AS Daily A Level Physics question
Two low-friction trolleys on a level track move in the same straight line and collide, latching together. During the brief collision, external effects like track friction and air resistance are negligible compared with the forces between the trolleys. Which statement must be true?
Answer
The correct answer is A.
Correct: A — The total momentum of A and B immediately after equals their total momentum immediately before. A is true because with negligible external forces the system’s momentum is conserved during the brief collision. B is wrong because sticking together is an inelastic collision, so total kinetic energy decreases as some becomes internal/thermal energy. C is wrong because although the interaction time is the same for both, ∆v = impulse/mass, so different masses generally do not have the same change in velocity. D is wrong because the impulses are equal in magnitude and opposite in direction by Newton’s third law.