Consider again a body at rest. It is possible to demonstrate that its resting mass m0 equals an intrinsic energy E0, expressed by:
Reactions that release energy do so due to the loss of mass, which is transformed into energy.
Solar energy, for example, comes from a reaction called Nuclear fusion, in which hydrogen nuclei associate to form a helium nucleus. However, the mass of helium is slightly smaller than the sum of the masses of hydrogen nuclei, due to the losses of released energy.
Another example of application of nuclear fusion occurs in the explosion of an atomic bomb. This leads us to infer that mass is a form of energy.
If the body is moving relative to a frame in which it has a resting mass m0, the total energy may be obtained by the expression:
M being the relativistic mass of the body, given by:
So the total energy AND will be the sum of the body's resting energy, AND0, with the kinetic energy ANDÇ: