AS Daily A Level Physics question
Two low-friction trolleys on a straight air track collide and stick together. Trolley A (0.40 kg) moves at 1.2 m/s to the right; Trolley B (0.80 kg) moves at 0.30 m/s to the left. External horizontal forces are negligible. Which option gives the direction and speed of the joined trolleys immediately after the collision?
Answer
The correct answer is D.
Correct: D — 0.20 m/s to the right. Net momentum = (0.40×1.2) − (0.80×0.30) = 0.48 − 0.24 = 0.24 kg m/s to the right; combined mass = 1.20 kg, so shared speed = 0.24/1.20 = 0.20 m/s to the right (sticking = inelastic, momentum conserved). A gives the wrong direction by assuming the heavier trolley’s motion must dominate despite the net momentum being to the right. B adds speeds or ignores mass and the opposing momentum, greatly overestimating the result. C divides only the first trolley’s momentum (0.48) by total mass, neglecting the second trolley’s opposing momentum. D matches the conserved total momentum and the fact they stick, giving the correct magnitude and direction.