The amount of work done has nothing at all
Mechanical kinetic energy can likewise be
to do with how long it takes to do it. When you
changed into mechanical potential energy. For
lift a weight of 1 pound through a distance of
example, suppose you throw a baseball straight
1 foot, you have done one foot-pound of work,
up in the air. The ball has kinetic energy while
regardless of whether you do it in half a second
it is in motion, but the amount of kinetic energy
or half an hour. The rate at which work is done
decreases and the amount of potential energy
is called POWER. The common unit of measure-
increases as the ball travels upward. When the ball
has reached its uppermost position, just before
ment for power is the HORSEPOWER (hp). By
definition, 1 horsepower is equal to 33,000 foot-
it starts its fall back to earth, it has only
potential energy. Then, as it falls back toward the
pounds of work per minute or 550 foot-pounds
earth, the potential energy is changed into kinetic
of work per second. Thus a machine that is
energy again.
capable of doing 550 foot-pounds of work per
Mechanical energy in transition is called
second is said to be a 1 horsepower machine. (As
WORK. When an object is moved through a dis-
you can see, your horsepower rating would not
tance against a resisting force, we say that work
be very impressive if you did 1 foot-pound of
has been done. The formula for calculating work
work in half an hour. Figure it out. It works out
to be just a little more than one-millionth of a
is
horsepower.)
W = F D,
where
Thermal Energy
W = work,
All substances are composed of very small
F = force, and
particles called molecules. The energy associated
with molecules is called thermal energy. Thermal
D = distance.
energy, like mechanical energy, exists in two
As you can see from this formula, you need
stored forms and in one transitional form. The
to know how much force is exerted and the
two stored forms of thermal energy are called
distance through which the force acts before you
(1) internal potential energy, and (2) internal
can find how much work is done. The unit of
kinetic energy. Thermal energy in transition is
force is the pound. When work is done against
called HEAT.
gravity, the force required to move an object is
Although molecules are too small to be seen,
equal to the weight of the object. Why? Because
they behave in some ways pretty much like the
weight is a measure of the force of gravity or, in
larger objects we considered in the discussion of
other words, a measure of the force of attraction
mechanical energy. Molecules have energy of
between an object and the earth. How much work
position (internal potential energy) because of the
will you do if you lift that 5-pound rock from the
forces which attract molecules to each other. In
bottom of the l00-foot cliff to the top? You will
this way, they are somewhat like the rock and the
do 500 foot-pounds of work-the weight of the
earth we considered before. Molecules have
object (5 pounds) times the distance (100 feet) that
energy of motion (internal kinetic energy) because
you move it against gravity.
they are constantly in motion. Thus, the two
We also do work against forces other than the
stored forms of thermal energy--internal poten-
force of gravity. When you push an object across
tial energy and internal kinetic energy--are in
the deck, you are doing work against friction. In
some ways similar to mechanical potential energy
this case, the force you work against is not only
and mechanical kinetic energy, except that
the weight of the object, but also the force
everything is on a smaller scale.
required to overcome friction and slide the
For most purposes, we will not need to
object over the surface of the deck.
distinguish between the two stored forms of
Notice that mechanical potential energy,
thermal energy. Therefore, instead of referring
mechanical kinetic energy, and work are all
to internal potential energy and internal kinetic
measured in the same unit, foot-pounds. One
energy, from now on we will simply use the term
foot-pound of work is done when a force of 1
internal energy. By internal energy, then, we will
pound acts through a distance of 1 foot. One foot-
mean the sum total of all internal energy stored
pound of mechanical potential energy or mechan-
in the substance or system because of the motion
ical kinetic energy is the amount of energy that
of the molecules and because of the forces of
is required to accomplish 1 foot-pound of work.
attraction between molecules. Although the term
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