From this we get directly that
What does this equation assert?
and
are the energy-values of the same point-mass, in the first place referred to a system with respect to which the point-mass moves, and in the second related to a system in which the point-mass is at rest. Hence the difference
, except for an additive constant, must be equal to the kinetic energy of the point-mass referred to the moving system. Thus, we may write
