In physics, the ability to do work. Objects can have energy by virtue of their motion (kinetic energy), by virtue of their position (potential energy), or by virtue of their mass (see E = mc2).
- The most important property of energy is that it is conserved — that is, the total energy of an isolated system does not change with time. This is known as the law of conservation of energy. Energy can, however, change form; for example, it can be turned into mass and back again into energy.