A cyclist travels 12 km due north to reach a town, then immediately turns around and cycles 5 km due south before stopping for a break. Describe the difference between the distance travelled and the displacement of the cyclist at the point where they stop, and explain why knowing only the speed of the cyclist is not enough to fully describe their motion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Distance travelled is the total length of the path: 12 km north + 5 km south = 17 km.
Displacement is the straight‑line change in position: 12 km north – 5 km south = 7 km north.
Displacement is a vector quantity – it has both magnitude (7 km) and direction (north).
Speed is a scalar quantity – it only tells how fast the cyclist is moving, with no direction.
To fully describe the motion we need velocity, which combines speed and direction.
Displacement is the straight‑line change in position: 12 km north – 5 km south = 7 km north.
Displacement is a vector quantity – it has both magnitude (7 km) and direction (north).
Speed is a scalar quantity – it only tells how fast the cyclist is moving, with no direction.
To fully describe the motion we need velocity, which combines speed and direction.
Examiner tips
- State the numerical values for distance and displacement; show the subtraction for displacement. Mention that displacement is a vector and speed is a scalar. Explain that velocity is required because it includes direction.
- common_mistakes
- :
- Confusing distance with displacement or giving only the magnitude of displacement. Forgetting to note that speed is scalar and velocity is needed to describe motion.
Mark scheme (5 marks)
- Distance is the total length of the path travelled (12 km + 5 km = 17 km)
- Displacement is 7 km due north (12 − 5 = 7 km in the original direction)
- Displacement is a vector quantity — it has both magnitude and direction
- Speed is a scalar quantity — it only gives the magnitude of how fast the cyclist is moving, with no direction
- Velocity is needed to fully describe the motion because it includes both speed and direction
Key terms in this question
distance · displacement · speed
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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