That's only when the speed is much smaller than the speed of light (v << c). The one you are after is: E^2 = m^2 c^4 + p^2 c^2 (Set momentum, p, to zero and

That's only when the speed is much smaller than the speed of light (v << c). The one you are after is: E^2 = m^2 c^4 + p^2 c^2 (Set momentum, p, to zero and divide everything by itself to get E = mc^2 again.) http://en.wikipedia.org/wiki/Energy–momentum_relation http://en.wikipedia.org/wiki/Energy–momentum_relation