6.4 
Motion in the Presence of Resistive Forces
165

Table 6.1

Terminal Speed for Various Objects Falling Through Air


Object

Skydiver
Baseball (radius 3.7 cm)
Golf ball (radius 2.1 cm)
Hailstone (radius 0.50 cm)
Raindrop (radius 0.20 cm)

Mass
Cross-Sectional Area
vT
(kg)(m 2)(m/s)

75 0.7060
0.145
4.2 3 102343
0.046
1.4 3 102344
24
4.8 3 10 7.9 3 102514
3.4 3 10251.3 3 10259.0

so


vT 5

2mg
Å DrA



(6.10)

Table 6.1 lists the terminal speeds for several objects falling through air.
Q uick Quiz 6.4  A baseball and a basketball, having the same mass, are dropped
through air from rest such that their bottoms are initially at the same height
above the ground, on the order of 1 m or more. Which one strikes the ground
first? (a) The baseball strikes the ground first. (b) The basketball strikes the
ground first. (c) Both strike the ground at the same time.

Conceptual Example 6.9

  The Skysurfer

Consider a skysurfer (Fig. 6.15) who jumps from a plane with his feet attached
firmly to his surfboard, does some tricks, and then opens his parachute.
Describe the forces acting on him during these maneuvers.
S o l u ti o n

Oliver Furrer/Jupiter Images


When the surfer first steps out of the plane, he has no vertical velocity. The
downward gravitational force causes him to accelerate toward the ground. As
his downward speed increases, so does the upward resistive force exerted by the
air on his body and the board. This upward force reduces their acceleration,
and so their speed increases more slowly. Eventually, they are going so fast that
the upward resistive force matches the downward gravitational force. Now the
net force is zero and they no longer accelerate, but instead reach their terminal
speed. At some point after reaching terminal speed, he opens his parachute,
resulting in a drastic increase in the upward resistive force. The net force (and
therefore the acceleration) is now upward, in the direction opposite the direction of the velocity. The downward velocity therefore decreases rapidly, and the
Figure 6.15  (Conceptual Example
resistive force on the parachute also decreases. Eventually, the upward resistive
6.9) A skysurfer.
force and the downward gravitational force balance each other again and a
much smaller terminal speed is reached, permitting a safe landing.
(Contrary to popular belief, the velocity vector of a skydiver never points upward. You may have seen a video in
which a skydiver appears to “rocket” upward once the parachute opens. In fact, what happens is that the skydiver slows
down but the person holding the camera continues falling at high speed.)


Example 6.10    Falling Coffee Filters  AM
The dependence of resistive force on the square of the speed is a simplification model. Let’s test the model for a specific
situation. Imagine an experiment in which we drop a series of bowl-shaped, pleated coffee filters and measure their terminal speeds. Table 6.2 on page 166 presents typical terminal speed data from a real experiment using these coffee filters as

continued


166Chapter 6 Circular Motion and Other Applications of Newton’s Laws
▸ 6.10 c o n t i n u e d
they fall through the air. The time constant t is small, so a dropped filter quickly reaches terminal speed. Each filter has a

mass of 1.64 g. When the filters are nested together, they combine in such a way that the front-facing surface area does not
increase. Determine the relationship between the resistive force exerted by the air and the speed of the falling filters.
S o l u ti o n

Conceptualize  Imagine dropping the coffee filters through the air. (If you have some coffee filters, try dropping
them.) Because of the relatively small mass of the coffee filter, you probably won’t notice the time interval during
which there is an acceleration. The filters will appear to fall at constant velocity immediately upon leaving your hand.
Categorize  Because a filter moves at constant velocity, we model it as a particle in equilibrium.
Analyze  At terminal speed, the upward resistive force on the filter balances the downward gravitational force so that
R 5 mg.
R 5 mg 5 1 1.64 g 2 a

Evaluate the magnitude of the resistive force:

Likewise, two filters nested together experience 0.032 2 N of resistive force, and so forth. These values of resistive force are shown in
the far right column of Table 6.2. A graph of the resistive force on
the filters as a function of terminal speed is shown in Figure 6.16a.
A straight line is not a good fit, indicating that the resistive force is
not proportional to the speed. The behavior is more clearly seen in
Figure 6.16b, in which the resistive force is plotted as a function of
the square of the terminal speed. This graph indicates that the resistive force is proportional to the square of the speed as suggested by
Equation 6.7.

0.18
0.16
0.14
0.12
0.10
0.08
0.06

0.04
0.02
0.00

The data points do not lie
along a straight line, but
instead suggest a curve.

0

1

2

Resistive force (N)

Resistive force (N)

Finalize  Here is a good opportunity for you to take some actual data
at home on real coffee filters and see if you can reproduce the results
shown in Figure 6.16. If you have shampoo and a marble as mentioned
in Example 6.8, take data on that system too and see if the resistive
force is appropriately modeled as being proportional to the speed.

3

4

1 000 g


Table 6.2

b 1 9.80 m/s2 2 5 0.016 1 N

Terminal Speed and

Resistive Force for Nested Coffee Filters
Number of
Filters

vT (m/s)a

R (N)

1.01
1.40
1.63
2.00
2.25
2.40
2.57
2.80
3.05
3.22

0.016 1
0.032 2
0.048 3
0.064 4
0.080 5

0.096 6
0.112 7
0.128 8
0.144 9
0.161 0

  1
  2
  3
  4
  5
  6
  7
  8
  9

10
a All

values of vT are approximate.

The fit of the straight line
to the data points indicates
that the resistive force is
proportional to the terminal
speed squared.

0

2


4

6

8

10

12

Terminal speed squared (m/s)2

Terminal speed (m/s)
a

0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00

1 kg

b


Figure 6.16  (Example 6.10) (a) Relationship between the resistive force acting on falling coffee filters and their terminal speed.
(b) Graph relating the resistive force to the square of the terminal speed.


Example 6.11    Resistive Force Exerted on a Baseball  AM
A pitcher hurls a 0.145-kg baseball past a batter at 40.2 m/s (5 90 mi/h). Find the resistive force acting on the ball at
this speed.


 
Summary
167

▸ 6.11 c o n t i n u e d
S o l u ti o n

Conceptualize  This example is different from the previous ones in that the object is now moving horizontally through
the air instead of moving vertically under the influence of gravity and the resistive force. The resistive force causes the
ball to slow down, and gravity causes its trajectory to curve downward. We simplify the situation by assuming the velocity vector is exactly horizontal at the instant it is traveling at 40.2 m/s.
Categorize  In general, the ball is a particle under a net force. Because we are considering only one instant of time, however, we are not concerned about acceleration, so the problem involves only finding the value of one of the forces.
Analyze  To determine the drag coefficient D, imagine
that we drop the baseball and allow it to reach terminal
speed. Solve Equation 6.10 for D:

D5

2mg
v T2rA
1 2mg

v 2
a 2 brAv 2 5 mg a b
vt
2 v t rA

Use this expression for D in Equation 6.7 to find an
expression for the magnitude of the resistive force:

R 5 12DrAv 2 5

Substitute numerical values, using the terminal speed
from Table 6.1:

R 5 1 0.145 kg 2 1 9.80 m/s2 2 a

40.2 m/s 2
b 5 1.2 N
43 m/s

Finalize  The magnitude of the resistive force is similar in magnitude to the weight of the baseball, which is about
1.4 N. Therefore, air resistance plays a major role in the motion of the ball, as evidenced by the variety of curve balls,
floaters, sinkers, and the like thrown by baseball pitchers.


Summary
Concepts and Principles
  A particle moving in uniform circular motion
has a centripetal acceleration; this acceleration
must be provided by a net force directed toward the
center of the circular path.


  An object moving through a liquid or gas experiences a
speed-dependent resistive force. This resistive force is in a
direction opposite that of the velocity of the object relative
to the medium and generally increases with speed. The
magnitude of the resistive force depends on the object’s size
and shape and on the properties of the medium through
which the object is moving. In the limiting case for a falling
object, when the magnitude of the resistive force equals the
object’s weight, the object reaches its terminal speed.

  An observer in a noninertial (accelerating)
frame of reference introduces fictitious forces
when applying Newton’s second law in that frame.

Analysis Model for Problem-Solving
  Particle in Uniform Circular Motion (Extension) With our new knowledge of forces, we can
extend the model of a particle in uniform circular motion, first introduced in Chapter 4. Newton’s second law applied to a particle moving in uniform circular motion states that the net force
causing the particle to undergo a centripetal acceleration (Eq. 4.14) is related to the acceleration according to
v
a F 5 ma c 5 m r
2



(6.1)

S

⌺F


S

ac

r

S

v


168Chapter 6 Circular Motion and Other Applications of Newton’s Laws
Objective Questions

1.  denotes answer available in Student Solutions Manual/Study Guide

1. A child is practicing
for a BMX race. His
B
speed remains conN
stant as he goes counA
C
terclockwise around
W
E
a level track with two
S
straight sections and
D

E
two nearly semicircular sections as shown in
Figure OQ6.1
the aerial view of Figure OQ6.1. (a) Rank
the magnitudes of his acceleration at the points A, B,
C, D, and E from largest to smallest. If his acceleration
is the same size at two points, display that fact in your
ranking. If his acceleration is zero, display that fact.
(b) What are the directions of his velocity at points A,
B, and C ? For each point, choose one: north, south,
east, west, or nonexistent. (c) What are the directions
of his acceleration at points A, B, and C ?
2.Consider a skydiver who has stepped from a helicopter
and is falling through air. Before she reaches terminal
speed and long before she opens her parachute, does
her speed (a) increase, (b) decrease, or (c) stay constant?
3.A door in a hospital has a pneumatic closer that pulls
the door shut such that the doorknob moves with constant speed over most of its path. In this part of its
motion, (a)  does the doorknob experience a centripetal acceleration? (b) Does it experience a tangential
acceleration?
4.A pendulum consists of a small object called a bob
hanging from a light cord of fixed length, with the top
end of the cord fixed, as represented in Figure OQ6.4.
The bob moves without friction, swinging equally
high on both sides. It moves from its turning point A
through point B and reaches its maximum speed at
point C. (a) Of these points, is there a point where
the bob has nonzero radial acceleration and zero tangential acceleration? If so, which point? What is the

Conceptual Questions


direction of its total acceleration
at this point? (b) Of these points,
is there a point where the bob
has nonzero tangential acceleration and zero radial acceleration? If so, which point? What is
the direction of its total acceleraA
tion at this point? (c)  Is there a
B
C
point where the bob has no acceleration? If so, which point? (d) Is
Figure OQ6.4
there a point where the bob has
both nonzero tangential and radial acceleration? If
so, which point? What is the direction of its total acceleration at this point?
5.As a raindrop falls through the atmosphere, its speed
initially changes as it falls toward the Earth. Before
the raindrop reaches its terminal speed, does the magnitude of its acceleration (a) increase, (b) decrease,
(c) stay constant at zero, (d) stay constant at 9.80 m/s2,
or (e) stay constant at some other value?
6.An office door is given a sharp push and swings open
against a pneumatic device that slows the door down
and then reverses its motion. At the moment the door
is open the widest, (a) does the doorknob have a centripetal acceleration? (b) Does it have a tangential
acceleration?
7.Before takeoff on an airplane, an inquisitive student
on the plane dangles an iPod by its earphone wire.
It hangs straight down as the plane is at rest waiting
to take off. The plane then gains speed rapidly as it
moves down the runway. (i) Relative to the student’s
hand, does the iPod (a)  shift toward the front of the

plane, (b) continue to hang straight down, or (c) shift
toward the back of the plane? (ii)  The speed of the
plane increases at a constant rate over a time interval
of several seconds. During this interval, does the angle
the earphone wire makes with the vertical (a) increase,
(b) stay constant, or (c) decrease?

