Newton's Third Law — AQA GCSE Physics
Test yourself on Newton's Third Law with AQA GCSE practice questions.
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Newton's Third Law explained
When two objects interact, each pushes or pulls on the other with a force of the same size but in the opposite direction.
Read the full explanation
In a collision between a bumper car and a barrier, the force of the car on the barrier equals the force of the barrier on the car, and the two point opposite ways. That is true whatever the masses involved: a lorry hitting a small car experiences exactly the same size of force as it exerts. What differs is the effect, because the same force produces a much larger acceleration on the smaller mass, from a = F ÷ m. The two forces never cancel because each acts on a different object. The two forces also act for the same length of time, so the changes of momentum are equal and opposite, which is why momentum is conserved.
Your focus
- State how Newton's Third Law applies to a named collision, identifying both forces and their directions.
- Explain why a small car is damaged more than a lorry even though the forces on them are equal.
- Identify which object each force of an interaction pair acts on in a given diagram.
Newton's Third Law exam tips
Marking Points
- one mark for naming both forces, such as the force of the car on the barrier and the force of the barrier on the car
- one mark for stating that the two forces are equal in size
- one mark for stating that the two forces act in opposite directions
- one mark for stating that the forces act on different objects and are the same type
Examiner Tips
- 💡One-mark third law questions still need both objects named; a bare 'equal and opposite' rarely scores.
- 💡Add 'and in the opposite direction' explicitly, since equal in size alone is incomplete.
- 💡If asked why the smaller vehicle suffers more, bring in a = F ÷ m rather than a bigger force.
Common Mistakes
- quoting 'every action has an equal and opposite reaction' without naming the two objects involved
- claiming the heavier vehicle exerts the larger force in a collision
- using the third law to argue that nothing can ever accelerate
- describing the balanced forces on a single object instead of an interaction pair