# Einstein's Equivalence of Energy and Mass

If a body is in motion with a very high velocity, *v*, (comparable to the speed of light *c*),
it has a relativistic mass, *m(v)*, that is different from the mass when it is at rest, *m _{0}*.
The relationship between the “rest mass” and the “relativistic mass” is shown in the equation below.

If *v/c* is « 1, the square root in the denominator can be expanded in a series and truncated after the second term.
This results in the following equation:

The factor *m*_{0}*v*^{2}/2 corresponds to the kinetic energy ΔE obtained as a result of the high velocity. The addition to the
rest mass, Δm, is therefore Δ*E*/*c*^{2}. These considerations led Albert Einstein (pictured above) to a postulate that formed
a cornerstone of his special theory of relativity:

Thus, already in 1905 the basic formula for the peaceful use of nuclear energy had been found, although it was not put into practice until almost 50 years later.