1.  denotes answer available in Student Solutions Manual/Study Guide

1.What forces cause (a) an automobile, (b) a propellerdriven airplane, and (c) a rowboat to move?

tion is constant in magnitude at all times and parallel
to the velocity.

2.A falling skydiver reaches terminal speed with her
parachute closed. After the parachute is opened, what
parameters change to decrease this terminal speed?

5.The observer in the accelerating elevator of Example
5.8 would claim that the “weight” of the fish is T, the
scale reading, but this answer is obviously wrong. Why
does this observation differ from that of a person outside the elevator, at rest with respect to the Earth?
6.If someone told you that astronauts are weightless in
orbit because they are beyond the pull of gravity, would
you accept the statement? Explain.
7.It has been suggested that rotating cylinders about
20 km in length and 8 km in diameter be placed in

3.An object executes circular motion with constant
speed whenever a net force of constant magnitude acts

perpendicular to the velocity. What happens to the
speed if the force is not perpendicular to the velocity?
4.Describe the path of a moving body in the event that
(a) its acceleration is constant in magnitude at all times
and perpendicular to the velocity, and (b) its accelera-




Problems

space and used as colonies. The purpose of the rotation is to simulate gravity for the inhabitants. Explain
this concept for producing an effective imitation of
gravity.
8.Consider a small raindrop and a large raindrop falling through the atmosphere. (a) Compare their terminal speeds. (b) What are their accelerations when they
reach terminal speed?
9. Why does a pilot tend to black out when pulling out of
a steep dive?

169

10.A pail of water can be whirled in a vertical path such
that no water is spilled. Why does the water stay in the
pail, even when the pail is above your head?
11.“If the current position and velocity of every particle in the Universe were known, together with the
laws describing the forces that particles exert on one
another, the whole future of the Universe could be calculated. The future is determinate and preordained.
Free will is an illusion.” Do you agree with this thesis?
Argue for or against it.


Problems
The problems found in this
  chapter may be assigned

online in Enhanced WebAssign

1. straightforward; 2. intermediate;
3. challenging
1. full solution available in the Student
Solutions Manual/Study Guide

AMT  
Analysis Model tutorial available in

Enhanced WebAssign

GP   Guided Problem
M  Master It tutorial available in Enhanced
WebAssign
W  Watch It video solution available in
Enhanced WebAssign

BIO
Q/C
S

Section 6.1 Extending the Particle in Uniform Circular
Motion Model
1. A light string can


AMT support a stationM ary hanging load

r

m

of 25.0  kg before
breaking. An object
of mass m 5 3.00  kg
attached to the string
rotates on a frictionless, horizontal table
in a circle of radius
r 5 0.800 m, and
Figure P6.1
the other end of the
string is held fixed
as in Figure P6.1. What range of speeds can the object
have before the string breaks?

2. Whenever two Apollo astronauts were on the surface of
the Moon, a third astronaut orbited the Moon. Assume
the orbit to be circular and 100 km above the surface
of the Moon, where the acceleration due to gravity is
1.52 m/s2. The radius of the Moon is 1.70 3 106 m.
Determine (a) the astronaut’s orbital speed and (b) the
period of the orbit.
3. In the Bohr model of the hydrogen atom, an electron
moves in a circular path around a proton. The speed
of the electron is approximately 2.20 3 10 6 m/s. Find
(a) the force acting on the electron as it revolves in a

circular orbit of radius 0.529 3 10210 m and (b) the
centripetal acceleration of the electron.
4. A curve in a road forms part of a horizontal circle. As a
car goes around it at constant speed 14.0 m/s, the total
horizontal force on the driver has magnitude 130 N.

What is the total horizontal force on the driver if the
speed on the same curve is 18.0 m/s instead?
5.In a cyclotron (one type of particle accelerator), a
deuteron (of mass 2.00 u) reaches a final speed of
10.0% of the speed of light while moving in a circular
path of radius 0.480 m. What magnitude of magnetic
force is required to maintain the deuteron in a circular path?
y
6.A car initially traveling
eastward
turns
north
by
W
traveling in a circular
path at uniform speed
x
as shown in Figure P6.6.
35.0Њ
C
O
The length of the arc
ABC is 235 m, and the
B

car completes the turn
in 36.0 s. (a) What is the
A
acceleration when the
car is at B located at an
Figure P6.6
angle of 35.08? Express
your answer in terms of the unit vectors i^ and j^. Determine (b) the car’s average speed and (c) its average
acceleration during the 36.0-s interval.

7.A space station, in the form of a wheel 120 m in
diameter, rotates to provide an “artificial gravity” of
3.00 m/s2 for persons who walk around on the inner
wall of the outer rim. Find the rate of the wheel’s
rotation in revolutions per minute that will produce
this effect.
8.Consider a conical pendulum (Fig. P6.8) with a bob
W of mass m 5 80.0 kg on a string of length L 5 10.0 m
that makes an angle of u 5 5.008 with the vertical. Determine (a) the horizontal and vertical components of the


170Chapter 6 Circular Motion and Other Applications of Newton’s Laws
16. A roller-coaster car (Fig. P6.16) has a mass of 500 kg

L

9. A coin placed 30.0 cm from the center
u
M of a rotating, horizontal turntable slips
m

when its speed is 50.0 cm/s. (a)  What
force causes the centripetal acceleration
when the coin is stationary relative to Figure P6.8
the turntable? (b) What is the coefficient of static friction between coin and turntable?
10. Why is the following situation
impossible? The object of mass
m 5 4.00  kg in Figure P6.10 is
,
attached to a vertical rod by two
m
strings of length , 5 2.00 m. The
d
strings are attached to the rod
at points a distance d 5 3.00 m
,
apart. The object rotates in a
horizontal circle at a constant
speed of v 5 3.00  m/s, and the
strings remain taut. The rod
Figure P6.10
rotates along with the object so
that the strings do not wrap onto the rod. What If?
Could this situation be possible on another planet?
11. A crate of eggs is located in the middle of the flatbed
W of a pickup truck as the truck negotiates a curve in the
flat road. The curve may be regarded as an arc of a
circle of radius 35.0 m. If the coefficient of static friction between crate and truck is 0.600, how fast can the
truck be moving without the crate sliding?
Section 6.2 Nonuniform Circular Motion
12.A pail of water is rotated in a vertical circle of radius

W 1.00 m. (a) What two external forces act on the water in
Q/C the pail? (b) Which of the two forces is most important
in causing the water to move in a circle? (c) What is
the pail’s minimum speed at the top of the circle if no
water is to spill out? (d) Assume the pail with the speed
found in part (c) were to suddenly disappear at the top
of the circle. Describe the subsequent motion of the
water. Would it differ from the motion of a projectile?
13. A hawk flies in a horizontal arc of radius 12.0 m at constant speed 4.00 m/s. (a) Find its centripetal acceleration. (b) It continues to fly along the same horizontal
arc, but increases its speed at the rate of 1.20 m/s2. Find
the acceleration (magnitude and direction) in this situation at the moment the hawk’s speed is 4.00 m/s.
14. A 40.0-kg child swings in a swing supported by two
M chains, each 3.00 m long. The tension in each chain at
the lowest point is 350 N. Find (a) the child’s speed at
the lowest point and (b) the force exerted by the seat
on the child at the lowest point. (Ignore the mass of
the seat.)
15. A child of mass m swings in a swing supported by two
S chains, each of length R. If the tension in each chain
at the lowest point is T, find (a) the child’s speed at the
lowest point and (b) the force exerted by the seat on the
child at the lowest point. (Ignore the mass of the seat.)

AMT when fully loaded with passengers. The path of the
W coaster from its initial point shown in the figure to point

B involves only up-and-down motion (as seen by the riders), with no motion to the left or right. (a) If the vehicle
has a speed of 20.0 m/s at point A, what is the force
exerted by the track on the car at this point? (b) What is
the maximum speed the vehicle can have at point B

and still remain on the track? Assume the roller-coaster
tracks at points A and B are parts of vertical circles of
radius r 1 5 10.0 m and r 2 5 15.0 m, respectively.
B

r2

r1
A

Figure P6.16  Problems 16 and 38.
17. A roller coaster at the Six

Q/C Flags Great America amuse-

ment park in Gurnee, Illinois, incorporates some
clever design technology and
some basic physics. Each vertical loop, instead of being
circular, is shaped like a teardrop (Fig. P6.17). The cars
ride on the inside of the loop
at the top, and the speeds
are fast enough to ensure the
Figure P6.17
cars remain on the track.
The biggest loop is 40.0 m high. Suppose the speed at
the top of the loop is 13.0 m/s and the corresponding
centripetal acceleration of the riders is 2g. (a) What is
the radius of the arc of the teardrop at the top? (b) If
the total mass of a car plus the riders is M, what force
does the rail exert on the car at the top? (c) Suppose

the roller coaster had a circular loop of radius 20.0 m.
If the cars have the same speed, 13.0 m/s at the top,
what is the centripetal acceleration of the riders at the
top? (d)  Comment on the normal force at the top in
the situation described in part (c) and on the advantages of having teardrop-shaped loops.
Frank Cezus/Getty Images

force exerted by the string on the pendulum and (b) the radial acceleration of
the bob.

18. One end of a cord is fixed and a small

Q/C 0.500-kg object is attached to the

other end, where it swings in a section
of a vertical circle of radius 2.00 m as
u
S
shown in Figure P6.18. When u 5 20.08,
v
the speed of the object is 8.00 m/s.
At this instant, find (a) the tension
in the string, (b) the tangential and
Figure P6.18
radial components of acceleration,
and (c) the total acceleration. (d) Is your answer
changed if the object is swinging down toward its





Problems

171

lowest point instead of swinging up? (e) Explain your
answer to part (d).

of kinetic friction mk between the backpack and the
elevator floor.

19.An adventurous archeologist (m 5 85.0 kg) tries to cross
a river by swinging from a vine. The vine is 10.0 m long,
and his speed at the bottom of the swing is 8.00 m/s.
The archeologist doesn’t know that the vine has a
breaking strength of 1 000 N. Does he make it across
the river without falling in?

