<P> (1) </P> <P> holds for a system, such as a particle or macroscopic body, having intrinsic rest mass m, total energy E, and a momentum of magnitude p, where the constant c is the speed of light, assuming the special relativity case of flat spacetime . </P> <P> The energy--momentum relation (1) is consistent with the familiar mass--energy relation in both its interpretations: E = mc relates total energy E to the (total) relativistic mass m (alternatively denoted m or m), while E = m c relates rest energy E to (invariant) rest mass which we denote m . Unlike either of those equations, the energy--momentum equation (1) relates the total energy to the rest mass m . All three equations hold true simultaneously . </P> <P> Special cases of the relation (1) include: </P>

Equation for the relativistic energy of an object with mass m
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