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AS Daily A Level Physics question

2025-09-16 OCR A Physical quantities & SI, Scalar vs vector, Kinematics (displacement–time) 3.1.1(c)

A trolley moves along a straight track. Its displacement–time graph is a straight line with a constant negative gradient. Which statement must be true?

  1. A Its speed decreases at a constant rate because the gradient is negative.
  2. B It moves with constant velocity in the negative direction; its acceleration is zero. (correct)
  3. C The area under the graph gives the distance travelled, which increases at a constant rate.
  4. D Its displacement becomes more negative over time, so its speed must be increasing.

Answer

The correct answer is B.

Correct: B — It moves with constant velocity in the negative direction; its acceleration is zero. A This confuses negative gradient with decreasing speed; the gradient is constant (not changing), so speed is constant, not decreasing. B This matches a straight-line displacement–time graph with negative slope: constant velocity directed opposite the positive displacement direction, so zero acceleration. C The area under a displacement–time graph does not represent distance; area is not meaningful here. D Becoming more negative does not imply speeding up; only the magnitude of the gradient indicates speed, and that magnitude is constant.

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