In physics, energy is a quantity that is often understood as the ability to perform work. This quantity can be assigned to any particle, object, or system of objects as a consequence of its physical state.

Different forms of energy include kinetic, potential, thermal, gravitational, sound, elastic and electromagnetic energy. The forms of energy are often named after a related force. German physicist Hermann von Helmholtz established that all forms of energy are equivalent — energy in one form can disappear but the same amount of energy will appear in another form. A restatement of this idea is that energy is subject to a conservation law over time.