AS Daily A Level Physics question
On a smooth straight track, a 0.40 kg trolley moving at 2.5 m/s to the right collides and sticks to a 0.60 kg trolley at rest. Neglect external forces during the brief collision. What is the speed and direction of the joined trolleys immediately after impact?
Answer
The correct answer is A.
Correct: A — 1.0 m/s to the right. Total momentum before is (0.40 × 2.5) = 1.0 kg m/s to the right; the combined mass is 1.00 kg, so speed = 1.0/1.00 = 1.0 m/s to the right. A is correct because the shared speed equals total momentum divided by total mass. B is wrong because it treats the final speed as a simple 0.40 fraction of the initial speed, ignoring that momentum (not speed) is shared across the total mass. C is wrong because the speed cannot remain 2.5 m/s when extra mass is added; that would increase total momentum without an external impulse. D is wrong because reversing direction would require negative final momentum, impossible here since initial momentum is to the right and there is no external impulse.