A lorry travelling at high speed along a motorway collides with a stationary car. After the collision, both vehicles move together in the same direction. Explain, using Newton's laws of motion and the concept of momentum, why the lorry slows down during the collision and why the combined speed of the vehicles after the collision is much lower than the lorry's original speed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A lorry of mass 10 000 kg is travelling at 30 m/s along a motorway. It collides with a stationary car of mass 1 500 kg. After the collision the two vehicles move together.
Model answer (5 marks)
During the collision the lorry pushes on the car, and by Newton's third law the car pushes back on the lorry with an equal and opposite force. This backwards force is a resultant force on the lorry, so by Newton's second law it causes a change in the lorry's momentum – the lorry decelerates.
Momentum is conserved in the collision. Before impact the only non‑zero momentum is that of the lorry: p_initial = 10 000 kg × 30 m s⁻¹ = 3.0×10⁵ kg m s⁻¹. The car has zero momentum. After impact the two vehicles move together, so the total mass is 10 000 kg + 1 500 kg = 11 500 kg. The conserved momentum is now shared by this larger mass, giving v_final = p_initial / 11 500 kg ≈ 26 m s⁻¹, which is much lower than the lorry's original 30 m s⁻¹.
Momentum is conserved in the collision. Before impact the only non‑zero momentum is that of the lorry: p_initial = 10 000 kg × 30 m s⁻¹ = 3.0×10⁵ kg m s⁻¹. The car has zero momentum. After impact the two vehicles move together, so the total mass is 10 000 kg + 1 500 kg = 11 500 kg. The conserved momentum is now shared by this larger mass, giving v_final = p_initial / 11 500 kg ≈ 26 m s⁻¹, which is much lower than the lorry's original 30 m s⁻¹.
Examiner tips
- Show the force pair from Newton's third law, then link to deceleration via Newton's second law.
- Use the conservation of momentum equation to calculate the final speed, emphasising the increased mass.
- Include the numerical values to demonstrate the calculation.
- Use correct units and UK spelling.
Common mistakes
- Ignoring the car’s zero initial momentum.
- Confusing the direction of the force (writing it as forwards instead of backwards).
- Failing to divide the initial momentum by the combined mass to find the final speed.
Mark scheme (5 marks)
- During the collision, the lorry exerts a force on the car (Newton's third law: the car exerts an equal and opposite force back on the lorry)
- The resultant (backwards) force on the lorry causes it to decelerate / slow down (Newton's second law: resultant force causes a change in momentum)
- Momentum is conserved in the collision (total momentum before = total momentum after)
- Before the collision only the lorry has momentum; the car is stationary so its momentum is zero
- After the collision the same total momentum is shared by a greater combined mass, so the combined speed is much lower than the lorry's original speed
Key terms in this question
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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