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Physics · Forces
Newton's Third Law
State and apply Newton's Third Law to pairs of interacting forces and to equilibrium situations.
Warm-up
Answer each one, then check.
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1
What is a force?
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A push or a pull
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2
What are balanced forces?
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Equal and opposite forces on the same object
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3
Give a contact force.
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Friction, tension or normal contact force
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4
What is a non-contact force?
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A force between objects that are not touching
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5
What is weight?
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The force of gravity on an object
Learning Objectives
- 1State Newton's Third Law.
- 2Identify pairs of forces between two interacting objects.
- 3Explain why interaction pairs do not cancel each other.
- 4Apply 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
Each force acts on a different object.
Finding an Interaction Pair
A book rests on a table. The table exerts an upward force on the book. What is the other force in this interaction pair?
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- 1 Type of force A contact (normal) force
- 2 Acts on The table
- 3 Direction Downwards
AnswerThe book pushes down on the table with an equal force.
Weight and Contact Force
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.
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- 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
AnswerNo. 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
A swimmer pushes backwards on the water. Explain why the swimmer moves forwards.
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- 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
AnswerThe water pushes back on the swimmer with an equal and opposite force.
Common Mistakes
Avoid these.
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Same object
Forces that cancel act on the same object; a Third-Law pair acts on different objects.
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Always equal
The forces in a pair are always equal, even if one object is much lighter.
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Same type
The pair is always the same type of force.
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Motion
Equal forces on different objects can give different accelerations (F = ma).
Find the Pair
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.
Can I...?
- 1State the third law.
- 2Name the object each force acts on.
- 3Identify a pair.
- 4Explain that pairs do not cancel.
- 5Apply it to a book on a table.
- 6Apply it to a swimmer.
- 7Apply it to gravity.
- 8Explain equilibrium.
Summary & Exam Focus
- 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.
Exam focus
State Newton's Third Law. (2 marks) (2 marks)
Forces equal in size and opposite in direction on different objects.
Key terms
The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.
- 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.
- Equilibrium
- When the forces on an object are balanced.
- Normal contact force
- The push from a surface on an object resting on it.
- Reaction force
- The force that forms a pair with another force.
- Different objects
- The two forces in a pair act on separate bodies.
Practice questions
Have a go at each one before you open its answer.
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Question 1 State 2 marks
State Newton's Third Law of motion.
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Model answer
Whenever two objects interact, the forces they exert on each other are equal and opposite.
Mark scheme
- Forces are equal — 1 mark
- and opposite — 1 mark
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Question 2 Explain 3 marks
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.
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Model answer
The book pushes down on the table (a contact force). It acts on the table.
Mark scheme
- Book pushes down — 1 mark
- On the table — 1 mark
- Equal in size and opposite in direction — 1 mark
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Question 3 Explain 3 marks
A swimmer pushes the water backwards and moves forwards. Explain why, using Newton's Third Law.
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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.
Mark scheme
- 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
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Question 4 Explain 3 marks
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.
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Model answer
The forces are equal and opposite (Newton's Third Law), however different the masses are.
Mark scheme
- Forces are equal — 1 mark
- Opposite in direction — 1 mark
- Newton's Third Law applies whatever the masses — 1 mark
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Question 5 Explain 4 marks
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.
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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.
Mark scheme
- 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
Quick check
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Newton's third law pairs are...
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A: equal and opposite
Equal in size, opposite in direction.
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The forces in a pair act on...
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B: different objects
Each force acts on the other object.
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A rocket moves up because the gas pushes...
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C: the rocket up
Equal and opposite force.
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If you push a wall, the wall pushes you...
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D: with an equal force
Third law.
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Interaction pairs are always...
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A: the same type of force
Both are contact, or both gravitational.
Downloads
Free to keep, print and annotate.
- Newtons Third Law.pptx Built from the lesson script on 30 September 2026. View
- Newtons Third Law - Completed Notes.docx The full notes for the lesson, to revise from. Built from the lesson script on 30 September 2026. View
- Newtons Third Law - Exam Questions.docx Built from the lesson script on 30 September 2026. View
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