A2 Daily A Level Physics question
Two students on low-friction trolleys push apart from rest in a straight line. Student A has mass 50 kg and moves east at 1.2 m/s immediately after the push. Student B has mass 75 kg. Air resistance is negligible. What is Student B’s speed and direction immediately after the push?
Answer
The correct answer is A.
Correct: A — 0.80 m/s west. The system starts at rest so total momentum is zero; after they push off, their momenta are equal and opposite, so v_B = (50/75)×1.2 = 0.8 m/s in the opposite direction (west). A is correct because the heavier student must move more slowly in the opposite direction to keep total momentum zero. B is wrong because equal and opposite forces do not give equal speeds when masses differ; speeds are inversely proportional to mass. C is wrong because it gives the correct magnitude but the wrong direction; Student B must move opposite to A. D is wrong because it adds speeds or misuses the mass ratio; the correct ratio (50/75) gives 0.8 m/s, not 1.8 m/s.