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288 Chapter 7 Conservation of Energy and an Introduction to Energy and Work

of interactions that result in zero motion). One example of doing work is lifting a book
from your desk to a high bookshelf; another is a football player pushing a blocking sled
across a field (Figure 7-1a). In each of these cases, the point of contact of where the force is
exerted on the object moves. The definitions of work and energy reference each other, but
things will become clearer as we move through this chapter and the next. We will see that if
an object or a system has energy, then that object or system may be able to do work.
One type of energy is kinetic energy, which is the energy that an object has due to its
motion (Figure 7-1b). An object with kinetic energy has the ability to do work; for example,
a moving ball has the ability to displace objects in its path (Figure 7-1c). It’s important to
remember that for anything for which we use the object model, the only type of energy
that it can have is kinetic energy. This is due to the fact that by the definition of an object,
we cannot change its shape, or the way its internal components are moving relative to each
other, because an object has no internal structure. Conversely, systems, because they have
internal structure, can have internal and potential energy, which we will define shortly.
Kinetic energy can be converted to other types of energy that can’t be used to do
work—such as when an egg thrown at high speed splatters against a wall. Before the egg
hits the wall, it can be modeled as an object because every point on the egg is moving in the
same trajectory. However, once the egg hits the wall, the points on the now-broken egg no
longer travel together as the egg changes shape. The egg can no longer, therefore, be mod-
eled as an object to describe it as it breaks. Breaking apart on the wall is an irreversible
change in shape, a change that disrupts the arrangement of the system in such a way that it
cannot simply return to its initial shape. For example, you could not easily reassemble the
egg, putting the yolk back in its sack, the egg white back into a smooth shield, and the shell
all into one piece. Such disordered changes dissipate energy into a type that is no longer
useful: The egg no longer has kinetic energy after it breaks. All the bits come to rest. The
kinetic energy went into breaking the egg. Conversely, if that egg is replaced with a rubber
ball, the ball bounces off the wall with nearly the same speed at which it hit the wall and
so we recover most of the ball’s kinetic energy. We say that as the ball compresses against
the wall, its kinetic energy is converted into potential energy—energy associated not with
the ball’s motion but with a reversible change in its shape, a change that will allow the rel-
ative position, the structural arrangement, of its parts to return to their original condition.
This potential energy is again converted into kinetic energy as the ball bounces back to
its original shape (Figure 7-1c or the photographs opening this chapter). Potential energy
is associated with a reversible change in shape of a system, which can be described as a
reversible change in the configuration of a system of objects. (We cannot use the object
model for the ball striking the wall during this process, because the various points on the
ball do not all move together at the same speed as the center of mass. The point in contact
with the wall has stopped, but the center of mass of the ball moves toward the wall as the
ball slows down and then away from the wall as the ball speeds back up.)

The football player exerts a force of magnitude F on the sled in its direction The girl does work on the basketball: She exerts a force
of motion while the point on which he is exerting the force moves a on the ball as she pushes it away from her. As a result
distance d. Hence he does an amount of work on the sled equal to Fd. the ball acquires kinetic energy (energy of motion).
(a) (b) (c)

F

AP Photo/Kevin Wolf

d Jens Karlsson/Getty Images BrunoWeltmann/Shutterstock

As the cue ball strikes a second pool ball, it stops and the second pool ball
moves off at nearly identical speed to that of the cue ball before the collision.
The cue ball exerts a force on the second ball as it comes to a stop.

Figure 7-1 Work and energy In this chapter we’ll explore the ideas of work and energy.

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