AS Daily A Level Physics question
Two trolleys A and B start from rest on a smooth bench. The same constant horizontal pull is applied to each for exactly 1.0 s. Trolley B has twice the mass of trolley A. Which statement must be true after the 1.0 s?
Answer
The correct answer is C.
Correct: C — B reaches half the speed of A. With the same pull, acceleration is inversely proportional to mass, so doubling the mass halves the acceleration and therefore halves the speed gained in the same time (e.g., if A gains 2 m/s in 1 s, B gains 1 m/s). A misinterprets the effect of mass, suggesting the heavier trolley speeds up more, which is opposite to the inverse relation. B ignores mass, but with half the acceleration, B cannot reach the same speed in the same time. C is correct by the inverse-mass proportionality and equal time. D is wrong because their speeds differ; for example, if A is 1 kg and reaches 2 m/s, KE_A = 1/2×1×2^2 = 2 J, while if B is 2 kg and reaches 1 m/s, KE_B = 1/2×2×1^2 = 1 J, not equal.