25. A small container of water is placed on a turntable
inside a microwave oven, at a radius of 12.0 cm from
the center. The turntable rotates steadily, turning one
revolution in each 7.25 s. What angle does the water
surface make with the horizontal?
Section 6.4 Motion in the Presence of Resistive Forces

Section 6.3 Motion in Accelerated Frames
20.An object of mass m 5
5.00  kg, attached to a
spring scale, rests on a
m

frictionless, horizontal
surface as shown in Figure P6.20. The spring
scale, attached to the
Figure P6.20
front end of a boxcar,
reads zero when the
car is at rest. (a) Determine the acceleration of the car
if the spring scale has a constant reading of 18.0 N
when the car is in motion. (b) What constant reading
will the spring scale show if the car moves with constant velocity? Describe the forces on the object as
observed (c) by someone in the car and (d) by someone at rest outside the car.
21. An object of mass m 5
M 0.500 kg is suspended
from the ceiling of an
accelerating truck as
shown in Figure P6.21.
Taking a 5 3.00 m/s2,
find (a) the angle u that
the string makes with
the vertical and (b) the
tension T in the string.

S

a

m

u


Figure P6.21

22. A child lying on her back experiences 55.0 N tension in
the muscles on both sides of her neck when she raises
her head to look past her toes. Later, sliding feet first
down a water slide at terminal speed 5.70 m/s and riding high on the outside wall of a horizontal curve of
radius 2.40 m, she raises her head again to look forward past her toes. Find the tension in the muscles on
both sides of her neck while she is sliding.
23.A person stands on a scale in an elevator. As the elevator
M starts, the scale has a constant reading of 591 N. As the
elevator later stops, the scale reading is 391 N. Assuming the magnitude of the acceleration is the same
during starting and stopping, determine (a) the weight
of the person, (b) the person’s mass, and (c) the acceleration of the elevator.
24. Review. A student, along with her backpack on the
S floor next to her, are in an elevator that is accelerating upward with acceleration a. The student gives her
backpack a quick kick at t 5 0, imparting to it speed
v and causing it to slide across the elevator floor.
At time t, the backpack hits the opposite wall a distance L away from the student. Find the coefficient

26.Review. (a) Estimate the terminal speed of a wooden
sphere (density 0.830 g/cm3) falling through air, taking its radius as 8.00 cm and its drag coefficient as
0.500. (b)  From what height would a freely falling
object reach this speed in the absence of air resistance?
27.The mass of a sports car is 1 200 kg. The shape of the
body is such that the aerodynamic drag coefficient
is 0.250 and the frontal area is 2.20 m2. Ignoring all
other sources of friction, calculate the initial acceleration the car has if it has been traveling at 100 km/h
and is now shifted into neutral and allowed to coast.
2 8.A skydiver of mass 80.0 kg jumps from a slow-moving
aircraft and reaches a terminal speed of 50.0 m/s.

(a) What is her acceleration when her speed is 30.0 m/s?
What is the drag force on the skydiver when her speed
is (b) 50.0 m/s and (c) 30.0 m/s?
29.Calculate the force required to pull a copper ball of
radius 2.00 cm upward through a fluid at the constant speed 9.00 cm/s. Take the drag force to be proportional to the speed, with proportionality constant
0.950 kg/s. Ignore the buoyant force.
30. A small piece of Styrofoam packing material is dropped
W from a height of 2.00 m above the ground. Until it
reaches terminal speed, the magnitude of its acceleration is given by a 5 g 2 Bv. After falling 0.500 m, the
Styrofoam effectively reaches terminal speed and then
takes 5.00 s more to reach the ground. (a) What is the
value of the constant B? (b) What is the acceleration at
t 5 0? (c) What is the acceleration when the speed is
0.150 m/s?
31. A small, spherical bead of mass 3.00 g is released from
M rest at t 5 0 from a point under the surface of a viscous liquid. The terminal speed is observed to be vT 5
2.00 cm/s. Find (a) the value of the constant b that
appears in Equation 6.2, (b) the time t at which the
bead reaches 0.632vT , and (c) the value of the resistive
force when the bead reaches terminal speed.
32. At major league baseball games, it is commonplace to
flash on the scoreboard a speed for each pitch. This
speed is determined with a radar gun aimed by an
operator positioned behind home plate. The gun uses
the Doppler shift of microwaves reflected from the
baseball, an effect we will study in Chapter 39. The gun
determines the speed at some particular point on the
baseball’s path, depending on when the operator pulls
the trigger. Because the ball is subject to a drag force
due to air proportional to the square of its speed given

by R 5 kmv 2, it slows as it travels 18.3 m toward the


172Chapter 6 Circular Motion and Other Applications of Newton’s Laws
plate according to the formula v 5 vie2kx . Suppose the
ball leaves the pitcher’s hand at 90.0 mi/h 5 40.2 m/s.
Ignore its vertical motion. Use the calculation of R for
baseballs from Example 6.11 to determine the speed of
the pitch when the ball crosses the plate.
33. Assume the resistive force acting on a speed skater is
S proportional to the square of the skater’s speed v and
is given by f 5 2kmv 2, where k is a constant and m is
the skater’s mass. The skater crosses the finish line of
a straight-line race with speed vi and then slows down
by coasting on his skates. Show that the skater’s speed
at any time t after crossing the finish line is v(t) 5
vi /(1 1 ktvi ).
3 4. Review. A window washer pulls a rubber squeegee

AMT down a very tall vertical window. The squeegee has

mass 160 g and is mounted on the end of a light rod.
The coefficient of kinetic friction between the squeegee and the dry glass is 0.900. The window washer
presses it against the window with a force having a
horizontal component of 4.00 N. (a) If she pulls the
squeegee down the window at constant velocity, what
vertical force component must she exert? (b) The window washer increases the downward force component
by 25.0%, while all other forces remain the same. Find
the squeegee’s acceleration in this situation. (c) The
squeegee is moved into a wet portion of the window,

where its motion is resisted by a fluid drag force R proportional to its velocity according to R 5 220.0v, where
R is in newtons and v is in meters per second. Find the
terminal velocity that the squeegee approaches, assuming the window washer exerts the same force described
in part (b).

35.A motorboat cuts its engine when its speed is 10.0 m/s
and then coasts to rest. The equation describing the
motion of the motorboat during this period is v 5
vie2ct , where v is the speed at time t, vi is the initial
speed at t 5 0, and c is a constant. At t 5 20.0 s, the
speed is 5.00 m/s. (a) Find the constant c. (b) What is
the speed at t 5 40.0 s? (c) Differentiate the expression
for v(t) and thus show that the acceleration of the boat
is proportional to the speed at any time.
36. You can feel a force of air drag on your hand if you
stretch your arm out of the open window of a speeding
car. Note: Do not endanger yourself. What is the order
of magnitude of this force? In your solution, state the
quantities you measure or estimate and their values.
Additional Problems
37.A car travels clockwise at constant speed around a circular
section of a horizontal road as
shown in the aerial view of Figure P6.37. Find the directions of
its velocity and acceleration at (a)
position A and (b) position B.

A
N
W


E
S

B

Figure P6.37
38.The mass of a roller-coaster car,
including its passengers, is
500 kg. Its speed at the bottom of the track in Figure
P6.16 is 19 m/s. The radius of this section of the track is

r 1 5 25 m. Find the force that a seat in the roller-coaster
car exerts on a 50-kg passenger at the lowest point.
39.A string under a tension of 50.0 N is used
m
to whirl a rock in a
R
horizontal circle of
radius 2.50 m at a
speed of 20.4 m/s on
a frictionless surface
as shown in Figure
P6.39. As the string
is pulled in, the
speed of the rock
Figure P6.39
increases. When the
string on the table is 1.00 m long and the speed of the
rock is 51.0 m/s, the string breaks. What is the breaking
strength, in newtons, of the string?

40.Disturbed by speeding cars outside his workplace,
Nobel laureate Arthur Holly Compton designed a
speed bump (called the “Holly hump”) and had it
in­stalled. Suppose a 1 800-kg car passes over a hump
in a roadway that follows the arc of a circle of radius
20.4 m as shown in Figure P6.40. (a) If the car travels at
30.0 km/h, what force does the road exert on the car as
the car passes the highest point of the hump?
S
v
(b) What If? What is
the maximum speed
the car can have without losing contact with
Figure P6.40 
the road as it passes this
Problems 40 and 41.
highest point?
41. A car of mass m passes over a hump in a road that folS lows the arc of a circle of radius R as shown in Figure
P6.40. (a) If the car travels at a speed v, what force does
the road exert on the car as the car passes the highest
point of the hump? (b) What If? What is the maximum
speed the car can have without losing contact with the
road as it passes this highest point?
m
42. A child’s toy consists of a small
S wedge that has an acute angle u
L
(Fig. P6.42). The sloping side of
the wedge is frictionless, and an
u

object of mass m on it remains
at constant height if the wedge
is spun at a certain constant
speed. The wedge is spun by
rotating, as an axis, a vertical
rod that is firmly attached to
the wedge at the bottom end.
Figure P6.42
Show that, when the object sits
at rest at a point at distance L up along the wedge, the
speed of the object must be v 5 (gL sin u)1/2.

43. A seaplane of total mass m lands on a lake with initial
S speed v i i^. The only horizontal force on it is a resistive
force on its pontoons from the water. The resistive
force
is proportional to the velocity of the seaplane:
S
R 5 2bS
v . Newton’s second law applied to the plane
is 2bv i^ 5 m 1 dv/dt 2 i^. From the fundamental theorem




Problems
of calculus, this differential equation implies that the
speed changes according to
dv
b

5 2 3 dt
3
m 0
vi v
v

t


(a) Carry out the integration to determine the speed of
the seaplane as a function of time. (b) Sketch a graph
of the speed as a function of time. (c) Does the seaplane come to a complete stop after a finite interval of
time? (d) Does the seaplane travel a finite distance in
stopping?
4 4. An object of mass m1 5
m1
W 4.00 kg is tied to an
String 1 ,
object of mass m 2 5
S
3.00 kg with String 1 of
m2
v
length , 5 0.500 m.
,
The combination is
String 2
swung in a vertical circular path on a second
Figure P6.44
string, String 2, of

length , 5 0.500 m. During the motion, the two strings
are collinear at all times as shown in Figure P6.44.
At the top of its motion, m 2 is traveling at v 5 4.00 m/s.
(a) What is the tension in String 1 at this instant?
(b) What is the tension in String 2 at this instant?
(c) Which string will break first if the combination is
rotated faster and faster?
45. A ball of mass m 5 0.275 kg swings
in a vertical circular path on a
string L 5 0.850 m long as in Figure P6.45. (a) What are the forces
L
acting on the ball at any point on
the path? (b) Draw force diagrams
m
for the ball when it is at the bottom
of the circle and when it is at the
Figure P6.45
top. (c)  If its speed is 5.20  m/s at
the top of the circle, what is the
tension in the string there? (d) If the string breaks when
its tension exceeds 22.5 N, what is the maximum speed
the ball can have at the bottom before that happens?
46. Why is the following situation impossible? A mischievous
child goes to an amusement park with his family. On
one ride, after a severe scolding from his mother, he
slips out of his seat and climbs to the top of the ride’s
structure, which is shaped like a cone with its axis vertical and its sloped sides making an angle of u 5 20.08
with the horizontal as shown in Figure P6.46. This part

d

u

Figure P6.46

173

of the structure rotates about the vertical central axis
when the ride operates. The child sits on the sloped
surface at a point d 5 5.32 m down the sloped side
from the center of the cone and pouts. The coefficient
of static friction between the boy and the cone is 0.700.
The ride operator does not notice that the child has
slipped away from his seat and so continues to operate
the ride. As a result, the sitting, pouting boy rotates in
a circular path at a speed of 3.75 m/s.
47. (a) A luggage carousel at an airport has the form of
a section of a large cone, steadily rotating about its
vertical axis. Its metallic surface slopes downward
toward the outside, making an angle of 20.08 with the
horizontal. A piece of luggage having mass 30.0 kg is
placed on the carousel at a position 7.46 m measured
horizontally from the axis of rotation. The travel bag
goes around once in 38.0 s. Calculate the force of static
friction exerted by the carousel on the bag. (b) The
drive motor is shifted to turn the carousel at a higher
constant rate of rotation, and the piece of luggage is
bumped to another position, 7.94 m from the axis of
rotation. Now going around once in every 34.0 s, the
bag is on the verge of slipping down the sloped surface.
Calculate the coefficient of static friction between the

bag and the carousel.
48. In a home laundry dryer, a cylindrical tub containing
wet clothes is rotated steadily about a horizontal axis
as shown in Figure P6.48. So that the clothes will dry
uniformly, they are made to tumble. The rate of rotation of the smooth-walled tub is chosen so that a small
piece of cloth will lose contact with the tub when the
cloth is at an angle of u 5 68.08 above the horizontal. If
the radius of the tub is r 5 0.330 m, what rate of revolution is needed?

