Problem: A particle with rest mass m0 and kinetic energy 3(m0)c^2 makes a complete inelastic collision with a stationary particle of rest mass 2m0. what are the velocity and rest mass of the composite particle? I tried to approach the problem first by identifying the velocity of the first particle before collision and got v=0.968c. Then i can't figure out why the rest mass of the composite particles is not simply 3m0, but this question is accompanied by a final answer, and it is not 3m0 for their composite rest mass. *BTW - after that I think that momentum conservation I will have no problem to find the new velocity... What am I missing?