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OVERVIEW

The term energy is closely associated with the concepts of force and motion. Naturally moving matter, such as the
wind or moving water, exerts forces. You have felt these forces if you have ever tried to walk against a strong wind or
stand in one place in a stream of rapidly moving water. The motion and forces of moving air and moving water are
used as energy sources (Figure 3.1). The wind is an energy source as it moves the blades of a windmill, performing
useful work. Moving water is an energy source as it forces the blades of a water turbine to spin, turning an electric
generator. Thus, moving matter exerts a force on objects in its path, and objects moved by the force can also be used
as an energy source.
Matter does not have to be moving to supply energy; matter contains energy. Food supplied the energy for the
muscular exertion of the humans and animals that accomplished most of the work before the twentieth century.
Today, machines do the work that was formerly accomplished by muscular exertion. Machines also use the energy
contained in matter. They use gasoline, for example, as they supply the forces and motion to accomplish work.
Moving matter and matter that contains energy can be used as energy sources to perform work. The concepts of
work and energy and the relationship to matter are the topics of this chapter. You will learn how energy flows in and
out of your surroundings as well as a broad, conceptual view of energy that will be developed more fully throughout
the course.

Mechanical work is the product of a force and the distance
3.1 WORK
an object moves as a result of the force. There are two important
You learned earlier that the term force has a special meaning in considerations to remember about this definition: (1) some-
science that is different from your everyday concept of force. thing must move whenever work is done, and (2) the movement
In everyday use, you use the term in a variety of associations must be in the same direction as the direction of the force. When
such as police force, economic force, or the force of an argu- you move a book to a higher shelf in a bookcase, you are doing
ment. Earlier, force was discussed in a general way as a push work on the book. You apply a vertically upward force equal to
or pull. Then a more precise scientific definition of force was the weight of the book as you move it in the same direction as
developed from Newton’s laws of motion—a force is a result of the direction of the applied force. The work done on the book
an interaction that is capable of changing the state of motion can therefore be calculated by multiplying the weight of the
of an object. book by the distance it was moved (Figure 3.2).
The word work represents another one of those concepts
that has a special meaning in science that is different from
your everyday concept. In everyday use, work is associated UNITS OF WORK
with a task to be accomplished or the time spent in perform- The units of work can be obtained from the definition of work,
ing the task. You might work at understanding physical sci- W = Fd. In the metric system, a force is measured in new-
ence, for example, or you might tell someone that physical tons (N), and distance is measured in meters (m), so the unit
science is a lot of work. You also probably associate physi- of work is
cal work, such as lifting or moving boxes, with how tired you
W = Fd
become from the effort. The definition of mechanical work
is not concerned with tasks, time, or how tired you become W = (newton)(meter)
from doing a task. It is concerned with the application of a W = (N)(m)
force to an object and the distance the object moves as a result
The newton-meter is therefore the unit of work. This derived
of the force. The work done on the object is defined as the
unit has a name. The newton-meter is called a joule (J) (pro-
product of the applied force and the parallel distance through
nounced “jool”).
which the force acts:
1 joule = 1 newton-meter
work = force × distance
2
The units for a newton are kg∙m/s , and the unit for a
W = Fd
meter is m. It therefore follows that the units for a joule
2
2
equation 3.1 are kg∙m /s .
62 CHAPTER 3 Energy 3-2