r
u

Figure P6.48
49.Interpret the graph in Figure 6.16(b), which describes
the results for falling coffee filters discussed in Example 6.10. Proceed as follows. (a) Find the slope of the
straight line, including its units. (b) From Equation
6.6, R 5 12 DrAv 2, identify the theoretical slope of a
graph of resistive force versus squared speed. (c) Set
the experimental and theoretical slopes equal to each
other and proceed to calculate the drag coefficient of
the filters. Model the cross-sectional area of the filters
as that of a circle of radius 10.5 cm and take the density of air to be 1.20 kg/m3. (d) Arbitrarily choose the
eighth data point on the graph and find its vertical


174Chapter 6 Circular Motion and Other Applications of Newton’s Laws

50. A basin surrounding a drain has the shape of a circular

Q/C cone opening upward, having everywhere an angle of


35.0° with the horizontal. A 25.0-g ice cube is set sliding around the cone without friction in a horizontal
circle of radius R. (a) Find the speed the ice cube must
have as a function of R. (b) Is any piece of data unnecessary for the solution? Suppose R is made two times
larger. (c) Will the required speed increase, decrease,
or stay constant? If it changes, by what factor? (d) Will
the time required for each revolution increase,
decrease, or stay constant? If it changes, by what factor?
(e) Do the answers to parts (c) and (d) seem contradictory? Explain.

S
51. A truck is moving with
a
S constant acceleration
a up a hill that makes
an angle f with the
u
m
horizontal as in Figure
P6.51. A small sphere
of mass m is suspended
f
from the ceiling of the
truck by a light cord. If
Figure P6.51
the pendulum makes a
constant angle u with the perpendicular to the ceiling,
what is a?

52. The pilot of an airplane executes a loop-the-loop

maneuver in a vertical circle. The speed of the airplane
is 300 mi/h at the top of the loop and 450 mi/h at the
bottom, and the radius of the circle is 1 200 ft. (a) What
is the pilot’s apparent weight at the lowest point if his
true weight is 160 lb? (b) What is his apparent weight
at the highest point? (c)  What If? Describe how the
pilot could experience weightlessness if both the
radius and the speed can be varied. Note: His apparent
weight is equal to the magnitude of the force exerted
by the seat on his body.
53. Review. While learning to drive, you are in a 1 200-kg

Q/C car moving at 20.0 m/s across a large, vacant, level

parking lot. Suddenly you realize you are heading
straight toward the brick sidewall of a large supermarket and are in danger of running into it. The pavement
can exert a maximum horizontal force of 7 000 N on
the car. (a) Explain why you should expect the force to
have a well-defined maximum value. (b) Suppose you
apply the brakes and do not turn the steering wheel.
Find the minimum distance you must be from the wall
to avoid a collision. (c) If you do not brake but instead
maintain constant speed and turn the steering wheel,
what is the minimum distance you must be from the
wall to avoid a collision? (d) Of the two methods in
parts (b) and (c), which is better for avoiding a collision? Or should you use both the brakes and the steering wheel, or neither? Explain. (e) Does the conclusion

in part (d) depend on the numerical values given in
this problem, or is it true in general? Explain.
54. A puck of mass m1 is tied


Q/C to a string and allowed
S to revolve in a circle of

m1
R

radius R on a frictionless, horizontal table.
The other end of the
string passes through a
m2
small hole in the center of the table, and
an object of mass m2 is
Figure P6.54
tied to it (Fig. P6.54).
The suspended object
remains in equilibrium while the puck on the tabletop
revolves. Find symbolic expressions for (a) the tension in
the string, (b) the radial force acting on the puck, and
(c) the speed of the puck. (d) Qualitatively describe what
will happen in the motion of the puck if the value of m2
is increased by placing a small additional load on the
puck. (e) Qualitatively describe what will happen in the
motion of the puck if the value of m2 is instead decreased
by removing a part of the hanging load.

55.Because the Earth rotates about its axis, a point on
M the equator experiences a centripetal acceleration of
0.033 7 m/s2, whereas a point at the poles experiences
no centripetal acceleration. If a person at the equator

has a mass of 75.0  kg, calculate (a) the gravitational
force (true weight) on the person and (b) the normal
force (apparent weight) on the person. (c) Which force
is greater? Assume the Earth is a uniform sphere and
take g 5 9.800 m/s2.
56. Galileo thought about whether acceleration should be

Q/C defined as the rate of change of velocity over time or as
S the rate of change in velocity over distance. He chose

the former, so let’s use the name “vroomosity” for the
rate of change of velocity over distance. For motion of
a particle on a straight line with constant acceleration,
the equation v 5 vi 1 at gives its velocity v as a function
of time. Similarly, for a particle’s linear motion with
constant vroomosity k, the equation v 5 vi 1 kx gives
the velocity as a function of the position x if the particle’s speed is vi at x 5 0. (a) Find the law describing the
total force acting on this object of mass m. (b) Describe
an example of such a motion or explain why it is unrealistic. Consider (c) the possibility of k positive and
(d) the possibility of k negative.

57. Figure P6.57 shows

AMT a photo of a swing
W ride at an amusement

park. The structure
consists of a horizontal, rotating, circular
platform of diameter
D from which seats

of mass m are suspended at the end
of massless chains
of length d. When
the system rotates at

Stuart Gregory/Getty Images

separation from the line of best fit. Express this scatter
as a percentage. (e) In a short paragraph, state what
the graph demonstrates and compare it with the theoretical prediction. You will need to make reference
to the quantities plotted on the axes, to the shape of
the graph line, to the data points, and to the results of
parts (c) and (d).

Figure P6.57




175

Problems
constant speed, the chains swing outward and make
an angle u with the vertical. Consider such a ride with
the following parameters: D 5 8.00  m, d 5 2.50 m,
m 5 10.0 kg, and u 5 28.08. (a) What is the speed of
each seat? (b) Draw a diagram of forces acting on the
combination of a seat and a 40.0-kg child and (c) find
the tension in the chain.


58. Review. A piece of putty is initially located at point A
GP on the rim of a grinding wheel rotating at constant
S angular speed about a horizontal axis. The putty is
dislodged from point A when the diameter through A
is horizontal. It then rises vertically and returns to A at
the instant the wheel completes one revolution. From
this information, we wish to find the speed v of the
putty when it leaves the wheel and the force holding it
to the wheel. (a)  What analysis model is appropriate
for the motion of the putty as it rises and falls? (b) Use
this model to find a symbolic expression for the time
interval between when the putty leaves point A and
when it arrives back at A, in terms of v and g. (c) What
is the appropriate analysis model to describe point A
on the wheel? (d) Find the period of the motion of
point A in terms of the tangential speed v and the
radius R of the wheel. (e) Set the time interval from
part (b) equal to the period from part (d) and solve
for the speed v of the putty as it leaves the wheel. (f) If
the mass of the putty is m, what is the magnitude of
the force that held it to the wheel before it was
released?
59.An amusement park ride
Q/C consists of a large vertical
S cylinder that spins about
its axis fast enough that
any person inside is held
up against the wall when
the floor drops away (Fig.
R

P6.59). The coefficient
of static friction between
person and wall is ms ,
and the radius of the cylFigure P6.59
inder is R. (a)  Show that
the maximum period of
revolution necessary to keep the person from falling is
T 5 (4p2Rms /g)1/2. (b) If the rate of revolution of the
cylinder is made to be somewhat larger, what happens to the magnitude of each one of the forces acting on the person? What happens in the motion of the
person? (c) If the rate of revolution of the cylinder is
instead made to be somewhat smaller, what happens to
the magnitude of each one of the forces acting on the
person? How does the motion of the person change?
60. Members of a skydiving club were given the following
data to use in planning their jumps. In the table, d is
the distance fallen from rest by a skydiver in a “freefall stable spread position” versus the time of fall t.
(a) Convert the distances in feet into meters. (b) Graph
d (in meters) versus t. (c) Determine the value of the
terminal speed vT by finding the slope of the straight
portion of the curve. Use a least-squares fit to determine this slope.

t (s)

d (ft)t (s)

d (ft)t (s)

0  0 7  652
1 16 8  808
2 62 9  971

3
138
10
1 138
4
242
11
1 309
5
366
12
1 483
6
504
13
1 657

14
15
16
17
18
19
20

d (ft)

1 831
2 005
2 179

2 353
2 527
2 701
2 875

61. A car rounds a banked curve as discussed in Example
S 6.4 and shown in Figure 6.5. The radius of curvature
of the road is R, the banking angle is u, and the coefficient of static friction is ms . (a) Determine the range
of speeds the car can have without slipping up or down
the road. (b) Find the minimum value for ms such that
the minimum speed is zero.
62. In Example 6.5, we investigated the forces a child expeW riences on a Ferris wheel. Assume the data in that example applies to this problem. What force (magnitude and
direction) does the seat exert on a 40.0-kg child when
the child is halfway between top and bottom?
63. A model airplane of mass 0.750 kg flies with a speed of
M 35.0 m/s in a horizontal circle at the end of a 60.0-m-long
control wire as shown in Figure P6.63a. The forces
exerted on the airplane are shown in Figure P6.63b: the
tension in the control wire, the gravitational force, and
aerodynamic lift that acts at u 5 20.08 inward from the
vertical. Compute the tension in the wire, assuming it
makes a constant angle of u 5 20.08 with the horizontal.
S

Circular path
of airplane

Flift
u


Wire
u
S

T
a

S

mg

b

Figure P6.63
6 4. A student builds and calibrates an accelerometer and
uses it to determine the speed of her car around a certain unbanked highway curve. The accelerometer is a
plumb bob with a protractor that she attaches to the
roof of her car. A friend riding in the car with the student observes that the plumb bob hangs at an angle
of 15.0° from the vertical when the car has a speed of
23.0 m/s. (a) What is the centripetal acceleration of the
car rounding the curve? (b) What is the radius of the
curve? (c) What is the speed of the car if the plumb
bob deflection is 9.00° while rounding the same curve?
Challenge Problems
65. A 9.00-kg object starting from rest falls through a viscous medium and experiences a resistive force given
by Equation 6.2. The object reaches one half its terminal speed in 5.54 s. (a) Determine the terminal speed.
(b) At what time is the speed of the object threefourths the terminal speed? (c) How far has the object
traveled in the first 5.54 s of motion?



