4. A bowling ball moving with speed, v collides head-on with a stationary tennis ball. The collision is elastic, and there is no friction. The bowling ball barely slows down. What is the speed of the tennis ball after the collision?
   A. nearly v
   B. nearly 2 v
   C. nearly 3 v
   D. nearly 4 v

In an elastic collision, both momentum and kinetic energy are conserved. Since the bowling ball is significantly more massive than the tennis ball and barely slows down, the speed of the tennis ball after the collision can be estimated using the conservation of momentum. The lighter ball (the tennis ball) will rebound with a speed that is nearly equal to that of the bowling ball prior to the collision. Hence, the answer is A. nearly v.

Another way to visualize this is to imagine two balls on a smooth surface. If a heavyweight guy gives a gentle push to a lightweight guy standing still, the lightweight guy will zoom off in the direction of the push, while the heavyweight guy hardly moves at all! It’s a fun analogy demonstrating how mass impacts motion during collisions!