Which of the following collisions is an elastic collision?

• A. Two bodies move towards each other, collide and then move away from each other. There is a rise in temperature.
• B. Two bodies collide and the sound of collision is heard by a blind man.
• C. Two steel balls collide such that their kinetic energy is conserved.
• D. A man jumps on to a moving cart

Answer: (C)

Elastic collisions are those in which the total kinetic energy of the system remains the same before and after the collision (momentum is conserved as well). The best example here is two steel balls colliding in which their kinetic energy is preserved, meaning after the impact they rebound without any loss of kinetic energy to other forms.

If a collision causes a rise in temperature, energy has been transferred from kinetic energy to internal energy, so the collision isn’t elastic. Hearing a sound from the collision indicates some kinetic energy has become sound energy, again not elastic. When a person jumps onto a moving cart, some energy goes into deformation, friction, and other non-recoverable forms, so the collision is not elastic.