AQA GCSE Combined Science (8464), Higher tier · Physics › Forces › Forces and motion › Forces, accelerations and Newton's Laws of motion
Practise Newton's Third Law. 11 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.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
4
State Newton's Third LawWhenever two objects interact, the forces they exert on each other are equal and opposite.
5
Identify the partner force in a Third Law pairIf a boy pushes a wall with 150 N, the wall pushes the boy with 150 N in the opposite direction.
6
Describe the features of a Third Law pairSame size, opposite directions, same type of force, acting on two different objects.
6
Explain motion using the Third LawA swimmer pushes water backwards; the water pushes the swimmer forwards.
7
Distinguish Third Law pairs from balanced forcesBalanced forces act on the same object; a Third Law pair acts on two different objects.
7
Apply the Third Law to equilibrium situationsA book on a table pushes down on the table, and the table pushes up on the book with an equal force.
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.
Equilibrium: do not mix up the pairs grade 7+
A book resting on a table is in equilibrium. Its weight (down) and the normal contact force from the table (up) are balanced. Both act on the book, so they are not a Third Law pair.
The Third Law partner of the book's weight is the gravitational pull of the book on the Earth, upwards.
The Third Law partner of the table's push up on the book is the book's push down on the table.
Cheatsheet
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
How to answer each type of question
Give the partner force
2 marks5
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 answer
150 N (1) away from the wall, in the opposite direction to the boy's push (1)
Explain motion using the Third Law
3 marks6
Say what the object pushes on, and in which direction.
Say that the other object pushes back with an equal force.
Say that this force is in the opposite direction and moves the object.
Example. Explain how a swimmer uses Newton's Third Law to move forwards.
Show the model answer
The swimmer's hands and feet push the water backwards (1). The water exerts an equal force on the swimmer (1) in the opposite direction, pushing the swimmer forwards (1).
Explain why two forces are not a Third Law pair
2 to 3 marks7
Check which object each force acts on.
Check whether the two forces are the same type.
Say what the forces actually are (e.g. balanced forces on one object).
Example. A lamp hangs at rest from a ceiling on a cable. A student says that the weight of the lamp and the tension in the cable are a Newton's Third Law pair, because they are equal and opposite. Explain why the student is wrong.
Show the model answer
Both forces act on the same object, the lamp (1). A Third Law pair acts on two different objects (1) and is the same type of force, but weight is gravitational and tension is a contact force (1).
Identify the forces in an equilibrium situation
3 to 4 marks7
Balancing force: a force on the same object, opposite to the one given.
Third Law partner: the same type of force, acting on the other object.
Give the size and direction.
Example. A book rests on a table. (a) Name the force that balances the weight of the book. (b) Describe the force that forms a Newton's Third Law pair with the weight of the book.
Show the model answer
(a) The normal contact force of the table on the book (1). (b) The gravitational force of the book on the Earth (1) equal in size to the book's weight (1) acting upwards (1).
Shortcuts and 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.
Where marks are lost
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.
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.
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 rope exerts on team A. Name the law that explains this.
1800 N towards team B (Newton’s Third Law).
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
Question 3Hard7 marks
Two teams, A and B, take part in a tug of war. At first the rope is stationary. Team A pulls on the rope with a force of 1800 N.
(a) What force does team B exert on the rope? Give a reason for your answer.[2]
(b) Give the size and direction of the force that the rope exerts on team A. Name the law that explains this.[2]
(c) Team A starts to pull team B towards them. A student says: ‘Team A must now be pulling on the rope harder than the rope pulls on team A.’ Explain why the student is wrong and what makes team A win.[3]
Show the answer and mark scheme
(a)Answer: 1800 N, because the rope is in equilibrium.
1800 N (in the opposite direction)
the rope is stationary / in equilibrium, so the resultant force on it is zero
(b)Answer: 1800 N towards team B (Newton’s Third Law).
1800 N towards team B
Newton’s Third Law
(c)Answer: The rope and team A always exert equal and opposite forces; team A wins because the ground pushes team A with a larger friction force, while team B’s friction is less than the rope’s pull.
the force of team A on the rope is always equal to the force of the rope on team A (Newton’s Third Law)
team A pushes harder (backwards) on the ground, so the ground exerts a larger (friction) force on team A
the friction force on team B is less than the pull of the rope on team B, so there is a resultant force on team B towards team A