AS Daily A Level Physics question
In a lab tracking experiment, the displacement s of a trolley along a straight track is plotted against time t. The graph has three sections: 0–3 s: s increases linearly from 0 m to +6 m; 3–5 s: s remains constant at +6 m; 5–9 s: s decreases smoothly from +6 m to –2 m and the curve gets progressively steeper as time increases. Which statement must be true?
Answer
The correct answer is B.
Correct: B — Between 5 s and 9 s the trolley is moving toward its starting point and speeding up. A The average velocity depends on net displacement, which is −2 m over 9 s, so it is not zero; finishing near the start does not make the average velocity zero. B This interval shows displacement decreasing with an increasingly steep slope, so velocity is negative (toward the start) and the speed increases. C This confuses distance with displacement: the trolley goes +6 m out and then 8 m back, so the total distance is greater than 8 m. D Speed is a scalar and cannot be negative; a flat displacement–time graph means zero velocity and therefore zero speed.