I am a bit confused about the definition of elastic collision. What i have studied is, elastic collision is one in which the kinetic energy is conserved. Now, some books also say that elastic collision is one in which both momentum and kinetic energy is conserved. But, is it true that momentum is always conserved in an elastic collision. I have an example where in the collision is elastic but the momentum is still not conserved. A rod of mass M is hanging from the hinge of the ceiling. Now a point mass 'm' collides elastically with the rod. Surely the energy is conserved. But is the momentum conserved?