Energy, from the Greek energeia (en, in; ergon, work), originally a technical term in Aristotelian philosophy denoting "actuality" or "existence in actuality," means, in general, activity or power of action. In the physical sciences it is defined as the capability to do work, as accumulated work or, in the words of Wilhelm Ostwald, as "that which is produced by work or which can be transformed into work." Energy is measured in terms of units of work, to overcome a resisting force of one dyne over a distance of one centimeter. (The joule = 10⁷ erg = the watt-second; the kilogram-meter = 9.81 × 10⁷ erg. In atomic physics the unit is the electron volt; ev = 1.6 × 10⁻¹² erg.

In physics, energy is either kinetic or potential. A body of mass m moving with a velocity v possesses, owing to its motion, the kinetic energy ½mv², which is the work necessary to overcome the inertial resistance in accelerating...