Eienstein has given the formula for converting the mass and energy into one another.
E = m c² , c = velocity of light in vacuum = 3 * 10^8 m/s
Any body with mass M0 and at rest has a rest energy = M0 c².
This equation has a very important significance in nuclear physics and nuclear fission reactions. Scientists could explain the amount of energy released and the reduction of mass of particles during the chain reaction, using this formula. This formula also has helped understanding small particle behaviorand how to predict their physical properties.
If a body is moving at a speed v then its total energy is addition of the rest energy and the kinetic energy Ek. Then
E = total energy = M0 c² + Ek
= Mr c² Mr is called the relative mass.
Mr is found to be = Mo / [ 1 - v²/c² ]^1/2
E = m c² implies that
when body is at rest m = M0 and its energy is M0 c², and
when it is moving its relative mass is Mr and relative energy is Mr c²