In physics, energy is described as the ability to do work by a force, measured in joules (J).
Several different forms of energy exist to describe various natural phenomena.

See also Photon Energy, Rest Energy, Binding Energy, Kinetic Energy and Effective Energy.

[Rest Mass Energy] The term refers to the concept of mass-energy equivalence. Einstein proposed that the equivalence of mass and energy is a general principle.
E=mc²
E = energy,
m = mass,
c = the speed of light in a vacuum.
c² is the conversion factor required to convert from units of mass to units of energy.
The rest energy of an electron or a positron for example amounts to 0.511 MeV.

See also Photon, Photon Energy.

The energy of a photon is direct proportional to its frequency.
E = h * v
E = Energy (J joule)
h = Planck's constant, 6.626 x 10^-34 J-s (Joule-seconds)
v = Frequency (Hz)
This relation between energy and frequency of a photon is known as Planck's relation.

See also Photon.

Binding energy is equal to the amount of energy which is used to free electrons or disintegrate nuclides from their atomic bond.
The electron binding energy of a hydrogen atom is with 13.6 eV very low. The nuclear binding energy of an alpha-particle, energy equivalent of the sum of the individual masses of nuclides minus the mass of the whole nucleus, is 28.3 MeV.

See also Alpha Decay, Beta Decay and Gamma Quantum.

The kinetic energy is the energy of motion, the energy an object possessed, e.g. a nucleon, a nucleus, an atom or another particle, because of its motion.