AS Daily A Level Physics question
On a low-friction track, a 0.50 kg glider A moves to the right at 2.0 m/s and collides centrally with a stationary 1.50 kg glider B. Two interchangeable bumpers are tested: - Foam bumper: the gliders stick together on impact. - Spring bumper: the collision is nearly elastic (kinetic energy essentially conserved). Ignore external forces during the brief impact. Which statement must be true?
Answer
The correct answer is B.
Correct: B — With the foam bumper, the two gliders stick and move right together at about 0.50 m/s; with the spring bumper, A reverses and moves left at about 1.0 m/s. A The total initial momentum is 0.50×2.0 = 1.0 kg m/s, so sticking gives v = 1.0/(0.50+1.50) = 0.50 m/s to the right; an elastic hit on a heavier, initially stationary target makes the lighter glider rebound, not continue right. B This matches momentum conservation for the inelastic case and the expected rebound of the lighter glider in a nearly elastic collision (A reverses and B takes forward motion). C A final speed of 1.0 m/s for the stuck pair would require 2.0 kg×1.0 m/s = 2.0 kg m/s of momentum, exceeding the initial 1.0 kg m/s; and in a nearly elastic collision the lighter glider does not simply stop—it rebounds. D Momentum is conserved in both cases, but B’s speed depends on collision type: with sticking B moves at 0.50 m/s with A, whereas a nearly elastic collision gives B about 1.0 m/s; assuming the same speed ignores the energy difference.