Thus the body has the same energy as a body of mass
moving with the velocity
. Hence we can say: If a body takes up an amount of energy
, then its inertial mass increases by an amount
; 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 of its energy. The law of the conservation of the mass of a system becomes identical with the law of the conservation of energy, and is only valid provided that the system neither takes up nor sends out energy. Writing the expression for the energy in the form