AS Daily A Level Physics question
A trolley moves in a straight line. Its displacement–time graph is described as: from t = 0 s to 4 s, displacement increases uniformly from 0 m to +8 m; from 4 s to 7 s, displacement remains constant at +8 m; from 7 s to 10 s, displacement decreases uniformly to +2 m. Which statement must be true?
Answer
The correct answer is A.
Correct: A — Between 7 s and 10 s, the trolley moves toward the origin with constant speed equal in magnitude to its speed between 0 s and 4 s. The graph’s gradients are constant on both segments and equal in magnitude but opposite in sign, and the displacement decreases while remaining positive, so it moves toward the origin with the same speed as before. A is correct for this reason. B is wrong because the flat section at t = 6 s has zero gradient, so the speed is zero even though the displacement is largest. C is wrong because average velocity depends on net displacement, which is not zero here; being “close” to the start does not make the average velocity zero. D is wrong because the displacement stays positive from 7 s to 10 s; direction of motion is indicated by the sign of the gradient (negative here), not by the sign of the displacement alone.