AQA GCSE Physics Foundation (8463), Foundation tier · Forces › Forces and motion › Newton's laws of motion
Practise Newton's Third Law. 6 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Newton's Third Law says that whenever two objects interact, the forces they exert on each other are equal and opposite. You need to identify these pairs of forces and not confuse them with balanced forces on a single object, especially in equilibrium situations.
Key facts
Third Law: interacting objects exert equal and opposite forces on each other
Third Law pair: same size, opposite directions, same type of force, different objects
Third Law pairs never cancel, because they act on different objects
Balanced forces act on the SAME object
Book on a table: weight of book ↔ book pulls Earth up; table pushes book up ↔ book pushes table down
Notes
The law
Whenever two objects interact, the forces they exert on each other are equal and opposite.
If object A exerts a force on object B, then object B exerts a force of the same size on object A, in the opposite direction.
Features of a Third Law pair
The two forces are the same size.
They act in opposite directions.
They are the same type of force (both gravitational, both normal contact forces, both frictional, and so on).
They act on different objects, so they never cancel each other out.
Examples
Walking: your foot pushes backwards on the ground; the ground pushes forwards on your foot (both are friction forces).
Rocket: the engine pushes exhaust gas backwards; the gas pushes the rocket forwards.
Swimming: your hands push the water backwards; the water pushes you forwards.
Gravity: the Earth pulls an apple down with a force equal to the apple's weight; the apple pulls the Earth up with a force of the same size.
How to answer each type of question
Give the partner force
2 marksGrade 5
Swap the two objects: 'A on B' becomes 'B on A'.
Give the same size and the opposite direction.
Example. A boy pushes on a wall with a force of 150 N. State the size and direction of the force that the wall exerts on the boy.
Show the model answerHide the model answer
150 N (1) away from the wall, in the opposite direction to the boy's push (1)
Don’t lose marks
Saying that the weight of a resting object and the normal contact force on it are a Third Law pair.
Thinking that Third Law pairs cancel, so nothing can move. They act on different objects, so they do not cancel.
Saying that in a collision the heavier or faster object exerts a bigger force. The forces on the two objects are always equal in size.
Giving the partner force in the same direction as the original force. It is always opposite.
More tips
Memory tricks
Third Law pairs: 'A on B' and 'B on A'. Just swap the names of the objects.
If both forces are drawn on the same object, they are not a Third Law pair.
Pair checklist: same size, same type, opposite directions, different objects.
Exam technique
When describing a Third Law pair, name both objects in each force: 'the force of the Earth on the ball' and 'the force of the ball on the Earth'.
Give both the size and the direction of the partner force.
Use the phrases 'equal and opposite' and 'act on different objects' in explanations.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
State Newton's Third LawWhenever two objects interact, the forces they exert on each other are equal and opposite.
Grade 5
Identify Third Law partner forcesIf a boy pushes a wall with 150 N, the wall pushes the boy with 150 N in the opposite direction.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
State Newton’s Third Law.
When two objects interact, the forces they exert on each other are equal and opposite.
Give the size and direction of the force that the exhaust gases exert on the engines.
45 kN forwards.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) State Newton’s Third Law.[2]
(b) A swimmer pushes backwards on the water with a force of 150 N. Give the size and direction of the force that the water exerts on the swimmer.[2]
Show the answer and mark scheme
(a)Answer: When two objects interact, the forces they exert on each other are equal and opposite.
when two objects interact, they exert forces on each other
that are equal (in size) and opposite (in direction)
(b)Answer: 150 N forwards.
150 (N)
forwards
Question 2Medium5 marks
A book rests on a table. The weight of the book is 8.0 N.
(a) The weight of the book is the gravitational force of the Earth on the book. Describe the force that forms a Newton’s Third Law pair with the weight of the book.[3]
(b) The table exerts a normal contact force of 8.0 N upwards on the book. A student says: ‘The weight of the book and the normal contact force are a Newton’s Third Law pair.’ Explain why the student is wrong.[2]
Show the answer and mark scheme
(a)Answer: The gravitational force of the book on the Earth: 8.0 N, upwards.
the gravitational force of the book on the Earth
8.0 N
upwards (towards the book)
(b)Answer: Both forces act on the book (a third-law pair acts on two different objects), and they are different types of force.
both forces act on the same object (the book), but the forces in a Newton’s Third Law pair act on different objects
they are different types of force (gravitational and contact) / the partner of the normal contact force is the downward push of the book on the table