Page:Relativity (1931).djvu/75

 fundamental equations of the electrodynamics of Maxwell: A body moving with the velocity $$v$$, which absorbs an amount of energy $$E_0$$ in the form of radiation without suffering an alteration in velocity in the process, has, as a consequence, its energy increased by an amount

In consideration of the expression given above for the kinetic energy of the body, the required energy of the body comes out to be

Thus the body has the same energy as a body of mass $$\left ( m + \tfrac{E_0}{c^2} \right )$$ moving with the velocity $$v$$. Hence we can say: If a body takes up an amount of energy $$E_0$$, then its inertial mass increases by an amount $$\tfrac{E_0}{c^2}$$; the inertial mass of a body is not a constant, but varies according to the change in the energy of the body. The inertial mass of a system of bodies can even be regarded as a measure