1. The problem statement, all variables and given/known data A mass m moving at speed 4c/5 collides with another mass m at rest. The collision produces a photon with energy E traveling perpendicular to the original direction, and a mass M traveling in another direction. In terms of E and m, what is M? What is the largest value of E (in terms of m) for which this setup is possible? 2. Relevant equations p=Ymv E=Ymc^2 (E^2-P^2c^2)_before = (E^2-P^2c^2)_after 3. The attempt at a solution I tried using momentum only, and I get the calculation of p_x=4/3mc, p_y=-E/c (I used four momenta) so now this doesn't give me much to go on alone since I'm looking for M first. I tried using equation 3 but it becomes so convoluted I'm not sure how that could help me with the answer at all. Any suggestions?