Newton’s Third Law: Whenever two objects interact, the forces they exert…

The idea

When two objects interact, each pushes or pulls on the other with a force of the same size but in the opposite direction. 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.

What earns the marks

• Naming both forces, such as the force of the car on the barrier and the force of the barrier on the car.
• Stating that the two forces are equal in size.
• Stating that the two forces act in opposite directions.
• Stating that the forces act on different objects and are the same type.

Common mistakes and how to put them right

• 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.

Exam 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.