In a collision with no external forces, which quantity remains constant?

• A. Conservation of Momentum
• B. Conservation of Energy
• C. Impulse
• D. Elastic Potential Energy

Answer: (B)

When there are no external forces acting, the total momentum of the system must stay unchanged. During a collision, the forces that particles exert on each other are internal to the system, so they can transfer momentum between the bodies, but the sum of their momenta remains the same.

Energy, however, can move between forms. In many collisions some kinetic energy is converted into deformation, heat, or sound, so the kinetic energy doesn’t have to stay constant. Only in a perfectly elastic collision is kinetic energy conserved as well. So the quantity that is guaranteed to stay constant in this scenario is the momentum, while energy conservation depends on the specifics of the collision.