1. The problem statement, all variables and given/known data Find the speed of a particle whose total energy is twice its rest energy. 2. Relevant equations E2 = p2c2 +m2c4 3. The attempt at a solution 4m2c4 = p2c2 +m2c4 3m2c4 = p2c2 SQRT(3)mc2 = pc SQRT(3)mc = p SQRT(3)mc = mv SQRT(3)c = v I know I can use E = gamma*mc2 to get the answer, but what have I done wrong in my approach? Thank you.