Say two balls of putty, moving relativistically near the speed of light, collide (although i understand this is not possible theoretically and realistically). They collide at a slight perpendicular displacement, instead of head-on, so that in the final state the stuck together system is rotating. Compared to the non-rotating head-on collision, how will this effect the final speed, and how will this effect the Mf' (final mass)? Imagine that you stop the lump from spinning, will its mass be great. less, or equal to Mf'? Here is my attempt; Vf remains the same, since rotation does not affect the conservation of momentum. However, less "energy" goes into the mass conversion, so there is less mass. If we stop it from spinning, we do work on the system, adding this rotational energy into the system and the mass willl then be greater to the final mass. I'm new to special relativity so I'm not exactly certain of my answer. Can anyone help me?