176Chapter 6 Circular Motion and Other Applications of Newton’s Laws
66. For t , 0, an object of mass m experiences no force and
S moves in the positive x direction with a constant speed
vi . Beginning at t 5 0, when the object passes position
x 5 0, it experiences a net S
resistive force proportional
to the square of its speed:  F net 5 2mkv 2 i^, where k is a
constant. The speed of the object after t 5 0 is given by
v 5 vi /(1 1 kvit). (a) Find the position x of the object as
a function of time. (b) Find the object’s velocity as a
function of position.
North
Radius of circular
67. A golfer tees off from
Pole
path of tee
a location precisely at
fi 5 35.08 north latiTee Golf ball
tude. He hits the ball
trajectory
RE cos fi
due south, with range
285  m. The ball’s iniHole
RE
tial velocity is at 48.08
above the horizontal.
fi
Suppose air resistance
Equator
is negligible for the golf

ball. (a) For how long
is the ball in flight?
Figure P6.67
The cup is due south
of the golfer’s location, and the golfer would have a
hole-in-one if the Earth were not rotating. The Earth’s
rotation makes the tee move in a circle of radius
R E cos fi 5 (6.37 3 106 m) cos 35.08 as shown in Figure P6.67. The tee completes one revolution each day.
(b) Find the eastward speed of the tee relative to the
stars. The hole is also moving east, but it is 285 m
farther south and thus at a slightly lower latitude ff .
Because the hole moves in a slightly larger circle, its
speed must be greater than that of the tee. (c) By how
much does the hole’s speed exceed that of the tee?
During the time interval the ball is in flight, it moves
upward and downward as well as southward with the
projectile motion you studied in Chapter 4, but it
also moves eastward with the speed you found in part
(b). The hole moves to the east at a faster speed, however, pulling ahead of the ball with the relative speed

68.

Q/C

69.

70.

you found in part (c). (d) How far to
the west of the hole does the ball land?

A single bead can slide with negligible
friction on a stiff wire that has been
u
bent into a circular loop of radius
15.0 cm as shown in Figure P6.68. The
circle is always in a vertical plane and
rotates steadily about its vertical diameter with a period of 0.450 s. The posi- Figure P6.68
tion of the bead is described by the
angle u that the radial line, from the center of the loop
to the bead, makes with the vertical. (a) At what angle
up from the bottom of the circle can the bead stay
motionless relative to the turning circle? (b) What If?
Repeat the problem, this time taking the period of the
circle’s rotation as 0.850 s. (c) Describe how the solution to part (b) is different from the solution to part
(a). (d) For any period or loop size, is there always an
angle at which the bead can stand still relative to the
loop? (e) Are there ever more than two angles? Arnold
Arons suggested the idea for this problem.
The expression F 5 arv 1 br 2v 2 gives the magnitude of
the resistive force (in newtons) exerted on a sphere of
radius r (in meters) by a stream of air moving at speed
v (in meters per second), where a and b are constants
with appropriate SI units. Their numerical values are
a 5 3.10 3 1024 and b 5 0.870. Using this expression,
find the terminal speed for water droplets falling under
their own weight in air, taking the following values for
the drop radii: (a) 10.0 mm, (b) 100 mm, (c) 1.00 mm.
For parts (a) and (c), you can obtain accurate answers
without solving a quadratic equation by considering
which of the two contributions to the air resistance is

dominant and ignoring the lesser contribution.
Because of the Earth’s rotation, a plumb bob does not
hang exactly along a line directed to the center of the
Earth. How much does the plumb bob deviate from a
radial line at 35.08 north latitude? Assume the Earth is
spherical.


Energy of a System

c h a p t e r

7

7.1 Systems and Environments
7.2 Work Done by a Constant
Force
7.3 The Scalar Product of Two
Vectors
7.4 Work Done by a Varying
Force
7.5 Kinetic Energy and the
Work–Kinetic
Energy Theorem
7.6 Potential Energy of a System
7.7 Conservative and
Nonconservative Forces
7.8 Relationship Between
Conservative Forces and
Potential Energy

7.9 Energy Diagrams and
Equilibrium
of a System

The definitions of quantities such as position, velocity, acceleration, and force and
associated principles such as Newton’s second law have allowed us to solve a variety of
problems. Some problems that could theoretically be solved with Newton’s laws, however,
are very difficult in practice, but they can be made much simpler with a different approach.
Here and in the following chapters, we will investigate this new approach, which will include
definitions of quantities that may not be familiar to you. Other quantities may sound familiar, but they may have more specific meanings in physics than in everyday life. We begin
this discussion by exploring the notion of energy.
The concept of energy is one of the most important topics in science and engineering. In
everyday life, we think of energy in terms of fuel for transportation and heating, electricity for lights and appliances, and foods for consumption. These ideas, however, do not truly
define energy. They merely tell us that fuels are needed to do a job and that those fuels provide us with something we call energy.
Energy is present in the Universe in various forms. Every physical process that occurs in
the Universe involves energy and energy transfers or transformations. Unfortunately, despite
its extreme importance, energy cannot be easily defined. The variables in previous chapters
were relatively concrete; we have everyday experience with velocities and forces, for example.
Although we have experiences with energy, such as running out of gasoline or losing our electrical service following a violent storm, the notion of energy is more abstract.

On a wind farm at the mouth of the
River Mersey in Liverpool, England,
the moving air does work on the
blades of the windmills, causing the
blades and the rotor of an electrical
generator to rotate. Energy is
transferred out of the system of the
windmill by means of electricity.
(Christopher Furlong/Getty Images)



177


178Chapter 7 

Energy of a System

 

The concept of energy can be applied to mechanical systems without resorting to Newton’s
laws. Furthermore, the energy approach allows us to understand thermal and electrical phenomena in later chapters of the book in terms of the same models that we will develop here in
our study of mechanics.
Our analysis models presented in earlier chapters were based on the motion of a particle
or an object that could be modeled as a particle. We begin our new approach by focusing our
attention on a new simplification model, a system, and analysis models based on the model of
a system. These analysis models will be formally introduced in Chapter 8. In this chapter, we
introduce systems and three ways to store energy in a system.

7.1 Systems and Environments
Pitfall Prevention 7.1
Identify the System  The most
important first step to take in solving a problem using the energy
approach is to identify the appropriate system of interest.

In the system model, we focus our attention on a small portion of the Universe—
the system—and ignore details of the rest of the Universe outside of the system.
A critical skill in applying the system model to problems is identifying the system.
A valid system
• may be a single object or particle

• may be a collection of objects or particles
• may be a region of space (such as the interior of an automobile engine combustion cylinder)
• may vary with time in size and shape (such as a rubber ball, which deforms
upon striking a wall)
Identifying the need for a system approach to solving a problem (as opposed to
a particle approach) is part of the Categorize step in the General Problem-Solving
Strategy outlined in Chapter 2. Identifying the particular system is a second part of
this step.
No matter what the particular system is in a given problem, we identify a system
boundary, an imaginary surface (not necessarily coinciding with a physical surface) that divides the Universe into the system and the environment surrounding
the system.
As an example, imagine a force applied to an object in empty space. We can
define the object as the system and its surface as the system boundary. The force
applied to it is an influence on the system from the environment that acts across the
system boundary. We will see how to analyze this situation from a system approach
in a subsequent section of this chapter.
Another example was seen in Example 5.10, where the system can be defined as
the combination of the ball, the block, and the cord. The influence from the environment includes the gravitational forces on the ball and the block, the normal
and friction forces on the block, and the force exerted by the pulley on the cord.
The forces exerted by the cord on the ball and the block are internal to the system
and therefore are not included as an influence from the environment.
There are a number of mechanisms by which a system can be influenced by its
environment. The first one we shall investigate is work.

7.2 Work Done by a Constant Force
Almost all the terms we have used thus far—velocity, acceleration, force, and so
on—convey a similar meaning in physics as they do in everyday life. Now, however,
we encounter a term whose meaning in physics is distinctly different from its everyday meaning: work.
To understand what work as an influence on a system
means to the physicist,

S
consider the situation illustrated in Figure 7.1. A force F is applied to a chalkboard


© Cengage Learning/Charles D. Winters

7.2 
Work Done by a Constant Force
179

a

b

c

Figure 7.1  ​A n eraser being pushed along a chalkboard tray by a force acting at different angles
with respect to the horizontal direction.
eraser, which we identify as the system, and the eraser slides along the tray. If we
want to know how effective the force is in moving the eraser, we must consider not
only the magnitude of the force but also its direction. Notice that the finger in Figure 7.1 applies forces in three different directions on the eraser. Assuming the magnitude of the applied force is the same in all three photographs, the push applied
in Figure 7.1b does more to move the eraser than the push in Figure 7.1a. On the
other hand, Figure 7.1c shows a situation in which the applied force does not move
the eraser at all, regardless of how hard it is pushed (unless, of course, we apply a
force so great that we break the chalkboard tray!). These results suggest that when
analyzing forces to determine the influence they have on the system, we must consider the vector nature of forces. We must
also consider the magnitude of the force.
S
Moving a force with a magnitude of 0 F 0 5 2 N through a displacement represents a
greater influence on the system than moving a force of magnitude 1 N through the

same displacement. The magnitude of the displacement is also important. Moving
the eraser 3 m along the tray represents a greater influence than moving it 2 cm if
the same force is used in both cases.
Let us examine the situation in Figure 7.2, where the object (the system) undergoes a displacement along a straight line while acted on by a constant force of magnitude F that makes an angle u with the direction of the displacement.

Pitfall Prevention 7.2
Work Is Done by . . . on . . .  Not
only must you identify the system,
you must also identify what agent
in the environment is doing work
on the system. When discussing
work, always use the phrase, “the
work done by . . . on . . . .” After
“by,” insert the part of the environment that is interacting directly
with the system. After “on,” insert
the system. For example, “the work
done by the hammer on the nail”
identifies the nail as the system,
and the force from the hammer
represents the influence from the
environment.

