In a closed system, momentum is conserved. How does this relate to elastic and inelastic collisions regarding kinetic energy?

• A. Elastic collisions conserve both momentum and kinetic energy; inelastic collisions conserve momentum but not kinetic energy.
• B. Elastic collisions conserve momentum only; inelastic conserve kinetic energy.
• C. Elastic collisions do not conserve momentum.
• D. Kinetic energy is conserved in all collisions.

Answer: (A)

In a closed system, momentum stays the same for any collision because there are no external forces acting on the pair of objects. How kinetic energy behaves, however, depends on the type of collision. In an elastic collision, the objects bounce off each other with no net loss of kinetic energy, so the total kinetic energy before and after is the same as well as the momentum. In an inelastic collision, the objects may deform or stick together, and some of that initial kinetic energy is transformed into other forms like heat, sound, or deformation energy, so the total kinetic energy is not conserved even though the total momentum is. A perfectly inelastic collision—where objects stick together—illustrates this clearly: momentum is conserved, but kinetic energy is not.