1. The problem statement, all variables and given/known data A photon strikes an electron of mass m that is initially at rest, creating an electron-positron pair. The photon is destroyed and the positron and two electrons move off at equal speeds along the initial direction of the photon. The energy of the photon was ? 3. The attempt at a solution are we suppose to use momentum conservation? or kinetic energy conservation? do we have to factor in relativistic effects? the answer is 4mc2 but i have totally no idea at all how to get that i think the final mass = 1.5 m ? where m = electron mass because positron and electron pair = 0.5 m , then add in the other stationary electron so 1.5m ? this is confusing . greatly appreciate any help thanks!