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7-2 The work done by a constant force exerted on a moving object 289

In this chapter we’ll look at the ways in which kinetic energy, potential energy,
and other types of internal energy—energy stored within a system, sometimes in a way
that can’t easily be extracted—can be converted from one type of energy into another
or transferred into or out of a system. As an example, the energy you need to make it
through today is extracted from energy stored in food that you consumed earlier. This
sort of internal energy is stored in chemical bonds, related to the structural arrange-
ment of atoms and molecules. Internal energy can also be due to motion: When the
molecules in water start moving more quickly, the water warms up. Temperature is
directly related to this sort of internal energy. Although it sounds like kinetic energy
because it involves motion, the motions are of microscopic objects inside the system
and are random and internal, not causing the system to move. In this course, we will
focus on work as the method of energy transfer through mechanical processes, but
when you continue your studies you will also learn how to calculate energy transfer
due to heating, which is transferring energy by thermal processes (and does not require
motion of the system, but just causes these random internal motions). In this course,
you just need to know this can happen, like when you boil water!

THE TAKEAWAY for Section 7-1

✔ Conservation laws constrain the possible behaviors of ✔ An object’s or system’s energy is related to its ability to
objects or systems. do work.
✔ Conservation of a quantity does not mean that the quantity in ✔ There are different types of energy, including kinetic,
a system cannot change, but that any change in that quantity potential, and internal. Energy can be converted from one
in a system must equal transfers into or out of the system. Con- of these types into another.
servation means the quantity is neither created nor destroyed.

Prep for the AP ® Exam
Building Blocks exchange energy with its surroundings but can exchange
matter. Explain why this category is not possible in terms
1. Describe how you know winds blowing across a field of of possible exchange processes.
tall grass have a property that we call energy and that
work is being done by the wind on the grass. Skills in Action
2. In everyday language, energy is consumed for transpor-
tation. In this language, if you say that energy should 4. An open system can exchange both energy and matter
be conserved, you might mean that you should drive a with its surroundings. A closed, isolated system cannot
smaller car or drive less. In physics, we look more deeply exchange energy or matter with its surroundings.
and see that the internal energy of the molecules of fuel is Categorize each of the following systems (a, b, c) as
converted to the motion of a car (the car’s kinetic energy) open or closed, isolated and describe evidence from
and into heating of the molecules of exhaust and of the your own experience to support your categorization.
engine. How does this conversion satisfy energy conserva- (a) Earth
tion? In your own words compare these two energy con- (b) You
servation statements. For example, can they be violated? (c) A very good ice chest while the lid is shut

Skill Builder

3. Missing from the definitions of open, closed, and iso-
lated systems given in Section 7-1 is a system that cannot

The work done by a constant force exerted on a moving
7-2 object depends on the magnitude of the force and the

distance the object moves in the direction of the force

The man depicted in Figure 7-2 is doing work as he pushes a crate up a ramp. The
amount of work that he does depends not only on how hard he pushes on the crate
(that is, on the magnitude of the force that he exerts) but also on the distance over
which he moves the point on which he is pushing the crate (and thus the crate since it

Uncorrected proofs have been used in this sample. Copyright © Bedford, Freeman & Worth Publishers.
Distributed by Bedford, Freeman & Worth Publishers. For review purposes only. Not for redistribution.

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