I figure this problem will not be easily solvable which is why I'll ask. Basically a moving mass M with a velocity along the x direction of .5c explodes into two fragments. Fragment A moves to the -x and fragment B moves to the +x. The sum of the mass is 1/4A + 3/4B = M. What are the final velocities of the two fragments? I made this question up. Most physics books tell you the Mass is at rest initially but what would happen if it moves and then explodes. What are the calculations for it?