AS Daily A Level Physics question
A 0.50 kg lab trolley travels at +2.0 m/s along a low-friction track and collides head-on with an interchangeable end bumper. For both bumpers, the contact time is 0.10 s. With soft bumper S the trolley sticks and comes to rest. With springy bumper B the trolley rebounds at −1.5 m/s. Which statement must be true about the average force on the trolley during the contact in S compared with B?
Answer
The correct answer is A.
Correct: A — The average force magnitude is 1.75 times larger with B than with S; in both cases it acts opposite the initial motion. A is correct because the momentum change in S is 0.50×(0−2.0)=−1.0 N s (magnitude 1.0), while in B it is 0.50×(−1.5−2.0)=−1.75 N s (magnitude 1.75); same contact time means the force ratio is 1.75:1, and the force opposes the initial motion in both cases. B is wrong because equal times do not imply equal forces; the rebound reverses velocity, giving a larger momentum change and thus a larger average force. C is wrong because force depends on rate of change of momentum, not on how much kinetic energy is lost; sticking dissipates more energy but produces a smaller momentum change than rebounding here. D is wrong because it ignores direction; using |vi|−|vf|=0.5 m/s underestimates the true velocity change, which is −3.5 m/s for the rebound.