A student is timing a remote-controlled car as it moves along a straight road. The car travels 30 m in one direction in 10 s, then immediately reverses and travels 10 m back along the same straight road in 5 s. Explain the difference between the car's average speed for the whole journey and its average velocity for the whole journey, and describe how each quantity is calculated using the car's motion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A remote-controlled car travels 30 m along a straight road in 10 s, then immediately reverses and travels 10 m back along the same straight road in 5 s.
Model answer (5 marks)
Average speed is a scalar and is calculated from the total distance travelled, regardless of direction. The car covers 30 m + 10 m = 40 m in 15 s, so
average speed = total distance ÷ total time = 40 m ÷ 15 s ≈ 2.7 m s⁻¹.
Average velocity is a vector and depends on displacement. The car ends 20 m from its starting point in the original direction, so the displacement is 20 m. Thus
average velocity = displacement ÷ total time = 20 m ÷ 15 s ≈ 1.3 m s⁻¹ in the original direction.
Speed has no direction; velocity has both magnitude and direction.
average speed = total distance ÷ total time = 40 m ÷ 15 s ≈ 2.7 m s⁻¹.
Average velocity is a vector and depends on displacement. The car ends 20 m from its starting point in the original direction, so the displacement is 20 m. Thus
average velocity = displacement ÷ total time = 20 m ÷ 15 s ≈ 1.3 m s⁻¹ in the original direction.
Speed has no direction; velocity has both magnitude and direction.
Examiner tips
- Show the two calculations separately, noting distance vs displacement; include the direction for velocity.
- Use the correct units (m s⁻¹) and round to one decimal as in the mark scheme.
Common mistakes
- Using total distance for both speed and velocity; forgetting the direction for velocity; mis‑calculating displacement as 30 m instead of 20 m.
Mark scheme (5 marks)
- Average speed uses total distance travelled (40 m), not displacement
- Average speed = total distance ÷ total time = 40 m ÷ 15 s (≈ 2.7 m/s)
- Displacement is 20 m (in the original direction of travel) because the car ends up 20 m from its start
- Average velocity = displacement ÷ total time = 20 m ÷ 15 s (≈ 1.3 m/s)
- Velocity is a vector quantity so it has both magnitude and direction / speed is a scalar and has no direction
Key terms in this question
average speed · average velocity
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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