Elastic vs inelastic collisions: Which statement is true about momentum and kinetic energy?

• A. Momentum is conserved in elastic collisions; kinetic energy is conserved in all collisions.
• B. Momentum is conserved in both elastic and inelastic collisions; kinetic energy is conserved only in elastic.
• C. Momentum is conserved only in elastic collisions; kinetic energy is conserved in both.
• D. Momentum is not conserved in any collision; kinetic energy is conserved in elastic.

Answer: (B)

Momentum conservation holds in collisions whenever there are no external forces acting on the system, so the total momentum before and after must be the same for both elastic and inelastic collisions. Kinetic energy, on the other hand, can be lost to other forms of energy in inelastic collisions—like deformation, heat, or sound—so the total kinetic energy after is not necessarily the same as before. In elastic collisions, no energy is lost to these forms, so kinetic energy is preserved as well. Therefore, the statement that momentum is conserved in both elastic and inelastic collisions while kinetic energy is conserved only in elastic collisions is the correct description. Think of pool balls as a close example of elastic interactions, where both momentum and kinetic energy stay the same overall, versus a car crash being a typical inelastic collision where some kinetic energy goes into deformation and heat.