AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER
Newton's Third Law
Forces · Lesson 16 of 20
Teacher copy - includes the notes for whoever is teaching from it.
Warm-up
Answer each one, then check.
1. What is a force?
A push or a pull
2. What are balanced forces?
Equal and opposite forces on the same object
3. Give a contact force.
Friction, tension or normal contact force
4. What is a non-contact force?
A force between objects that are not touching
5. What is weight?
The force of gravity on an object
Learning Objectives
1. State Newton's Third Law.
2. Identify pairs of forces between two interacting objects.
3. Explain why interaction pairs do not cancel each other.
4. Apply the law to objects in equilibrium.
Newton's Third Law
Whenever two objects interact, the forces they exert on each other are equal and opposite.
The two forces in an interaction pair act on different objects, are the same type of force, and are equal in size and opposite in direction.
Interaction Pairs
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Each force acts on a different object. |
Examples of pairs of equal and opposite forces: a book on a table, a person pushing a wall and the Earth and Moon.
Finding an Interaction Pair
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A book rests on a table. The table exerts an upward force on the book. What is the other force in this interaction pair? |
1. Type of force
A contact (normal) force
2. Acts on
The table
3. Direction
Downwards
Answer: The book pushes down on the table with an equal force.
Weight and Contact Force
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A book at rest on a table has a weight of 10 N. Is the normal contact force from the table the Third-Law pair of the weight? Explain. |
1. Weight
The Earth pulls the book down; the pair is the book pulling the Earth up
2. Normal contact force
The table pushes the book up: a different kind of force
3. So
They are equal and opposite because the book is in equilibrium, not because of the Third Law
Answer: No. The pair of the weight is the book's gravitational pull on the Earth. The contact force balances the weight (Newton's First Law).
Why Things Move
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A swimmer pushes backwards on the water. Explain why the swimmer moves forwards. |
1. Action
The swimmer pushes the water backwards
2. Reaction
The water pushes the swimmer forwards with an equal force
3. Resultant
Acts on the swimmer and causes acceleration
Answer: The water pushes back on the swimmer with an equal and opposite force.
Common Mistakes
Avoid these.
▸ Same object. Forces that cancel act on the same object; a Third-Law pair acts on different objects.
▸ Always equal. The forces in a pair are always equal, even if one object is much lighter.
▸ Same type. The pair is always the same type of force.
▸ Motion. Equal forces on different objects can give different accelerations (F = ma).
Key Terms
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Newton's Third Law The forces two interacting objects exert on each other are equal and opposite. |
Interaction pair The two forces in an interaction between two objects. |
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Equilibrium When the forces on an object are balanced. |
Normal contact force The push from a surface on an object resting on it. |
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Reaction force The force that forms a pair with another force. |
Different objects The two forces in a pair act on separate bodies. |
Your Task: Find the Pair
10 minutes
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For each, state the other force in the pair: (a) a foot pushes backwards on the ground (b) the Sun pulls the Earth (c) a rocket pushes gas out of its engine. 1. Swap the objects. 2. Keep the same type of force. |
A good answer shows: (a) The ground pushes the foot forwards. (b) The Earth pulls the Sun. (c) The gas pushes the rocket forwards.
Note: Ask why the rocket accelerates but the gas accelerates differently.
Can I...?
☐ State the third law.
☐ Name the object each force acts on.
☐ Identify a pair.
☐ Explain that pairs do not cancel.
☐ Apply it to a book on a table.
☐ Apply it to a swimmer.
☐ Apply it to gravity.
☐ Explain equilibrium.
Summary
✓ Interaction pairs: equal and opposite, on different objects.
✓ Same type of force in a pair.
✓ Weight and normal force are not a Third-Law pair.
✓ Equilibrium: balanced forces on one object.
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EXAM FOCUS State Newton's Third Law. (2 marks) Forces equal in size and opposite in direction on different objects. |
Exam Practice: Newton's Third Law
Answer all questions. Use the mark allocation as a guide to how much to write. · 16 minutes
▸ Question 1 · 2 marks · State. State Newton's Third Law of motion.
▸ Question 2 · 3 marks · Explain. A book rests on a table. The table pushes up on the book. State the force that forms a Newton's Third Law pair with this force, and the…
▸ Question 3 · 3 marks · Explain. A swimmer pushes the water backwards and moves forwards. Explain why, using Newton's Third Law.
▸ Question 4 · 3 marks · Explain. The Earth pulls the Moon with a gravitational force. A student says that the Moon pulls on the Earth with a much smaller force because the…
▸ Question 5 · 4 marks · Explain. A book of weight 10 N rests on a table. A student says the weight and the upward force from the table are a Newton's Third Law pair…
Question 1 · 2 marks · State
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“State Newton's Third Law of motion.” |
HOW TO ANSWER IT Command word: State. Worth 2 marks, so plan before writing.
Question 1 · mark scheme
2 marks available. Award a mark for each point made.
▸ Forces are equal. 1 mark
▸ and opposite. 1 mark
▸ Model answer. Whenever two objects interact, the forces they exert on each other are equal and opposite.
Question 2 · 3 marks · Explain
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“A book rests on a table. The table pushes up on the book. State the force that forms a Newton's Third Law pair with this force, and the object it acts on.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 2 · mark scheme
3 marks available. Award a mark for each point made.
▸ Book pushes down. 1 mark
▸ On the table. 1 mark
▸ Equal in size and opposite in direction. 1 mark
▸ Model answer. The book pushes down on the table (a contact force). It acts on the table.
Question 3 · 3 marks · Explain
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“A swimmer pushes the water backwards and moves forwards. Explain why, using Newton's Third Law.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 3 · mark scheme
3 marks available. Award a mark for each point made.
▸ Swimmer pushes water backwards. 1 mark
▸ Water pushes swimmer forwards with an equal force. 1 mark
▸ Forces act on different objects or make the swimmer accelerate. 1 mark
▸ Model answer. The swimmer pushes the water backwards. The water exerts an equal and opposite force on the swimmer, forwards, so the swimmer moves forwards.
Question 4 · 3 marks · Explain
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“The Earth pulls the Moon with a gravitational force. A student says that the Moon pulls on the Earth with a much smaller force because the Moon is much less massive. Explain why the student is wrong.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 4 · mark scheme
3 marks available. Award a mark for each point made.
▸ Forces are equal. 1 mark
▸ Opposite in direction. 1 mark
▸ Newton's Third Law applies whatever the masses. 1 mark
▸ Model answer. The forces are equal and opposite (Newton's Third Law), however different the masses are.
Question 5 · 4 marks · Explain
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“A book of weight 10 N rests on a table. A student says the weight and the upward force from the table are a Newton's Third Law pair. Explain why the student is wrong. Name the force that does form a pair with the weight.” |
HOW TO ANSWER IT Command word: Explain. Worth 4 marks, so plan before writing.
Question 5 · mark scheme
4 marks available. Award a mark for each point made.
▸ Both forces act on the book. 1 mark
▸ Third Law pairs act on different objects. 1 mark
▸ Equal because of equilibrium. 1 mark
▸ Book pulls the Earth upwards. 1 mark
▸ Model answer. Both forces act on the same object (the book), but a Third-Law pair acts on different objects. They are equal because the book is in equilibrium. The pair for the weight (the Earth pulling the book down) is the book pulling the Earth up.