E=mc^2 proof. Why use Newtonian kinetic energy??? In "Does the Inertia of a Body Depend Upon its Energy-Content?" Einstein says that the difference in kinetic energy of a body before and after it releases some energy is... K(0)-K(1)=E(gamma-1) In his previous paper, he worked out the kinetic energy of masses as... K=mc2(gamma-1) Can't we immediately compare the two equations and conclude that E=mc^2? Why does Einstein bother converting the first equation above into K(0)-K(1)=(1/2)(E/c2)v2 ? It seems to me that this is a less precise method since he had to neglect magnitudes of fourth and higher order to get it. **I know that me writing out gamma looks really dumb but I was having trouble with Latex.