Answer:
Kinetic and Potential Energy/Conservation of Energy
Intro
Associate energy usually with activity or motion:
Falling stone possesses energy, energetic person constantly doing things
Foods are rich in energy, the Earth receives radiant energy from the sun
Relation between piece of pie and a falling stone:
The ability to do work
Work
Changes that take place in physical world are result of forces. Forces are needed
to pick things up, move things, squeeze things, stretch things, etc. If push against a stone
wall, nothing happens. However, if we apply the same force to one of the stones that
make up the wall, the stone flies through the air for some distance - something has been
accomplished from our push. The difference between the two situations is that our hand
did not move when it pushed against the wall. The force was stationary. In the second
case, when we throw the stone, our hand moved while the force was being applied, and
before the stone actually left our grasp. The motion of the force is what constitutes the
difference between the two cases and is responsible for the difference in the two results.
Almost all forces that produce effects such as motion or distortions in an object
undergo displacements in doing so - a moving force accomplishes something, while a
stationary force does not, which we define as work. Work done by a force is equal to the
magnitude of the force multiplied by the distance through which the force acts. If the
distance is zero, no work is done by the force, no matter how great it is. If something
moves a distance, it does not do work unless it exerts a force on something else.
W = Fd
F is assumed to be in the same direction as d. If not, W = F* d*cosθ. Thus, a force that
is perpendicular to the motion of an object can do no work upon it ( at 90 degrees).
Therefore, gravity does no work on objects moving along the Earth's surface. However,
if we drop an object, as it falls to the ground work is done upon it.
In the metric system, work is measured in Joules, 1 Joule is the amount of work
done by a force of 1 Newton that acts through a distance of 1m.
1 J = 1 N * m
1 J = 0.738 ft * lb
1 ft * lb = 1.36 J
Power
Power is the rate at which work is being done by some force.
P = W / t
Metric System, unit of power is the watt:
1 watt = 1 J / sec
Origin: in order to see the steam engines he had invented, James Watt was usually
compelled to compare its power output with that of a horse. After various tests he found
that a typical horse could perform works at the rate of 22,000 ft *lb / min for as much as
10 hr / day. To avoid possible disputes about full measure, Watt increased this figure by
one-half in establishing the unit that he called the horsepower. Watt's horse power
represents a rate of doing work of 33,000 (ft *lb) / min or 550 (ft * lb) / sec.
1 hp = 746 W
Kinetic Energy
What makes it possible for a force to do work? The answer is energy. Energy
may be thought of as the property of something which enables it to do work. When we
say that something has energy, we suggest that it is capable of exerting a force on
something else and performing work on it. When work is done something, energy has
been added to it. Energy is measured in Joules, the same units as work.
Energy takes on many forms. One type is the energy a moving body possesses by
virtue of its motion. Every moving object has the capacity to do work. By striking
another object that is free to move, the moving object can exert a force and cause the
second object to shift its position. While the object is moving, it has the capacity for
doing work. Energy means the ability to do work, so all moving things have energy by
virtue of their motion. This type of energy is called kinetic energy.
The energy that an object possesses because it is moving, it's kinetic energy, is
defined as :
KE = 1/2 mv2
