A Not-So-
Simple
Machine
How does a multispeed
bicycle let a cyclist ride
over any kind of terrain
with the least effort?
➥ Look at the text
on page 238 for
the answer.
WHAT YOU’LL LEARN
• You will recognize that work
and power describe how
energy moves through the
environment.
• You will relate force to work
and explain how machines
make work easier by chang-
ing forces.
WHY IT’S IMPORTANT
• A little mental effort in iden-
tifying the right machine for
a task can save you much
physical effort. From open-
ing a can of paint to releas-
ing a car stuck in the mud
to sharpening a pencil,
machines are a part of
everyday life.
CHAPTER
223
PHYSICS
To find out more about work,
energy, and machines, visit the
Glencoe Science Web site at
science.glencoe.com
Energy, Work,
and Simple
Machines
10
W
hat is energy? Energy is needed to make cars run, to heat
or cool our homes, and to make computers hum. Solar
energy is required for crops and forests to grow. The energy
stored in food gives you the energy needed to play sports or walk
to the store. Note, however, all these statements indicate that hav-
ing energy enables something to perform an action, rather than
saying directly what energy is. It is hard to give a good definition
of energy without examples of how energy is used and the result-
ing changes.
In this chapter, you’ll concentrate on one method of changing
the energy of a system—work. You’ll need to be careful here. You
may think you’re doing work when you put forth a physical effort.
For example, you and a friend may try to move a stalled car, but
the car doesn’t budge. You feel as though you’ve done work
because you’re out of breath and your arms ache. However, to a
scientist, work is defined in terms of force as well as a change in
position. If the car didn’t move, no work was done!
For thousands of years, doing work has been of vital concern
to the human race. However, the forces the human body can exert
are limited by physical strength and body design. Consequently,
humans have developed machines that in