Hi, you guys! :) I am studying Elastic/Inelastic Collision my Physics class right now. So I know: Elastic Collision: P(initial) = P(final) ; KE(initial) = KE(final) Inelastic Collison: P(initial) = P(final) ; KE(initial) ≠ KE(final) I'm wondering how I am supposed to know right when I see a type of problem, whether elastic or inelastic collision is taking place? There was a pool ball example problem that I was working, and it required that you know it is elastic collion (and therefore, KE(i) and KE(f) are equal) taking place. ....In that particular problem, the collision was not head-on, so the two balls moved in different directions after the collision. But Newton's Cradle is an example of elastic collision also, right? If one ball hits the other balls, the middle ones stay stationary, but the end ball swings out and continues that pattern. So if the pool balls don't have a head-on collision, resulting in the striking ball to become stationary, and to transfer all of it's kinetic energy to the other ball, how could an elastic collision be taking place??? Please help me to understand this better! Thank you so much!