MJ)/(1 + 3.8) = 1.58 MJ.
Then P= (1.58 MJ)/(3600 s) = 440 W.
E24-31 K= (260 K)/(298 K 260 K) = 6.8.
E24-32 K= (0.85)(270K)/(299K270K) = 7.91. In 15 minutes the motor can do (210 W)(900 s) =
1.89×105J of work. Then
|QL|=K|W|= (7.91)(1.89×105J) = 1.50×106J.
E24-33 The Carnot engine has an efficiency
= 1 T2
T1
=|W|
|Q1|.
The Carnot refrigerator has a coefficient of performance
K=T4
T3T4
=|Q4|
|W|.
Lastly, |Q4|=|Q3| − |W|. We just need to combine these three expressions into one. Starting with
the first, and solving for |W|,
|W|=|Q1|T1T2
T1
.
Then we combine the last two expressions, and
T4
T3T4
=|Q3|−|W|
|W|=|Q3|
|W|1.
307
Finally, combine them all,
T4
T3T4
=|Q3|
|Q1|
T1
T1T21.
Now, we rearrange,
|Q3|
|Q1|=T4
T3T4
+ 1T1T2
T1
,
=T3
T3T4T1T2
T1
,
= (1 T2/T1)/(1 T4/T3).
E24-34 (a) Integrate:
ln N!ZN
1
ln x dx =Nln NN+ 1 Nln NN.
(b) 91, 752, and about 615,000. You will need to use the Stirling approximation extended to a
double inequality to do the last two:
2πnn+1/2en+1/(12n+1) < n!<2πnn+1/2en+1/(12n).
E24-35 (a) For this problem we don’t care how the particles are arranged inside a section, we
only care how they are divided up between the two sides.
Consequently, there is only one way to arrange the particles: you put them all on one side, and
you have no other choices. So the multiplicity in this case is one, or w1= 1.
(b) Once the particles are allowed to mix we have more work in computing the multiplicity.
Using Eq. 24-19, we have
w2=N!
(N/2)!(N/2)! =N!
((N/2)!)2
(c) The entropy of a state of multiplicity wis given by Eq. 24-20,
S=kln w
For part (a), with a multiplicity of 1, S1= 0. Now for part (b),
S2=kln N!
((N/2)!)2!=kln N!2kln(N/2)!
and we need to expand each of those terms with Stirling’s approximation.
Combining,
S2= = k(Nln NN)2k((N/2) ln(N/2) (N/2)) ,
=kNlnN kN kN ln N+kN ln 2 + kN,
=kN ln 2
Finally, ∆S=S2S1=kN ln 2.
(d) The answer should be the same; it is a free expansion problem in both cases!
308
P24-1 We want to evaluate
S=ZTf
Ti
nCVdT
T,
=ZTf
Ti
nAT 3dT
T,
=ZTf
Ti
nAT 2dT ,
=nA
3Tf3Ti3.
Into this last expression, which is true for many substances at sufficiently low temperatures, we
substitute the given numbers.
S=(4.8 mol)(3.15×105J/mol ·K4)
3(10 K)3(5.0 K)3= 4.41×102J/K.
P24-2
P24-3 (a) Work is only done along path ab, where Wab =pV=3p0V0. So Wabc =3p0V0.
(b) ∆ERbc =3
2nRTbc, with a little algebra,
Eintbc =3
2(nRTcnRTb) = 3
2(pcVcpbVb) = 3
2(8 4)p0V0= 6p0V0.
Sbc =3
2nR ln(Tc/Tb),with a little algebra,
Sbc =3
2nR ln(pc/pb) = 3
2nR ln 2.
(c) Both are zero for a cyclic process.
P24-4 (a) For an isothermal process,
p2=p1(V1/V2) = p1/3.
For an adiabatic process,
p3=p1(V1/V2)γ=p1(1/3)1.4= 0.215p1.
For a constant volume process,
T3=T2(p3/p2) = T1(0.215/0.333) = 0.646T1.
