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kinetic energy, so it has the ability to do work. An object with
potential energy has energy of position, and it, too, has the abil-
ity to do work. You could say that energy fl owed into and out
of an object during the entire process. The following energy
d
F scheme is intended to give an overall conceptual picture of en-
d ergy flow. Use it to develop a broad view of energy. You will
F
learn the details later throughout the course.
Inertia Gravity
A B
ENERGY FORMS
Energy comes in various forms, and different terms are used to dis-
tinguish one form from another. Although energy comes in various
forms, this does not mean that there are different kinds of energy.
d d
The forms are the result of the more common fundamental forces—
F F gravitational, electromagnetic, and nuclear—and objects that are
interacting. Energy can be categorized into five forms: (1) mechani-
cal, (2) chemical, (3) radiant, (4) electrical, and (5) nuclear. The
Friction Shape of spring
following is a brief discussion of each of the five forms of energy.
C D
Mechanical energy is the form of energy of familiar objects
FIGURE 3.9 Examples of working against (A) inertia, and machines (Figure 3.10). A car moving on a highway has kinetic
(B) gravity, (C) friction, and (D) shape. mechanical energy. Water behind a dam has potential mechani-
cal energy. The spinning blades of a steam turbine have kinetic
mechanical energy. The form of mechanical energy is usually as-

4. Work against shape. The force that is needed to stretch sociated with the kinetic energy of everyday-sized objects and the
or compress a spring is working against the shape of the potential energy that results from gravity. There are other possibili-
spring. Other examples of work against shape include ties (e.g., sound), but this description will serve the need for now.
compressing or stretching elastic materials. If the elastic Chemical energy is the form of energy involved in chemi-
limit is reached, then the work goes into deforming or cal reactions (Figure 3.11). Chemical energy is released in the
breaking the material. chemical reaction known as oxidation. The fire of burning wood
5. Work against any combination of inertia, fundamental is an example of rapid oxidation. A slower oxidation releases
forces, friction, and/or shape. It is a rare occurrence on energy from food units in your body. As you will learn in the
Earth that work is against only one type of resistance. chemistry unit, chemical energy involves electromagnetic forces
Pushing on the back of a stalled automobile to start it between the parts of atoms. Until then, consider the following
moving up a slope would involve many resistances. Th is
is complicated, however, so a single resistance is usually
singled out for discussion.
Work is done against a resistance, but what is the result? The
result is that some kind of energy change has taken place. Among
the possible energy changes are the following:

1. Increased kinetic energy. Work against inertia results in an

increase of kinetic energy, the energy of motion.
2. Increased potential energy. Work against gravity and work
against shape result in an increase of potential energy, the
energy of position.
3. Increased temperature. Work against friction results
in an increase in the temperature. Temperature is a
manifestation of the kinetic energy of the particles making
up an object, as you will learn in chapter 4.
4. Increased combinations of kinetic energy, potential energy,

and/or temperature. Again, isolated occurrences are more
the exception than the rule. In all cases, however, the sum
of the total energy changes will be equal to the work done.
Work was done against various resistances, and energy was FIGURE 3.10 Mechanical energy is the energy of motion,
increased as a result. The object with increased energy can now or the energy of position, of many familiar objects. This boat has
do work on some other object or objects. A moving object has energy of motion.

70 CHAPTER 3 Energy 3-10