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.
Students should be able to apply Newton’s Third Law to examples of equilibrium situations.
Equilibrium questions ask you to keep two ideas apart. A book resting on a table is in equilibrium because the forces acting on the book, its weight downwards and the normal contact force from the table upwards, are equal and opposite, so the resultant force on the book is zero. Those two forces are not a third law pair: they act on the same object. The third law pairs are different. The book pushes down on the table and the table pushes up on the book; the Earth pulls down on the book and the book pulls up on the Earth. Each pair acts on two different objects, so a pair can never be responsible for equilibrium on its own. In a tug of war that nobody is winning, the resultant force on the rope is zero, while the pull of each team on the rope is paired with the rope's pull on that team.
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.
Show all 6 objectives
- Identify the third law partner of the normal contact force acting on a book resting on a table.
- Explain why the weight of a resting object and the contact force on it are not an interaction pair.
- Draw a free body diagram for an object in equilibrium and show that the resultant force is zero.
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
- one mark for identifying the two forces acting on the object itself and stating they are equal and opposite
- one mark for stating that the resultant force on the object is therefore zero
- one mark for identifying the correct third law partner of a named force, acting on the other object
- one mark for stating that the two forces of a third law pair never act on the same object
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.
- 💡Draw the object alone and mark only the forces acting on it; a third law partner never appears on that diagram.
- 💡Use the phrase 'resultant force is zero' for equilibrium and 'equal and opposite on different objects' for the third law.
- 💡Check the type of each force: the partner of a gravitational pull is another gravitational pull.
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
- calling the weight of a book and the table's contact force a Newton's Third Law pair
- assuming that because forces are equal and opposite they must be an interaction pair
- leaving out the object a force acts on, so the examiner cannot tell which pair is meant
- saying an object in equilibrium has no forces acting on it