I suppose this sounds a little silly, but: Momentum has to be conserved, but often at the cost of some kinetic energy. For example, if someone throws a 1kg mass at 50kg person at 10 m/s, he catches it: mv=mv (1)10=v(51) v=10/51 KE1=.5(1)(10)^2=50j KE2=.5(51)(10/51)^2=.98j Heat is thus created, but my question is: where? I suppose in this situation it would be in the glove... but how exactly does that heat come about? Heat is, I guess, kinetic energy on a smaller scale (moving particles). How do these particles begin to move?