1. The problem statement, all variables and given/known data I have to protons coliding - they have same magnitude of velocity and momentum and they collide head-on: p + p -> p + (p + p_) (the "p_" is an anti-proton). I have to find the minimum combined kinetic energy of the two protons for this process to run. 3. The attempt at a solution I use P_total = P_1 + P_2 <=> (all the masses squared ...) * c^2 = (P_1 + P_2)^2. But this is where I encounter my problem. The 4-vector for e.g. P_1 is [E/c ; gamma*m*v] - I don't know v? Is there another way of doing this? - another thing: When I have found the total energy, do I have to subtract it with the rest mass of a proton x 2 (2*m_p*c^2) to find combined kinetic energy ?