In this example, we are ignoring the fact that
potential energy will be changed to mechanical
some work is done against the resistance of the
kinetic energy. Another way of saying this is that
air and that some heat results from the passage
the energy of position will be changed to the
of the bullet through the air. But this does not
energy of motion.
change the basic idea that kinetic energy is stored
In these examples, you will notice that an
energy rather than energy in transition. The air
external source of energy is used to get things
must be regarded as a resisting object, which
started. Energy from some outside source is
causes some of the stored kinetic energy of the
required to push the rock, open the gate of the
bullet to be converted into energy in transition
dam, or let go of the sled. All real machines and
(heat and work) while the bullet is passing through
processes require this kind of boost from an
the air. However, the major part of the stored
energy source outside the system. For example,
kinetic energy does not become energy in
there is a tremendous amount of chemical energy
transition until the bullet strikes an object firmer
stored in fuel oil; but this energy will not turn the
than air which resists its passage.
power turbine until you have expended some
Mechanical potential energy is measured in
energy to start the oil burning. Similarly, the
foot-pounds. Consider, for example, the rock at
energy in any one system affects other energy
the top of the cliff. If the rock weighs 5 pounds
systems. However, it is easier to learn the basic
and if the distance from the rock to the earth at
principles of energy if we forget about all the
the base of the cliff is 100 feet, 500 foot-pounds
energy systems that might be involved in or
of mechanical potential energy exist because of
affected by each energy process. In the examples
the relative positions of the rock and the earth.
given in this chapter, therefore, we will consider
Another way of expressing this idea is by the
only one energy process or energy system at a
formula
time, disregarding both the energy boosts that
may be received from outside systems and the
PE = W x D,
energy transfers that may take place between the
system we are considering and other systems.
where
Notice that both mechanical potential energy
PE = total potential energy of the object (in
and mechanical kinetic energy are stored forms
foot-pounds),
of energy. It is easy to see why we regard
mechanical potential energy as being stored, but
W = total weight of the object (in pounds),
it is not so easy to see the same thing about
and
mechanical kinetic energy. Part of the trouble
comes about because mechanical kinetic energy
D = distance between the earth and the
is often referred to as the energy of motion, thus
object (in feet).
leading to the false conclusion that energy in
transition is somehow involved. This is not the
Mechanical kinetic energy is also measured in
case, however. Work is the only form of
foot-pounds. The amount of kinetic energy
mechanical energy that can be properly considered
present at any one time is directly related to the
as energy in transition.
velocity of the moving object and to the weight
of the moving object.
If you have trouble with the idea that
Mechanical potential energy can be changed
mechanical kinetic energy is stored, rather than
into mechanical kinetic energy. If you push that
in transition, think of it like this: A bullet that
5-pound rock over the edge of the 100-foot cliff,
has been fired from a gun has mechanical kinetic
it begins to fall, and as it falls, it loses potential
energy because it is in motion. The faster the bullet
energy and gains kinetic energy. At any given
is moving, the more kinetic energy it has. There
moment, the total amount of mechanical energy
is no doubt in anybody's mind that the bullet has
(potential plus kinetic) stored in the system is the
the capacity to produce an effect, so we may safely
same--that is, 500 foot-pounds. But the pro-
say that it has energy. Although the bullet is not
portions of potential energy and kinetic energy
in transition, the energy of the bullet is not
are changing all the time as the rock is falling.
transferred to any other object or system until the
Just before the rock hits the earth, all the stored
bullet strikes some object that resists its passage.
mechanical energy is kinetic energy. As the rock
When the bullet strikes against a resisting object,
hits the earth, the kinetic energy is changed into
then, and only then, can we say that energy in
energy in transition--that is, work and heat.
transition exists, in the form of heat and work.
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