question_answer 8)

Answer carefully, with reasons: (a) In an elastic collision of two billiard balls, is the total kinetic energy conserved during the short time of collision of the balls (i.e., when they are in contact) ? [Delhi 12] (b) Is the total linear momentum conserved during the short time of an elastic collision of two balls ? (c) What are the answers to (a) and (b) for an inelastic collision ? (d) If the potential energy of two billiard balls depends only on the separation distance between their centres, is the collision elastic or inelastic ? (Note, we are talking here of potential energy corresponding to the force during collision, not gravitational potential energy.)  

Answer:

(a) During the short time of collision when the balls are in contact, the kinetic energy of the balls gets converted into potential energy. In an elastic collision, though the kinetic energy before collision is equal to the kinetic energy after the collision but kinetic energy is not conserved during the short time of collision. (b) Yes, the total linear momentum is conserved during the short time of an elastic collision of two balls. (c) In an inelastic collision, the total K.E. is not conserved during collision as well as even after the collision. But the total linear momentum of the two balls is conserved. (d) The collision is elastic because the forces involved are conservative.