I'm not sure this is the right place to post this question but here goes: In feynman vol.1 39-9 there is a situation where two molecules are about to collide in a CM frame, the frame has velocity Vcm and the two molecules have respective velocities v1 and v2, then there is no correlation between Vcm and the relative velocity, w (w=v1-v2) "coming in". what does Feynman mean by "coming in"? I find this a very difficult argument to understand, specifically, is w really relative velocity *before* the collision? in that case how can it be independent of Vcm, surely if one of the molecules is very heavy and very fast and the other slow and light, then Vcm is moving very fast in roughly the same direction of the heavy molecule and a correlation exists. If someone could look this up I'd appreciate it as I've been trying to get my head around it for ages, many thanks.