an equation expressing the mass of an object as a function of its velocity: as the velocity v of an object increases, its mass m increases from its rest mass m₀ according to the equation: m = m₀/1 - v2/c2 where c is the speed of light in a vacuum. If the speed of an object were to become equal to that of light, it can be seen that the mass would be infinite, from which it follows that nothing can accelerate up to or beyond the speed of light.  [PJC]