*** Elastic and Inelastic Collision explanation *** 1) Why energy and momentum are conserved in elastic collision? 2) Why only momentum is conserved in inelastic collision? Why energy is not conserved? Answers: Conservation of momentum is a consequence of Newton's third law. From the derivation, we see that only internal forces act on a system and the net external forces act on the system are zero, then we can apply conservation of momentum. 1) In the case of elastic collision, only internal forces act when the two bodies collide and come to a temporary halt and the external forces (e.g. gravitational force and normal force are zero) , so we can say firmly that momentum is conserved. I can only explain physically why momentum is conserved for the elastic collision. I somehow couldn't find a satisfactory explanation to why energy is conserved for elastic collision and question 2. For question 2, my textbook explains mathematically why energy is not conserved, but how about physically. A trolley that collides with another trolley and then stick together will result in a loud sound which explains why energy is not conserved. but how about if a big asteroid collideswith the earth(inelastic collision) will the energy conserved? All suggestions and ideas are welcome. Thank you.