(b) The easiest ones first: Eint12 = 0, W23 = 0, Q31 = 0, S31 = 0. The next easier ones:
Eint23 =5
2nRT23 =5
2nR(0.646T1T1) = 0.885p1V1,
Q23 = ∆Eint23 W23 =0.885p1V1,
Eint31 =Eint23 Eint12 = 0.885p1V1,
W31 = ∆Eint31 Q31 = 0.885p1V1.
309
Finally, some harder ones:
W12 =nRT1ln(V2/V1) = p1V1ln(3) = 1.10p1V1,
Q12 = ∆Eint12 W12 = 1.10p1V1.
And now, the hardest:
S12 =Q12/T1= 1.10nR,
S23 =S12 S31 =1.10nR.
P24-5 Note that TA=TB=TC/4 = TD.
Process I: ABC
(a) QAB =WAB =nRT0ln(VB/VA) = p0V0ln 2. QBC =3
2nR(TCTB) = 3
2(pCVCpBVB) =
3
2(4p0V0p0V0) = 4.5p0V0.
(b) WAB =nRT0ln(VB/VA) = p0V0ln 2. WBC = 0.
(c) Eint =3
2nR(TCTA) = 3
2(pCVCpAVA) = 3
2(4p0V0p0V0) = 4.5p0V0.
(d) ∆SAB =nR ln(VB/VA) = nR ln 2; SBC =3
2nR ln(TC/TB) = 3
2nR ln 4 = 3nR ln 2.Then
SAC = 4nR.
Process II: ADC
(a) QAD =WAD =nRT0ln(VD/VA) = p0V0ln 2. QDC =5
2nR(TCTD) = 5
2(pCVC
pDVD) = 5
2(4p0V0p0V0) = 10p0V0.
(b) WAB =nRT0ln(VD/VA) = p0V0ln 2. WDC =pV=p0(2V0V0/2) = 3
2p0V0.
(c) Eint =3
2nR(TCTA) = 3
2(pCVCpAVA) = 3
2(4p0V0p0V0) = 4.5p0V0.
(d) ∆SAD =nR ln(VD/VA) = nR ln 2; SDC =5
2nR ln(TC/TD) = 5
2nR ln 4 = 5nR ln 2.Then
SAC = 4nR.
P24-6 The heat required to melt the ice is
Q=m(cwT23 +L+ciT12),
= (0.0126 kg)[(4190 J/kg ·K)(15 C) + (333×103J/kg) + (2220 J/kg ·K)(10 C)],
= 5270 J.
The change in entropy of the ice is
Si=m[cwln(T3/T2) + L/T2+ciln(T2/T1)],
= (0.0126kg)[(4190J/kg·K) ln(288/273) + (333×103J/kg)/(273K),
+(2220J/kg·K) ln(273/263)],
= 19.24 J/K
The change in entropy of the lake is ∆Sl= (5270 J)/(288 K) = 18.29 J/K. The change in entropy
of the system is 0.95 J/kg.
P24-7 (a) This is a problem where the total internal energy of the two objects doesn’t change,
but since no work is done during the process, we can start with the simpler expression Q1+Q2= 0.
The heat transfers by the two objects are
Q1=m1c1(T1T1,i),
Q2=m2c2(T2T2,i).
Note that we don’t call the final temperature Tfhere, because we are not assuming that the two
objects are at equilibrium.
310
We combine these three equations,
m2c2(T2T2,i) = m1c1(T1T1,i),
m2c2T2=m2c2T2,i+m1c1(T1,iT1),
T2=T2,i+m1c1
m2c2
(T1,iT1)
As object 1 “cools down”, object 2 “heats up”, as expected.
(b) The entropy change of one object is given by
S=ZTf
Ti
mc dT
T=mc ln Tf
Ti
,
and the total entropy change for the system will be the sum of the changes for each object, so
S=m1c1ln T1
Ti,1
+m2c2ln T2
Ti,2
.
Into the this last equation we need to substitute the expression for T2in as a function of T1. There’s
no new physics in doing this, just a mess of algebra.
