Which statement about collisions is true?

• A. Kinetic energy is always conserved in all collisions.
• B. Momentum is conserved in every collision.
• C. Total energy is conserved in all collisions, but kinetic energy may not be.
• D. Energy can be created or destroyed in collisions.

Answer: (C)

In a collision, the total energy of the system, if you account for all forms of energy, stays the same. That means kinetic energy, internal energy, deformation energy, heat, sound, and other forms can exchange energy among themselves, but the sum remains constant. If the bodies deform or heat up, some of the kinetic energy is converted into other forms, so the kinetic energy after the collision is not the same as before. Only in an ideal elastic collision would the kinetic energy be preserved as well.

So the best statement is that total energy is conserved in all collisions, but kinetic energy may not be. The idea to keep in mind is that energy can change form during a collision, but the total amount doesn’t vanish or appear from nowhere. The other options either claim kinetic energy is always conserved (which isn’t true for inelastic collisions) or claim momentum is conserved in every collision without qualification (external forces can affect momentum exchange), or say energy can be created or destroyed (which would violate energy conservation).