mass energy
The equation explained
In wordsAs the observer speed shrinks, twice the kinetic-energy drop over its square settles on the emitted energy divided by the square of the speed of light.Full explanationAt any finite speed the quotient is a little larger than L/c². As the speed shrinks it settles on L/c², the coefficient Newtonian mechanics reads as the mass the body lost. The limit is taken, not set at v = 0, where the quotient is not defined.