Explain why the work done by the normal contact force on a car travelling along a horizontal road is zero, even though the car moves a considerable distance.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A car travels at constant speed along a straight, horizontal road. The car's weight acts vertically downward and a normal contact force acts vertically upward from the road surface.
Model answer (4 marks)
Work is defined as the product of force and displacement in the direction of the force. The normal contact force acts vertically upward, perpendicular to the horizontal road. The car’s displacement is horizontal, so the angle between the normal force and the displacement is 90°. Because cos 90° = 0, the work done by the normal contact force is zero, no matter how large the force or the distance travelled.
Examiner tips
- Use the definition of work and the angle between force and displacement.
- Show that the normal force is perpendicular to the displacement.
- Explain that cos 90° = 0 gives zero work.
- Mention that the magnitude of the force or distance does not matter.
Common mistakes
- Confusing the normal force with the weight or friction force.
- Assuming work is zero because the car moves horizontally, without citing the angle or cosine.
- Forgetting to state that the normal force is vertical and the displacement is horizontal.
Mark scheme (4 marks)
- Work is defined as the product of force and the displacement in the direction of the force (or the component of force in the direction of displacement).
- The normal contact force acts vertically upward (perpendicular to the road surface).
- The displacement of the car is horizontal, so the angle between the normal contact force and the displacement is 90°.
- Because cos 90° = 0, the work done by the normal contact force is zero regardless of the magnitude of the force or the distance travelled.
Key terms in this question
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →