I have looked at the Einsteins Derivation of the Relation E=mc2. http://www.adamauton.com/warp/emc2.html But I don't if this shorthand derivation is O.k. or not. like the derivation, I use maxwells law and say the momentum p is given by p = E / c ---> 1 Now from newtonian mechanics, we have p= m* v or, p=mc (here v=c) --->2 Combining 1 and 2 mc = E / c therfore, E = mc2 What's wrong here? If its wrong, why is the result correct? If its correct, why didn't Einstein use it?