पृष्ठ 64 / 212

In consideration of the expression given above for the kinetic

energy of the body, the required energy of the body comes out to

be

image025

Thus the body has the same energy as a body of mass

image026

moving with the velocity v. Hence we can say: If a body takes up

an amount of energy E0, then its inertial mass increases by an

amount

image027

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

image028

we see that the term mc2, which has hitherto attracted our

attention, is nothing else than the energy possessed by the

body[12] before it absorbed the energy E0.