Explain why a passenger in a car experiences a sideways force from the car seat when the car travels around a horizontal circular bend at constant speed, even though the passenger's speed is constant.

IB DP Physics Higher Level (2023 syllabus) — A.2 Forces and momentum · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

A passenger’s velocity is a vector; when the car turns the direction of the velocity changes even though the speed is constant.
1. The change in direction means the passenger is accelerating – the acceleration is centripetal and directed towards the centre of the circular path.
2. By Newton’s second law a net force is required to produce this acceleration, and that force must also point towards the centre.
3. The only contact force that can supply this inward force is the seat’s normal reaction on the passenger.
4. Therefore the seat pushes the passenger sideways (inward) to provide the centripetal force needed for the circular motion.

Examiner tips

  • Use the term ‘velocity is a vector’ to show you understand the change in direction; link this to acceleration. Mention Newton’s second law explicitly and state the direction of the required force. Identify the seat’s normal reaction as the source of the centripetal force. Keep the answer concise – 4 points, one sentence each.

Common mistakes

  • Confusing speed with velocity and saying the passenger is not accelerating. Forgetting to specify that the seat’s normal reaction is the inward force. Using vague language such as ‘the car pushes the passenger’ instead of ‘the seat’s normal reaction provides the centripetal force’.

Mark scheme (4 marks)

  1. Velocity is a vector quantity, so changing direction means the velocity is changing even if the speed is constant.
  2. A changing velocity means the passenger is accelerating (centripetal acceleration directed towards the centre of the circular path).
  3. By Newton's second law, a net force is required to produce this acceleration, directed towards the centre of the circular path.
  4. The seat (contact force / normal reaction from the seat) provides this centripetal force, pushing the passenger inwards / towards the centre.

Related

More Forces and momentum questions

▶ Try answering this question with AI marking (free) →