S

F

u

S


The work W done on a system by an agent exerting a constant force on the
system is the product of the magnitude F of the force, the magnitude Dr of
the displacement of the point of application of the force, and cos u, where u is
the angle between the force and displacement vectors:


W ; F Dr cos u

(7.1)

Notice in Equation 7.1 that work is a scalar, even though it is defined in terms
S
r . In Section 7.3, we explore how to
of two vectors, a force F and a displacement DS
combine two vectors to generate a scalar quantity.
Notice also that the displacement in Equation 7.1 is that of the point of application
of the force. If the force is applied to a particle or a rigid object that can be modeled
as a particle, this displacement is the same as that of the particle. For a deformable
system, however, these displacements are not the same. For example, imagine pressing in on the sides of a balloon with both hands. The center of the balloon moves
through zero displacement. The points of application of the forces from your hands
on the sides of the balloon, however, do indeed move through a displacement as
the balloon is compressed, and that is the displacement to be used in Equation 7.1.
We will see other examples of deformable systems, such as springs and samples of
gas contained in a vessel.
As an example of the distinction between the definition of work and our everyday understanding of the word, consider holding a heavy chair at arm’s length for
3 min. At the end of this time interval, your tired arms may lead you to think you

⌬r

Figure 7.2  ​A n object undergoes

a displacement DS
r under the
S
action of a constant force  F.
WW
Work done by a
constant force


180Chapter 7 

Energy of a System

S

F is the only force
that does work on
the block in this
situation.
S

F

S

n

u

S


⌬r
S

mg

Figure 7.3  ​A n object is displaced on a frictionless, horizonn
tal surface. The normal force S
g do
and the gravitational force mS
no work on the object.
Pitfall Prevention 7.3
Cause of the Displacement  We can
calculate the work done by a force
on an object, but that force is not
necessarily the cause of the object’s
displacement. For example, if you
lift an object, (negative) work is
done on the object by the gravitational force, although gravity is
not the cause of the object moving
upward!

d

Q uick Quiz 7.1 ​The gravitational force exerted by the Sun on the Earth holds the
Earth in an orbit around the Sun. Let us assume that the orbit is perfectly circular. The work done by this gravitational force during a short time interval in
which the Earth moves through a displacement in its orbital path is (a) zero
(b) positive (c) negative (d) impossible to determine

F


S

S

⌬r

⌬r
S

F

S

F

S

S

⌬r

⌬r
c

Figure 7.4  ​(Quick Quiz 7.2)
A block is pulled by a force in four
different directions. In each case,
the displacement of the block
is to the right and of the same

magnitude.

Example 7.1

W 5 F Dr

b

S

a



The units of work are those of force multiplied by those of length. Therefore,
the SI unit of work is the newton ? meter (N ? m 5 kg ? m2/s2). This combination of
units is used so frequently that it has been given a name of its own, the joule ( J).
An important consideration for a system approach to problems is that work is an
energy transfer. If W is the work done on a system and W is positive, energy is transferred to the system; if W is negative, energy is transferred from the system. Therefore, if a system interacts with its environment, this interaction can be described
as a transfer of energy across the system boundary. The result is a change in the
energy stored in the system. We will learn about the first type of energy storage in
Section 7.5, after we investigate more aspects of work.

S

F

have done a considerable amount of work on the chair. According to our definition, however, you have done no work on it whatsoever. You exert a force to support
the chair, but you do not move it. A force does no work on an object if the force
does not move through a displacement. If Dr 5 0, Equation 7.1 gives W 5 0, which is

the situation depicted in Figure 7.1c.
Also notice from Equation 7.1 that the work done by a force on a moving object
is zero when the force applied is perpendicular to the displacement of its point of
application. That is, if u 5 908, then W 5 0 because cos 908 5 0. For example, in
Figure 7.3, the work done by the normal force on the object and the work done by
the gravitational force on the object are both zero because both forces are perpendicular to the displacement and have zero components along an axis in the direction of DS
r.
S
The sign of the work also depends on the direction of F relative to DS
rS
. The work
done by the applied force on a system is positive when the projection of F onto DS
r
is in the same direction as the displacement. For example, when an object is lifted,
the work done by the applied force on the object is positive because the direction
of that force is upward, in the sameSdirection as the displacement of its point of
application. When the projection of F onto DS
r is in the direction opposite the displacement, W is negative. For example, as an object is lifted, the work done by the
gravitational force on the object is negative. The factor cos u in the definition of W
(Eq. 7.1) automaticallyStakes care of the sign.
If an applied force F is in the same direction as the displacement DS
r , then u 5
0 and cos 0 5 1. In this case, Equation 7.1 gives

Q uick Quiz 7.2 ​Figure 7.4 shows four situations in which a force is applied to an
object. In all four cases, the force has the same magnitude, and the displacement of the object is to the right and of the same magnitude. Rank the situations in order of the work done by the force on the object, from most positive to
most negative.

  Mr. Clean


A man cleaning a floor pulls a vacuum cleaner with a force of magnitude F 5 50.0 N at an angle of 30.08 with the horizontal (Fig. 7.5). Calculate the work done by the force on the vacuum cleaner as the vacuum cleaner is displaced 3.00 m
to the right.


7.3 
The Scalar Product of Two Vectors
181

▸ 7.1 c o n t i n u e d
50.0 N

S o l u ti o n

Conceptualize  ​Figure 7.5 helps conceptualize the
situation. Think about an experience in your life in
which you pulled an object across the floor with a
rope or cord.
Categorize  ​We are asked for the work done on

Figure 7.5  ​(Example 7.1) A
vacuum cleaner being pulled
at an angle of 30.08 from the
horizontal.

S

n

30.0Њ


S
an object by a force and are given the force on
mg
the object, the displacement of the object, and
the angle between the two vectors, so we categorize this example as a substitution problem. We identify the vacuum
cleaner as the system.

W 5 F Dr cos u 5 1 50.0 N 2 1 3.00 m 2 1 cos 30.08 2
5 130 J

Use the definition of work (Eq. 7.1):

S

n and the gravitational  F g 5 mS
g do no work on the vacuum cleaner
Notice in this situation that the normal force S
because these forces are perpendicular to the displacements of their points of application. Furthermore, there was
no mention of whether there was friction between the vacuum cleaner and the floor. The presence or absence of friction is not important when calculating the work done by the applied force. In addition, this work does not depend on
whether the vacuum moved at constant velocity or if it accelerated.


Pitfall Prevention 7.4

7.3 The Scalar Product of Two Vectors
Because of the way the force and displacement vectors are combined in Equation
7.1, it is helpful to use a convenient mathematical tool
called
theSscalar
product of

S
S
S
two vectors. We write this scalar product of vectors  A  and  B  as  A ? B . (Because of
the dot symbol, the scalar product is often S
called the
dot product.)
S
The scalar product of any two vectors A  and  B  is defined as a scalar quantity
equal to the product of the magnitudes of the two vectors and the cosine of the
angle u between them:
S

S

A ? B ; AB cos u



S

(7.2)

S

As is the case with any multiplication,  A  and  B  need not have the same units.
By comparing this definition with Equation 7.1, we can express Equation 7.1 as a
scalar product:
S


W 5 F Dr cos u 5 F ? DS
r


S

S

(7.3)

S

S

S

S

S

B
u

S

S
A . B = AB cos u

B cos u
S


S

A

A?B 5 B?A

S

S

statement is equivalent to stating that  A ? B  equals the product of the magnitude of  B  and the projection of  A
onto B .

2In

WW
Scalar product of any two
S
S
vectors  A  and  B

S

In other words,  F ? D r is a shorthand notation for F Dr cos u.
Before continuing with our discussion of work,Slet us investigate
some properties
S
of the dot product. Figure 7.6 shows two vectors  A  and  B  and the angle u between
them used

in the
definition of the dot product. In Figure
7.6, B cos u is the projecS
S
S S
tion of B  onto SA . Therefore, EquationS7.2 means
that
A
?
B is the product of the
S
magnitude of  A  and the projection of B  onto  A .1
From the right-hand side of Equation 7.2, we also see that the scalar product is
commutative.2 That is,

1This

Work Is a Scalar  Although Equation 7.3 defines the work in terms
of two vectors, work is a scalar;
there is no direction associated
with it. All types of energy and
energy transfer are scalars. This
fact is a major advantage of the
energy approach because we don’t
need vector calculations!

S

Chapter 11, you will see another way of combining vectors that proves useful in physics and is not commutative.


Figure
7.6  ​The scalar productS
S S

A ? B  equals the magnitude of  A  
multiplied by B cos u, which is the
S
S
projection of B  onto  A .


182Chapter 7 

Energy of a System

Finally, the scalar product obeys the distributive law of multiplication, so
S

S

S

S

S

S

S


A ?1B 1 C2 5 A ? B 1 A ? C

S

The scalar product is simple
to evaluate from Equation
7.2 when  A  is S
either
perS
S
S
S
B
B
pendicular or parallel
to 
.
If 
 is
perpendicular
to 
 (u
5
908),
then
A
?
B
 5
0.

A
S S
S
(The
equality A
?
B
 5
0
also
holds
in
the
more
trivial
case
in
which
either A
 
S
S
S
or B  is zero.) If vector 
A
 is
parallel
to
vector 
 and

the
two
point
in
the
same
direcB
S S
S
S
tion (u 5 0), then  A ? B  5 AB. If vector 
A  is parallel to vector B  but the two point
S S
in opposite directions (u 5 1808), then  A ? B  5 2AB. The scalar product is negative
when 908 , u # 1808.
The unit vectors ^i, ^j, and k^ , which were defined in Chapter 3, lie in the positive
x, y, and z directions, respectively,
of a right-handed coordinate system. Therefore, it
S S
follows from the definition of  A ? B  that the scalar products of these unit vectors are
Scalar products of  
unit vectors



i^ ? i^ 5 j^ ? j^ 5 k^ ? k^ 5 1



i^ ? j^ 5 i^ ? k^ 5 j^ ? k^ 5 0

S

(7.4)
(7.5)
S

Equations 3.18 and 3.19 state that two vectors  A  and  B  can be expressed in unitvector form as
S

A 5 Ax i^ 1 Ay j^ 1 Az k^
S

B 5 Bx i^ 1 By j^ 1 Bz k^

Using these expressions for the vectors and
the Sinformation given in Equations 7.4
S
and 7.5 shows that the scalar product of  A  and  B  reduces to
S

S

A ? B 5 Ax Bx 1 Ay By 1 Az Bz



(7.6)

(Details of the derivation S
are left

for you in Problem 7 at the end of the chapter.) In
S
the special case in which A 5 B , we see that
S

S

A ? A 5 A2x 1 A2y 1 A2z 5 A2

Q uick Quiz 7.3 ​Which of the following statements is true about the relationship
between the dot product
of two vectors and the
product of the magni­tudes S S
S S
S S
of the vectors? (a)  A ? B  is larger than AB. (b)  A ? B  is smaller than AB. (c)  A ? B  
could
be larger or smaller than AB, depending on the angle between the vectors.
S S
(d)  A ? B  could be equal to AB.

Example 7.2
S

   The Scalar Product
S

S

S


The vectors  A  and  B  are given by  A 5 2 i^ 1 3 ^j  and  B 5 2 i^ 1 2 ^j .
S

S

(A)  ​Determine the scalar product  A ? B .
S o l u ti o n

Conceptualize  ​There is no physical system to imagine here. Rather, it is purely a mathematical exercise involving two
vectors.

Categorize   ​Because we have a definition for the scalar product, we categorize this example as a substitution problem.
S

S

Substitute the specific vector expressions for  A  and  B :

S

S

A ? B 5 1 2 i^ 1 3 j^ 2 ? 1 2 i^ 1 2 j^ 2

5 22 i^ ? i^ 1 2 i^ ? 2 j^ 2 3 j^ ? i^ 1 3 j^ ? 2 ^j
5 22(1) 1 4(0) 2 3(0) 1 6(1) 5 22 1 6 5 4

The same result is obtained when we use Equation 7.6 directly, where Ax 5 2, Ay 5 3, Bx 5 21, and By 5 2.



