Newton's Third Law
Revise Newton's Third Law for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Newton's third law states that when two objects interact they exert equal and opposite forces on each other.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Newton's third law states that whenever two objects interact, the forces they exert on each other are equal and opposite.
The crucial detail is that the two forces act on different objects. This is why they do not cancel each other out — if they acted on the same object, nothing could ever accelerate.
When a person pushes on a wall, the wall pushes back on the person with an equal force in the opposite direction. When a swimmer pushes water backwards, the water pushes the swimmer forwards. Identifying which force acts on which object is what these questions test.
Revision notes
The law
Whenever two objects interact, the forces they exert on each other are equal in size and opposite in direction.
The two forces are always of the same type — if one is a contact force, so is the other.
Why they do not cancel
The two forces act on DIFFERENT objects.
Forces only cancel when they act on the same object. Because these act on different objects, each can cause its own object to accelerate. This is the point students most often miss.
Everyday examples
A swimmer pushes water backwards; the water pushes the swimmer forwards.
A person pushes on a wall; the wall pushes back equally. A rocket pushes exhaust gases downwards; the gases push the rocket upwards.
Key points
- Interacting objects exert equal and opposite forces.
- The two forces act on different objects.
- They do not cancel because they act on different objects.
- The forces are always the same type.
- A swimmer pushes water back; water pushes the swimmer forward.
- Identify which force acts on which object.
Worked examples
Example 1
State Newton's third law. [2 marks]
Model answer
Whenever two objects interact, the forces they exert on each other are equal in size (1 mark) and opposite in direction (1 mark).
Example 2
Explain why the two forces in an interaction pair do not cancel each other out. [2 marks]
Model answer
The two forces act on different objects (1 mark), and forces can only cancel when they act on the same object (1 mark).
Example 3
A swimmer pushes backwards against the water. Explain how this makes the swimmer move forwards. [2 marks]
Model answer
The swimmer exerts a backwards force on the water (1 mark), and by Newton's third law the water exerts an equal and opposite forward force on the swimmer, pushing them forwards (1 mark).
Common mistakes
Saying the two forces cancel.
They act on different objects, so they do not cancel.
Naming both forces as acting on the same object.
Each force acts on the other object in the interaction.
Saying the forces are unequal.
They are always equal in size and opposite in direction.
Confusing this with balanced forces.
Balanced forces act on one object; a third law pair acts on two.
Exam tips
- Always state which force acts on which object.
- Explain non-cancellation through different objects.
- Learn one clear everyday example.
- Keep third law pairs distinct from balanced forces.
Key terms
- Newton's third law
- Interacting objects exert equal and opposite forces.
- Interaction pair
- The two equal and opposite forces in an interaction.
- Equal and opposite
- Same magnitude, opposite direction.
- Different objects
- Where the two paired forces act.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.