Explain why a passenger in a car experiences a forward lurch when the car brakes suddenly to a stop.

IB DP Physics Standard 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).

A car travelling at constant velocity brakes sharply and comes to rest in a short time interval.

Model answer (4 marks)

Before braking the passenger moves with the car at the same velocity. When the brakes are applied the car’s wheels exert a horizontal force on the car, causing a deceleration. The passenger’s body, however, experiences only the normal force from the seat and the small friction of the seat‑belt; this horizontal force is insufficient to decelerate the passenger at the same rate as the car. According to Newton’s second law, a net force is required to change momentum, so the passenger’s momentum remains approximately constant. Consequently the passenger tends to keep moving forward at the original speed, which makes them lurch forward relative to the decelerating car.

Examiner tips

  • Use the phrase ‘net (resultant) force is required to change momentum’ to show understanding of Newton’s second law.
  • Explain that the passenger’s horizontal force is insufficient, linking to the seat‑belt/normal force.
  • Show the passenger’s tendency to continue at the original velocity to justify the forward lurch.
  • Keep the answer concise – 4 points, no extra words.

Common mistakes

  • Confusing the passenger’s motion with the car’s motion; not recognising the passenger’s inertia.
  • Forgetting to mention the insufficient horizontal force or the role of the seat‑belt.
  • Using vague terms like ‘force’ without linking to Newton’s second law or impulse-momentum theorem.

Mark scheme (4 marks)

  1. Before braking, the passenger is moving at the same velocity as the car.
  2. A net (resultant) force is required to change the momentum of an object (Newton's second law / impulse-momentum theorem).
  3. The car decelerates but no sufficient horizontal force acts on the passenger to decelerate them at the same rate as the car.
  4. The passenger tends to continue moving forward (at the original velocity) relative to the car, so they appear to lurch forward relative to the decelerating car.

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