7.4 
Work Done by a Varying Force
183

▸ 7.2 c o n t i n u e d
S

S

(B)  Find the angle u between  A  and  B .
Solution
S

S

A 5 "A2x 1 A2y 5 " 1 2 2 2 1 1 3 2 2 5 "13

Evaluate the magnitudes of  A  and  B  using the Pythagorean theorem:

B 5 "B2x 1 B2y 5 " 1 21 2 2 1 1 2 2 2 5 "5
S

cos u 5



Example 7.3

S


A?B
4
4
5
5
AB
"65
"13"5
4
5 60.38
u 5 cos21
!65

Use Equation 7.2 and the result from part (A) to find the
angle:

   Work Done by a Constant Force

A particle moving in the xy plane undergoes a displacement given Sby DS
r 5 1 2.0 i^ 1 3.0 j^ 2 m as a constant force 
S
^
^
1
2
F 5 5.0 i 1 2.0 j N acts on the particle. Calculate the work done by  F  on the particle.
Solution

Conceptualize  ​A lthough this example is a little more physical than the previous one in that it identifies a force and a

displacement, it is similar in terms of its mathematical structure.
Categorize  ​Because we are given force and displacement vectors and asked to find the work done by this force on the
particle, we categorize this example as a substitution problem.
S

Substitute the expressions for  F  and DS
r into
Equation 7.3 and use Equations 7.4 and 7.5:

S
W 5 F ? DS
r 5 3 1 5.0 i^ 1 2.0 j^ 2 N 4 ? 3 1 2.0 i^ 1 3.0 j^ 2 m 4

5 1 5.0 i^ ? 2.0 i^ 1 5.0 i^ ? 3.0 j^ 1 2.0 j^ ? 2.0 i^ 1 2.0 j^ ? 3.0 j^ 2 N # m
5 [10 1 0 1 0 1 6] N ? m 5 16 J



7.4 Work Done by a Varying Force
Consider a particle being displaced along the x axis under the action of a force that
varies with position. In such a situation, we cannot use Equation 7.1 S
to calculate the
work done by the force because this relationship applies only when F is constant in
magnitude and direction. Figure 7.7a (page 184) shows a varying force applied on
a particle that moves from initial position xi to final position xf . Imagine a particle
undergoing a very small displacement Dx, shown in the figure. The x component
Fx of the force is approximately constant over this small interval; for this small displacement, we can approximate the work done on the particle by the force using
Equation 7.1 as

W < Fx Dx

which is the area of the shaded rectangle in Figure 7.7a. If the Fx versus x curve is
divided into a large number of such intervals, the total work done for the displacement from xi to xf is approximately equal to the sum of a large number of such
terms:
W < a Fx Dx
xf



xi


184Chapter 7 

Energy of a System

The total work done for the
displacement from xi to xf is
approximately equal to the sum
of the areas of all the rectangles.

If the size of the small displacements is allowed to approach zero, the number of
terms in the sum increases without limit but the value of the sum approaches a definite value equal to the area bounded by the Fx curve and the x axis:
lim a Fx Dx 5 3 Fx dx
Dx S 0

Area = Fx ⌬x

Fx

xf


xf
xi

xi

Therefore, we can express the work done by Fx on the system of the particle as it
moves from xi to xf as
W 5 3 Fx dx
xf

Fx



(7.7)

xi

xi

xf

x

⌬x
a
The work done by the component
Fx of the varying force as the particle moves from xi to xf is exactly
equal to the area under the curve.


This equation reduces to Equation 7.1 when the component Fx 5 F cos u remains
constant.
If more than one force acts on a system and the system can be modeled as a particle,
the total work done on the system is just the work done by the net force. If we
express the net force in the x direction as o Fx , the total work, or net work, done as
the particle moves from xi to xf is
a W 5 Wext 5 3 1 a Fx 2 dx (particle)
xf

For the general case of a net force g F whose magnitude and direction may both
vary, we use the scalar product,
xi

Fx

S

xi

S
(7.8)
a W 5 Wext 5 3 1 a F 2 ? d r  (particle)
where the integral is calculated over the path that the particle takes through space.
The subscript “ext” on work reminds us that the net work is done by an external
agent on the system. We will use this notation in this chapter as a reminder and to
differentiate this work from an internal work to be described shortly.
If the system cannot be modeled as a particle (for example, if the system is
deformable), we cannot use Equation 7.8 because different forces on the system
may move through different displacements. In this case, we must evaluate the work

done by each force separately and then add the works algebraically to find the net
work done on the system:
S



Work
xf

x

b

Figure 7.7  ​(a) The work done on
a particle by the force component
Fx for the small displacement Dx is
Fx Dx, which equals the area of the
shaded rectangle. (b) The width Dx
of each rectangle is shrunk to zero.



S
a W 5 Wext 5 a a 3 F ? d r b  (deformable system)
S

forces

Example 7.4


   Calculating Total Work Done from a Graph

A force acting on a particle varies with x as shown in Figure 7.8. Calculate the
work done by the force on the particle as it moves from x 5 0 to x 5 6.0 m.
S o l u ti o n

Conceptualize  ​Imagine a particle subject to the force in Figure 7.8. The force
remains constant as the particle moves through the first 4.0 m and then decreases
linearly to zero at 6.0 m. In terms of earlier discussions of motion, the particle could
be modeled as a particle under constant acceleration for the first 4.0 m because
the force is constant. Between 4.0 m and 6.0 m, however, the motion does not fit
into one of our earlier analysis models because the acceleration of the particle is
changing. If the particle starts from rest, its speed increases throughout the motion,
and the particle is always moving in the positive x direction. These details about its
speed and direction are not necessary for the calculation of the work done, however.

The net work done by this force
is the area under the curve.
Fx (N)
5

0

A

B

C
1


2

3

4

5

6

x (m)

Figure 7.8  ​(Example 7.4) The
force acting on a particle is constant
for the first 4.0 m of motion and then
decreases linearly with x from x B 5
4.0 m to x C 5 6.0 m.

Categorize  ​Because the force varies during the motion of the particle, we must
use the techniques for work done by varying forces. In this case, the graphical representation in Figure 7.8 can be used
to evaluate the work done.


7.4 
Work Done by a Varying Force
185

▸ 7.4 c o n t i n u e d
Analyze  ​The work done by the force is equal to the area under the curve from x A 5 0 to x C 5 6.0 m. This area is equal
to the area of the rectangular section from A to B plus the area of the triangular section from B to C.

Evaluate the area of the rectangle:

W A to B 5 (5.0 N)(4.0 m) 5 20 J

Evaluate the area of the triangle:

W B to C 5 12(5.0 N)(2.0 m) 5 5.0 J

Find the total work done by the force on the particle:

W A to C 5 W A to B 1 W B to C 5 20 J 1 5.0 J 5 25 J

Finalize  ​Because the graph of the force consists of straight lines, we can use rules for finding the areas of simple geometric models to evaluate the total work done in this example. If a force does not vary linearly as in Figure 7.7, such
rules cannot be used and the force function must be integrated as in Equation 7.7 or 7.8.


Work Done by a Spring
A model of a common physical system on which the force varies with position is
shown in Figure 7.9. The system is a block on a frictionless, horizontal surface and
connected to a spring. For many springs, if the spring is either stretched or compressed a small distance from its unstretched (equilibrium) configuration, it exerts
on the block a force that can be mathematically modeled as


(7.9)

Fs 5 2kx

WW
Spring force


where x is the position of the block relative to its equilibrium (x 5 0) position and k
is a positive constant called the force constant or the spring constant of the spring.
In other words, the force required to stretch or compress a spring is proportional
to the amount of stretch or compression x. This force law for springs is known as
Hooke’s law. The value of k is a measure of the stiffness of the spring. Stiff springs
have large k values, and soft springs have small k values. As can be seen from Equation 7.9, the units of k are N/m.
xϭ0

S

Fs

a

x

When x is positive
(stretched spring), the
spring force is directed
to the left.

x

When x is zero
(natural length of the
spring), the spring
force is zero.

x


b
S

Fs

c

x

When x is negative
(compressed spring),
the spring force is
directed to the right.

x
Fs
kx max
d

0
x max

x
Fs ϭ Ϫkx

The work done by the
spring force on the
block as it moves from
Ϫx max to 0 is the area
of the shaded triangle,

1
2
Ϫ
2 kx max.

Figure 7.9  The force exerted
by a spring on a block varies with
the block’s position x relative to
the equilibrium position x 5 0.
(a) x is positive. (b) x is zero. (c) x
is negative. (d) Graph of Fs versus
x for the block–spring system.


186Chapter 7 

Energy of a System

The vector form of Equation 7.9 is
S

F s 5 Fs i^ 5 2kx ^i



(7.10)

where we have chosen the x axis to lie along the direction the spring extends or
compresses.
The negative sign in Equations 7.9 and 7.10 signifies that the force exerted by

the spring is always directed opposite the displacement from equilibrium. When
x . 0 as in Figure 7.9a so that the block is to the right of the equilibrium position,
the spring force is directed to the left, in the negative x direction. When x , 0 as in
Figure 7.9c, the block is to the left of equilibrium and the spring force is directed
to the right, in the positive x direction. When x 5 0 as in Figure 7.9b, the spring
is unstretched and Fs 5 0. Because the spring force always acts toward the equilibrium position (x 5 0), it is sometimes called a restoring force.
If the spring is compressed until the block is at the point 2x max and is then
released, the block moves from 2x max through zero to 1x max. It then reverses direction, returns to 2x max, and continues oscillating back and forth. We will study these
oscillations in more detail in Chapter 15. For now, let’s investigate the work done by
the spring on the block over small portions of one oscillation.
Suppose the block has been pushed to the left to a position 2x max and is then
released. We identify the block as our system and calculate the work Ws done by the
spring force on the block as the block moves from xi 5 2x max to xf 5 0. Applying
Equation 7.8 and assuming the block may be modeled as a particle, we obtain
S
Ws 5 3 Fs ? dS
r 5 3 1 2kx i^ 2 ? 1 dx i^ 2 5 3
xf



xi

0

2x max

1 2kx 2 dx 5 12kx 2max

(7.11)


where we have used the integral e xn dx 5 xn11/(n 1 1) with n 5 1. The work done by
the spring force is positive because the force is in the same direction as its displacement (both are to the right). Because the block arrives at x 5 0 with some speed, it
will continue moving until it reaches a position 1x max. The work done by the spring
2
force on the block as it moves from xi 5 0 to xf 5 x max is Ws 5 212kx max
. The work is
negative because for this part of the motion the spring force is to the left and its
displacement is to the right. Therefore, the net work done by the spring force on the
block as it moves from xi 5 2x max to xf 5 x max is zero.
Figure 7.9d is a plot of Fs versus x. The work calculated in Equation 7.11 is the
area of the shaded triangle, corresponding to the displacement from 2x max to 0.
2
Because the triangle has base x max and height kx max, its area is 12kx max
, agreeing with
the work done by the spring as given by Equation 7.11.
If the block undergoes an arbitrary displacement from x 5 xi to x 5 xf , the work
done by the spring force on the block is
Ws 5 3 1 2kx 2 dx 5 12kx i 2 2 12kx f 2
xf

