• Momentum is always conserved. When solving conservation of momentum problems, remember that
momentum is a vector quantity.
• In an elastic collision, kinetic energy is conserved. When two objects collide and bounce off each
other, without losing any energy (to heat, sound, etc.), they have engaged in an elastic collision. In an
inelastic collision, kinetic energy is not conserved. The extreme case is a perfectly inelastic collision.
When two objects collide and stick together, they have engaged in a perfectly inelastic collision.