So the thing I have issues with in this problem is that it's about 2 particles travelling relative to each other. Been going though the basics of relativity, relativistic lengths, times, kinetic energy, work and force. But this problem states two particles and I'm not sure how to approach it. The word "common" is where I get confused. 1. The problem statement, all variables and given/known data A 3.0 MeV photon is moving in positive x-direction and an electron in the opposite direction at a velocity of 0.995c. Calculate their common total energy, momentum and total rest mass. 2. Relevant equations Fundamental relativistic equations.