A ball is thrown vertically upwards and returns to its starting position. Explain why the average speed of the ball over the entire journey is greater than zero whilst its average velocity is zero.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Average velocity is zero because the ball returns to its starting position, so the net displacement is zero.
Average velocity is defined as displacement divided by time; with zero displacement the average velocity is zero regardless of the time taken.
Average speed is greater than zero because the ball travels a non‑zero total distance (upward plus downward).
Average speed is defined as total distance divided by time; it is a scalar, so the distances on the way up and the way down add together rather than cancel.
Average velocity is defined as displacement divided by time; with zero displacement the average velocity is zero regardless of the time taken.
Average speed is greater than zero because the ball travels a non‑zero total distance (upward plus downward).
Average speed is defined as total distance divided by time; it is a scalar, so the distances on the way up and the way down add together rather than cancel.
Examiner tips
- State that displacement is zero for average velocity; state that total distance is non‑zero for average speed.
- Use the definitions of average velocity and average speed explicitly.
- Show that the scalar nature of speed means distances add, not cancel.
Common mistakes
- Confusing displacement with distance; writing that average velocity is zero because the ball stops at the top.
- Claiming average speed is zero because the ball returns to the start; ignoring the upward and downward travel.
- Using vector notation for speed or omitting the definition of average speed.
Mark scheme (4 marks)
- Average velocity is zero because displacement is zero — the ball returns to its starting position, so net change in position is zero.
- Average velocity is defined as displacement divided by time, so zero displacement gives zero average velocity regardless of the time taken.
- Average speed is greater than zero because the ball has travelled a non-zero total distance (up and then back down).
- Average speed is defined as total distance divided by time, which is a scalar quantity, so the distances on the way up and the way down add together rather than cancelling.
Key terms in this question
average velocity · average speed
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Kinematics questions
- A cyclist travels along a straight road. For the first part of the journey the c…
- A car travels along a straight road. Its velocity changes from +20 m s⁻¹ to −8 m…
- A sprinter accelerates uniformly from rest and reaches a maximum velocity of 11 …
- Explain why the area under a velocity–time graph between two instants gives the …