Elastic and inelastic collisions — conservation of momentum and energy
Total momentum p = m₁v₁ + m₂v₂ is always conserved in collisions. In elastic collisions, kinetic energy is also conserved. In perfectly inelastic collisions, the objects stick together and KE is lost (converted to heat/sound). After elastic collision: v₁' = (m₁−m₂)v₁/(m₁+m₂) + 2m₂v₂/(m₁+m₂).