Energy is the ability or capacity to do work. Several forms of energy
include heat, chemical energy, and, according to the theory of
relativity and mass; other forms of energy are
associated with the transmission of light, sound, and electricity. Energy
and work are measured in the same units: joules, ergs, electron-volts,
calories, foot-pounds, or some other, depending on the system of
measurement being used. When a force acts on a body, the work performed
(and the energy expended) is the product of the force and the distance
over which it is exerted. Potential energy is the capacity for doing work
that a body possesses because of its position or condition. For example,
a weight lifted to a certain height has potential energy because of its
position in earth's gravitational field. Kinetic energy, the energy a
body possesses because it is in motion, is equal to 1/2mv2, where m is
its mass and v is its velocity. The average kinetic energy of the atoms
or molecules of a body is measured by the temperature of the body. Energy
(or its equivalent in mass) can be neither created nor destroyed
(law of conservation of matter and energy), but it can be changed from one
form into another.
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