Not exactly a homework question, so sorry if I'm in the wrong forum 1. The problem statement, all variables and given/known data I'm trying to understand the difference between KE and momentum. 2. Relevant equations It seems to me that: In a collision, KE can be converted into heat, which is a kind of KE/potential energy because it is the vibration of atoms: ie, energy is conserved, but it can change into a different kind of energy. But what about momentum? As far as I understand, momentum is always conserved, even in inelastic collisions. But if some of the KE has become heat after a collision, why can't some of the momentum become heat, too. Or don't vibrating atoms have momentum? 3. The attempt at a solution OK. Maybe I'm seeing a possible answer here: because those atoms are vibrating randomly in different directions, the aggregate momentum of the body warmed after the collision does not change due to the warming. Momentum can't be changed into heat and the body has to change its overall motion instead. Is this right?