Does the inertia of a body depend upon its energy content?”
, to which his answer was “yes”. The first record of the relationship of mass and energy explicitly in the form E = mc2 was written by Einstein in a review of relativity in 1907. If this formula is taken to include kinetic energy then it is only valid for relativistic mass, but it can also be taken as valid in the rest frame for invariant mass. Einstein’s conventions and interpretations were sometimes ambivalent and varied a little over the years, however examination of Einstein’s papers and books on relativity show that he almost never used relativistic mass himself. Whenever the symbol m for mass appears in his equations it is always invariant mass. He did not introduce the notion that the mass of a body increases with velocity, just that it increases with energy content. The equation E = mc2 was only meant to be applied in the rest frame of the particle. Perhaps Einstein’s only definite reference to mass increasing with kinetic energy is in his “autobiographical notes”.
