3.3.3—That, for an elastic collision, total kinetic energy is conserved and the

An elastic collision satisfies momentum conservation and also conserves total kinetic energy: KE_before=KE_after.

Use momentum and kinetic-energy equations together only when the collision is identified as elastic; otherwise kinetic energy may become thermal, sound or deformation energy.

For two identical smooth balls in a head-on elastic collision, exchanging their velocities satisfies both conservation laws.

Momentum conservation alone does not prove a collision is elastic; most real collisions are not perfectly elastic.