Past exam of the physics 1 paper of the University of Oxford / 2017 / 9 / a / i / Solution

For a massive free particle with rest mass $m$ and velocity $\mathbf{v}$,The relativistic energy-momentum relation follows directly:This is the squared Minkowski norm of the four-momentum multiplied by $c^2$. The Lorentz transformation preserves that norm, so it has the same value in every inertial frame. In the particle's rest frame, the momentum is zero and the total energy is its rest energy $m{c}^2$.