Relativistic Energy

Equation (3.169) can be written

$$m\,c^2 = K + m_0\,c^2.$$ (3.173)

Let us define the relativistic energy, $E$, of our particle as

$$E = m\,c^2.$$ (3.174)

The previous two equations suggest that the particle possesses two types of energy. First, the particle possesses kinetic energy, $K$, by virtue of its motion. Second, the particle possesses rest mass energy,

$$E_0 = \frac{1}{2}\,m_0\,c^2,$$ (3.175)

by virtue of its rest mass. The conjecture that mass is a form of energy was first made by Einstein in 1905. Incidentally, Equation (3.174) implies that conservation of energy in relativistic dynamics is equivalent to conservation of relativistic mass.