Imagine two particles of uneven masses colliding at relativistic velocities. What is the final velocity of each particle after the collision (no they do not stick together). This question pertains to the relativistic treatment of a one dimensional collision. Basically I tried to derive the equations of motion for the relativistic case but I'm stuck. I attached what I've done so far. I thought that solving for the Newtonian conservation of momentum case would be easy simply by putting in the gammas and solving for a simultaneous equation except that I end up with a simultaneous equation for two unknown masses (these are the relativistic masses) while the final rest masses are known. In the attached picture the unknowns are the final relativistic masses while the initial rest masses and the initial relativistic masses are known (please see the last picture at the last line). Thanks.