In the special theory of relativity, mass and energy are equivalent (as can be seen by calculating the work required to accelerate an object). When an object's velocity increases, so does its energy and hence its mass equivalent (inertia). It thus requires more force to accelerate it the same amount than it did at a lower velocity. Newton's Second Law, F = d p d t , remains valid because it is a mathematical definition. But for momentum to be conserved at relativistic relative velocity, v , momentum must be redefined as: p = m 0 v 1 − − v 2 / c 2 , where m 0 is the rest mass and c the speed of light.