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AS Daily A Level Physics question

2026-06-14 OCR A Mechanics & Materials (M3) OCR A H156 Module 3.5 Newton’s laws of motion — Third law Module 3.2 Forces in action — interaction pairs (qualitative)

A small car collides head-on with a much heavier lorry on a straight road. During the brief impact, which statement must be true?

  1. A The force on the car from the lorry is equal in magnitude to the force on the lorry from the car. (correct)
  2. B The lorry exerts a larger force on the car because it has a larger mass.
  3. C If the car is moving faster, it exerts a larger force on the lorry than the lorry exerts on it.
  4. D The lighter car experiences a smaller force but a larger acceleration than the lorry.

Answer

The correct answer is A.

Correct: A — The force on the car from the lorry is equal in magnitude to the force on the lorry from the car. A states Newton’s third law: interaction forces come in equal and opposite pairs on different objects. B is wrong because mass does not set the size of the interaction force; it affects acceleration, not the third‑law force. C is wrong because the relative speeds do not unbalance an action–reaction pair; the forces remain equal at each instant of contact. D is wrong because the lighter car does not experience a smaller force; both experience the same force, but the car’s acceleration is larger due to its smaller mass.

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