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.