AS Daily A Level Physics question
A small rubber ball is fired horizontally at a stationary lab trolley on a smooth track. In trial 1 the ball sticks to the trolley. In trial 2 the ball bounces straight back from the trolley. The ball is launched at the same speed in both trials, and the trolley is much more massive than the ball. Which statement must be true about the trolley’s speed immediately after the impact?
Answer
The correct answer is D.
Correct: D — The trolley is faster when the ball bounces back, because reversing the ball’s direction gives the trolley a larger change in momentum. A Momentum is conserved overall, but that does not mean the trolley’s speed is the same; a bounce reverses the ball’s momentum and transfers more to the trolley. B Sticking does not guarantee more kinetic energy is passed to the trolley; in fact sticking typically loses kinetic energy internally and transfers less momentum than a bounce. C In a bounce the ball changes velocity more (it reverses), so the trolley gains more momentum and is not slower. D Reversal of the ball’s motion means a larger change in the ball’s momentum, so the trolley receives a larger impulse and ends up faster.