There are two identical ball,ball A with velocity v and ball B is at rest. Ball A hit ball B directly. How will the two balls move after the collision? In my mind there are three cases: 1. Ball A stops and ball B moves with the velocity v. 2. Two balls move together with same speed v/2. 3. Two ball moves with different velocity that B is faster. Case 1 is elastic collision Case 2 is perfectly inelastic collision Case 3 is inelastic collision What is the factor affecting the resulting movement of the two balls? How do I know if it is case 1 2 or 3 ? Since the difference between them is energy loss, I'm guessing if it is based on the source of energy loss. For example, it is likely to be case 1 if the collision is in space since no sound is produced and the energy loss is reduced. You could say there is still energy loss in heat or radiation but there are ways to reduce them. My main point is that the movement of the balls is determined by the energy loss while the momentum is still conserved. Am I correct? If I am correct,then something strange happened. If two balls have glue on them and they must move together after the collision, which means it has to be case 2, does that mean more energy is lost because of the glue? Before sticking them together, may be it is case 3 and some energy is lost by heat or radiation. After adding the glue, more radiation and heat is produced? Why? Or the energy is lost in the glue? Where is the energy? That confused me. Please help.