Work done by a spring  



(7.12)

xi

From Equation 7.12, we see that the work done by the spring force is zero for any

motion that ends where it began (xi 5 xf ). We shall make use of this important
result in Chapter 8 when we describe the motion of this system in greater detail.
Equations 7.11 and 7.12 describe the work done by the spring on the block. Now
let us consider the work done on the block by an external agent as the agent applies
a force on the block and the block moves very slowly from xi 5 2x max to xf 5 0 as
in Figure 7.10. We canScalculate this work by noting that at any value of the position, the applied
force SF app is equal in magnitude
and opposite in direction to the
S
S
spring force F s , so F app 5 Fapp i^ 5 2 F s 5 2 1 2kx i^ 2 5 kx ^i. Therefore, the work
done by this applied force (the external agent) on the system of the block is


Wext 5 3 F app ? dS
r 5 3 1 kx i^ 2 ? 1 dx i^ 2 5 3
S

xf

xi

0

2
kx dx 5 212kx max


2x max



7.4 
Work Done by a Varying Force
187

This work is equal to the negative of the work done by the spring force for this displacement (Eq. 7.11). The work is negative because the external agent must push
inward on the spring to prevent it from expanding, and this direction is opposite
the direction of the displacement of the point of application of the force as the
block moves from 2x max to 0.
For an arbitrary displacement of the block, the work done on the system by the
external agent is
Wext 5 3 kx dx 5 12kx f 2 2 12kx i 2

If the process of moving the
block is carried out very slowly,
S
then Fapp is equal in magnitude
S
and opposite in direction to Fs
at all times.
S

xf



S

Fapp


Fs

(7.13)

xi

Notice that this equation is the negative of Equation 7.12.
Q uick Quiz 7.4 ​A dart is inserted into a spring-loaded dart gun by pushing the
spring in by a distance x. For the next loading, the spring is compressed a distance 2x. How much work is required to load the second dart compared with
that required to load the first? (a) four times as much (b) two times as much
(c) the same (d) half as much (e) one-fourth as much

Example 7.5

xi ϭ Ϫx max

xf ϭ 0

Figure 7.10  ​A block moves from
xi 5 2x max to xf 5 0 on a frictionS
less surface as a force  F app is
applied to the block.

  Measuring k for a Spring  AM

A common technique used to measure the force constant of a spring is demonstrated by the setup in Figure 7.11. The spring is hung vertically (Fig. 7.11a), and
an object of mass m is attached to its lower end. Under the action of the “load” mg,
the spring stretches a distance d from its equilibrium position (Fig. 7.11b).

S


Fs

d

(A)  I​ f a spring is stretched 2.0 cm by a suspended object having a mass of
0.55 kg, what is the force constant of the spring?
S o l u ti o n

Conceptualize  ​Figure 7.11b shows what happens to the spring when the object is
attached to it. Simulate this situation by hanging an object on a rubber band.

Categorize  ​The object in Figure 7.11b is at rest and not accelerating, so it is mod-

The elongation d is
caused by the weight mg
of the attached object.
a

S

mg

c

b

eled as a particle in equilibrium.

Figure 7.11  ​(Example 7.5) Deter-


Analyze  ​Because the object is in equilibrium, the net force on it is zero and the

mining the force constant k of a
spring.

upward spring force balances the downward gravitational force mS
g (Fig. 7.11c).

Apply the particle in equilibrium model to the object:
Apply Hooke’s law to give Fs 5 kd and solve for k:

S

g 50
F s 1 mS

k5

mg
d

5

S

Fs 2 mg 5 0

1 0.55 kg 2 1 9.80 m/s2 2
2.0 3 1022 m


S

Fs 5 mg

5 2.7 3 102 N/m

(B)  ​How much work is done by the spring on the object as it stretches through this distance?
S o l u ti o n

Use Equation 7.12 to find the work done by the spring
on the object:

Ws 5 0 2 12kd 2 5 212 1 2.7 3 10 2 N/m 2 1 2.0 3 1022 m 2 2
5 25.4 3 1022 J

Finalize  ​This work is negative because the spring force acts upward on the object, but its point of application (where
the spring attaches to the object) moves downward. As the object moves through the 2.0-cm distance, the gravitational
force also does work on it. This work is positive because the gravitational force is downward and so is the displacement
continued


188Chapter 7 

Energy of a System

▸ 7.5 c o n t i n u e d
of the point of application of this force. Would we expect the work done by the gravitational force, as the applied force
in a direction opposite to the spring force, to be the negative of the answer above? Let’s find out.
Evaluate the work done by the gravitational force on the

object:

S

r 5 1 mg 2 1 d 2 cos 0 5 mgd
W 5 F ? DS

5 (0.55 kg)(9.80 m/s2)(2.0 3 1022 m) 5 1.1 3 1021 J

If you expected the work done by gravity simply to be that done by the spring with a positive sign, you may be surprised
by this result! To understand why that is not the case, we need to explore further, as we do in the next section.


7.5 K
 inetic Energy and the Work–Kinetic
Energy Theorem
⌬x
S

⌺F
m

S

vi

S

vf


Figure 7.12  ​A n object undergo-

r 5 Dx ^i and
ing a displacement DS
a change in velocity under the
S
action of a constant net force g F .

We have investigated work and identified it as a mechanism for transferring energy
into a system. We have stated that work is an influence on a system from the environment, but we have not yet discussed the result of this influence on the system.
One possible result of doing work on a system is that the system changes its speed.
In this section, we investigate this situation and introduce our first type of energy
that a system can possess, called kinetic energy.
Consider a system consisting of a single object. Figure 7.12 shows a block of
mass m moving
through a displacement directed to the right under the action of a
S
net force g F , also directed to the right. We know from Newton’s second law that
S
the block moves with an acceleration a
. If the block (and therefore the force) moves
S
^
through a displacement DSr 5 Dx i 5 1 x f 2 x i 2 ^i, the net work done on the block by
the external net force g F  is
Wext 5 3 a F dx
xf




(7.14)

xi

Using Newton’s second law, we substitute for the magnitude of the net force o F 5
ma and then perform the following chain-rule manipulations on the integrand:


f
f
f
f
dv
dv dx
Wext 5 3 ma dx 5 3 m
dx 5 3 m
dx 5 3 mv dv
dt
dx dt
xi
xi
xi
vi



Wext 5 12mv f 2 2 12mv i 2

x


x

x

v

(7.15)

where vi is the speed of the block at x 5 xi and vf is its speed at xf .
Equation 7.15 was generated for the specific situation of one-dimensional
motion, but it is a general result. It tells us that the work done by the net force on a
particle of mass m is equal to the difference between the initial and final values of
a quantity 12mv 2. This quantity is so important that it has been given a special name,
kinetic energy:
Kinetic energy  



K ; 12mv 2

(7.16)

Kinetic energy represents the energy associated with the motion of the particle.
Note that kinetic energy is a scalar quantity and has the same units as work. For
example, a 2.0-kg object moving with a speed of 4.0 m/s has a kinetic energy of 16 J.
Table 7.1 lists the kinetic energies for various objects.
S
Equation 7.15 states that the work done on a particle by a net force g F acting
on it equals the change in kinetic energy of the particle. It is often convenient to
write Equation 7.15 in the form



Wext 5 K f 2 K i 5 DK

(7.17)

Another way to write it is Kf 5 Ki 1 Wext, which tells us that the final kinetic energy
of an object is equal to its initial kinetic energy plus the change in energy due to
the net work done on it.


7.5 Kinetic Energy and the Work–Kinetic Energy heorem

Table 7.1
Object

189

Kinetic Energies for Various Objects
Mass (kg)

Earth orbiting the Sun
5.97
Moon orbiting the Earth
7.35
Rocket moving at escape speed
500
Automobile at 65 mi/h
000
Running athlete

70
1.0
Stone dropped from 10 m
Golf ball at terminal speed
0.046
Raindrop at terminal speed   3.5
Oxygen molecule in air
   5.3

Speed (m/s)

Kinetic Energy (J)

2.98
2.65
1.02
3.82
1.12
 3.14
29
  8.4
10
3 500
14
98
44
45
9.0   1.4
500  6.6


28

21

Escape speed is the minimum speed an object must reach near the Earth’s surface to move infinitely far away from
the Earth.

We have generated Equation 7.17 by imagining doing work on a particle. We
could also do work on a deformable system, in which parts of the system move with
respect to one another. In this case, we also find that Equation 7.17 is valid as long
as the net work is found by adding up the works done by each force and adding, as
discussed earlier with regard to Equation 7.8.
Equation 7.17 is an important result known as the work–kinetic energy theorem:
When work is done on a system and the only change in the system is in its
speed, the net work done on the system equals the change in kinetic energy of
the system, as expressed by Equation 7.17:
The work–kinetic energy theorem indicates that the speed of a system increases if
the net work done on it is positive because the final kinetic energy is greater than
the initial kinetic energy. The speed decreases if the net work is negative because the
final kinetic energy is less than the initial kinetic energy.
Because we have so far only investigated translational motion through space,
we arrived at the work–kinetic energy theorem by analyzing situations involving
translational motion. Another type of motion is rotational motion, in which an
object spins about an axis. We will study this type of motion in Chapter 10. The
work–kinetic energy theorem is also valid for systems that undergo a change in
the rotational speed due to work done on the system. The windmill in the photo
graph at the beginning of this chapter is an example of work causing rotational
motion.
The work–kinetic energy theorem will clarify a result seen earlier in this chapter
that may have seemed odd. In Section 7.4, we arrived at a result of zero net work

done when we let a spring push a block from
max to
max. Notice that
because the speed of the block is continually changing, it may seem complicated
to analyze this process. The quantity in the work–kinetic energy theorem, how
ever, only refers to the initial and final points for the speeds; it does not depend on
details of the path followed between these points. Therefore, because the speed
is zero at both the initial and final points of the motion, the net work done on
the block is zero. We will often see this concept of path independence in similar
approaches to problems.
Let us also return to the mystery in the Finalize step at the end of Example 7.5.
Why was the work done by gravity not just the value of the work done by the spring
with a positive sign? Notice that the work done by gravity is larger than the magni
tude of the work done by the spring. Therefore, the total work done by all forces
on the object is positive. Imagine now how to create the situation in which the only
forces on the object are the spring force and the gravitational force. You must sup
port the object at the highest point and then remove your hand and let the object
fall. If you do so, you know that when the object reaches a position 2.0 cm below
your hand, it will be moving, which is consistent with Equation 7.17. Positive net

WW
Work–kinetic energy
theorem

Pitfall Prevention 7.5
Conditions for the Work–Kinetic
Energy Theorem The work–
kinetic energy theorem is important but limited in its application;
it is not a general principle. In
many situations, other changes in

the system occur besides its speed,
and there are other interactions
with the environment besides
work. A more general principle
involving energy is conservation of
energy in Section 8.1.

Pitfall Prevention 7.6
The Work–Kinetic Energy
Theorem: Speed, ot Velocity
The work–kinetic energy theorem
relates work to a change in the
speed of a system, not a change
in its velocity. For example, if
an object is in uniform circular
motion, its speed is constant. Even
though its velocity is changing, no
work is done on the object by the
force causing the circular motion.