A car travelling at high speed brakes suddenly and comes to a stop. Explain, using Newton's laws of motion, why the passengers continue to move forwards after the car stops, and describe how seat belts reduce the risk of injury to passengers.

OCR A-Level Physics A (H556) — 3.5 Newton's laws and momentum · Explain · 5 marks · View as Markdown

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

Modern cars are fitted with seat belts designed to keep passengers safe during sudden deceleration. Safety engineers consider both the forces acting on passengers and the time over which those forces act when designing restraint systems.

Model answer (5 marks)

According to Newton’s First Law a passenger will keep moving at the car’s original speed until a force acts on them. When the car brakes the seat belt is the only force that can change the passenger’s motion. By Newton’s Second Law the belt exerts a decelerating force on the passenger, which changes their velocity. The belt is designed to stretch slightly, which increases the time over which this force is applied. Because the change in momentum is the same, a longer stopping time means a smaller average force on the passenger, thereby reducing the risk of injury.

Examiner tips

  • Use the phrase ‘Newton’s First Law – inertia’ to justify forward motion.
  • Show the belt provides a force that satisfies F=ma (Newton’s Second Law).
  • Explain that increasing stopping time reduces force (F=Δp/Δt).

Common mistakes

  • Claiming the car’s deceleration directly stops the passenger – ignores the need for a force.
  • Forgetting to mention the belt’s role in increasing stopping time.
  • Using the wrong sign for the force (e.g., saying the belt pushes the passenger forward).

Mark scheme (5 marks)

  1. Newton's First Law: passengers tend to remain in their state of motion (continue moving forwards) when the car stops, because no resultant force is acting on them to change their motion
  2. The car decelerates but no force has yet acted on the passenger to decelerate them, so they continue at the original speed
  3. Newton's Second Law / the seat belt exerts a resultant force on the passenger to decelerate them (force causes change in velocity / acceleration)
  4. The seat belt stretches / increases the time over which the passenger decelerates
  5. A longer stopping time means a smaller force acts on the passenger (for the same change in momentum), reducing injury

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