A car travels along a straight road. Its velocity changes from +20 m s⁻¹ to −8 m s⁻¹ in a time of 7.0 s. Explain what the change in sign of the velocity indicates, and state what quantity the magnitude of the rate of change of velocity represents. Then explain why it is possible for the car's average speed over this interval to be non-zero even if its displacement over the same interval happened to be zero.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. The change from +20 m s⁻¹ to −8 m s⁻¹ means the car has reversed its direction of motion – it was moving forward, then turned and moved backward.
2. The magnitude of the rate of change of velocity is the acceleration (|a|).
3. Average speed is the total distance travelled divided by the total time.
4. Even if the displacement is zero (the car returns to its starting point), the path length is still positive, so the average speed is non‑zero.
2. The magnitude of the rate of change of velocity is the acceleration (|a|).
3. Average speed is the total distance travelled divided by the total time.
4. Even if the displacement is zero (the car returns to its starting point), the path length is still positive, so the average speed is non‑zero.
Examiner tips
- Use the word ‘reverse’ or ‘change direction’ for the sign change; link it to opposite velocity vectors. State ‘acceleration’ explicitly and include the unit m s⁻². Explain the difference between distance and displacement to justify non‑zero average speed.
Common mistakes
- Confusing velocity with speed when describing the sign change. Forgetting to mention the unit for acceleration. Saying the average speed is zero because displacement is zero, ignoring the path length concept.
Mark scheme (4 marks)
- A change in sign of velocity indicates the car has reversed its direction of motion (is now moving in the opposite direction to its initial motion).
- The magnitude of the rate of change of velocity represents acceleration (magnitude of acceleration).
- Average speed is total distance travelled divided by total time, whereas average velocity is total displacement divided by total time.
- If the displacement is zero the car has returned to its starting position, but the total distance (path length) travelled is still positive (non-zero), so the average speed is non-zero.
Key terms in this question
velocity · displacement · average speed
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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