energy SI unit joule (J). The capacity to do work is defined as energy. It can be due to the motion possessed by a particle, which is called kinetic energy. It is given by the formula (1/2)mv2. A particle or a system of particles can posses energy by virtue of its position. For example a particle raised to a height h above the ground, has an inherent energy mgh. This energy is called potential energy.
According to the theory of relativity mass and energy are inter convertible. The total energy E can be found from the equation
E2= p2c2+ m02c4 (e7)
where p is the momentum of a particle with rest mass m0, and c is the velocity of light (see special theory of relativity).