Is elastic collision defined as a collision where no energy is lost or where no momentum is lost? We had to do a lab in class and one question was whether the collison was elastic. I think we all got it wrong because we sayed it was elastic because momemtum was conserved. But we were supposed to check if energy was (apparently it wasn't) conserved. (It seems kind of stupid because nowhere in the lab did we have to find the energy so we would have to find it separately to find whether it was elastic or not.) So is elasticity defined by momentum or energy?