 Solution

The relativistic energy of a particle of rest mass $m$ is $E=\gamma m{c}^2$, where the Lorentz factor is $\gamma =(1-v^2/c^2{)}^{-1/2}$. Using the binomial series at small $v/c$ givesThe second term is the familiar Newtonian kinetic energy. The first is the rest energy, which remains when $v=0$. Thus the new physical idea is mass-energy equivalence: a body's mass represents energy $m{c}^2$ even at rest. The total energy is not merely energy due to motion.