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A constant intrinsic to a body which determines its inertial and energy-momentum properties. It is a fundamental concept of special relativity, and in particular it determines the internal energy content of a body. It is the same as the inertial mass of classical mechanics. According to the principle of equivalence, the basic physical principle of general relativity, the inertial mass of a body is also equal to its gravitational mass. See Classical mechanics, Gravitation, Relativity
The rest mass or inertial mass of a body, m, is a measure of its resistance to being accelerated at a by a force F ; in classical mechanics the relation between inertial mass, acceleration, and force is given by Newton's law, Eq. (1).
Associated with the rest mass of a body, there is an internal or rest energy. In the system where the body is at rest, the energy of the body is given
The experimental realization of the interconversion of mass and energy is accomplished in the reactions of nuclei and elementary particles. In particular, the energy source of nuclear bombs and nuclear fission reactors is a small decrease in the total mass of the interacting nuclei, which gives rise to a large energy release because of the large numerical value of c2. See Elementary particle, Nuclear fission