Before the advent of this theory
of special relativity it was presumed  that mass and energy were conserved
separately in a reaction However, Einstein showed that mass is another
form of energy and one can convert mass-energy into other forms of
energy, say kinetic energy and vice-versa Einstein gave the famous mass-energy equivalence relation
E = mc 2
(13 6)
Here the energy equivalent of mass m is related by the above equation
and c is the velocity of light in vacuum and is approximately equal to
3×108 m s–1