(c) We want to evaluate d(∆S)/dT1. To save on algebra we will work with the last expression,
remembering that T2is a function, not a variable. Then
d(∆S)
dT1
=m1c1
T1
+m2c2
T2
dT2
dT1
.
We’ve saved on algebra, but now we need to evaluate dT2/dT1. Starting with the results from part
(a),
dT2
dT1
=d
dT1T2,i+m1c1
m2c2
(T1,iT1),
=m1c1
m2c2
.
Now we collect the two results and write
d(∆S)
dT1
=m1c1
T1
+m2c2
T2m1c1
m2c2,
=m1c11
T11
T2.
We could consider writing T2out in all of its glory, but what would it gain us? Nothing. There is
actually considerably more physics in the expression as written, because...
(d) ...we get a maximum for ∆Swhen d(∆S)/dT1= 0, and this can only occur when T1=T2
according to the expression.
P24-8 Tb= (10.4×1.01×105Pa)(1.22 m3)/(2 mol)(8.31 J/mol ·K) = 7.71×104K.Maybe not so
realistic? Tacan be found after finding
pc=pb(Vb/Vc)γ= (10.4×1.01×105Pa)(1.22/9.13)1.67 = 3.64×104Pa,
Then
Ta=Tb(pa/pb) = (7.71×104K)(3.64×104/1.05×106) = 2.67×103K.
311
Similarly,
Tc=Ta(Vc/Va) = (2.67×103K)(9.13/1.22) = 2.00×104K.
(a) Heat is added during process ab only;
Qab =3
2nR(TbTa) = 3
2(2 mol)(8.31 J/mol ·K)(7.71×104K2.67×103K) = 1.85×106J.
(b) Heat is removed during process ca only;
Qca =5
2nR(TaTc) = 5
2(2 mol)(8.31 J/mol ·K)(2.67×103K2.00×104K) = 0.721×106J.
(c) W=|Qab|−|Qca|= (1.85×106J) (0.721×106J) = 1.13×106J.
(d) e=W/Qab = (1.13×106)/(1.85×106) = 0.611.
P24-9 The pV diagram for this process is Figure 23-21, except the cycle goes clockwise.
(a) Heat is input during the constant volume heating and the isothermal expansion. During
heating,
Q1=3
2nRT=3
2(1 mol)(8.31 J/mol ·K)(600 K 300 K) = 3740 J;
During isothermal expansion,
Q2=W2=nRT ln(Vf/V i) = (1 mol)(8.31 J/mol ·K)(600 K) ln(2) = 3460 J;
so Qin = 7200 J.
(b) Work is only done during the second and third processes; we’ve already solved the second,
W2=3460 J;
W3=pV=paVcpaVa=nR(TcTa) = (1 mol)(8.31 J/mol ·K)(600 K 300 K) = 2490 J;
So W=970 J.
(c) e=|W|/|Qin|= (970 J)/(7200 J) = 0.13.
P24-10 (a) Tb=Ta(pb/pa) = 3Ta;
Tc=Tb(Vb/Vc)γ1= 3Ta(1/4)0.4= 1.72Ta;
pc=pb(Vb/Vc)γ= 3pa(1/4)1.4= 0.430pa;
Td=Ta(Va/Vd)γ1=Ta(1/4)0.4= 0.574Ta;
pd=pa(Va/Vd)γ=pa(1/4)1.4= 0.144pa.
(b) Heat in occurs during process ab, so Qi=5
2nRTab = 5nRTa; Heat out occurs during
process cd, so Qo=5
2nRTcd = 2.87nRTa. Then
e= 1 (2.87nRTa/5nRTa) = 0.426.
P24-11 (c) (VB/VA)=(pA/pB) = (0/0.5) = 2. The work done on the gas during the isothermal
compression is
W=nRT ln(VB/VA) = (1 mol)(8.31 J/mol ·K)(300 K) ln(2) = 1730 J.
Since ∆Eint = 0 along an isotherm Qh= 1730 J.
The cycle has an efficiency of e= 1 (100/300) = 2/3. Then for the cycle,
W=eQh= (2/3)(1730 J) = 1150 J.
312

Navigation menu