Potential energy

The quantity describing a body’s ability to do work is called energy; its unit is the joule (J). Energy that depends on the mutual positions of bodies or of parts of a body is potential energy. For a body of mass m raised to a height h in the Earth’s gravity field E_p = mgh, where h is measured from a zero level (chosen freely, so what matters is the change in E_p, not E_p itself). When the body falls, gravity does work and the potential energy decreases by that amount. A stretched or compressed spring also has potential energy: the more it is deformed, the more work it can do. The work done in lifting a body equals the increase of its potential energy.