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AQA GCSE Physics · Topic 4.5 Forces · Combined and Triple
Edexcel International GCSE Physics · Section 1 Forces and motion · Physics only
Newton’s Third Law
Physics only Newton’s third law (1.29P) is Physics only — Double Award
students can skip this page.
In this lesson you'll learn to: state Newton’s third law; name every force as
“force on ___ due to ___” and find its partner; tell a third-law pair apart from
balanced forces; use W = m g to turn a mass-balance reading into a force; and explain why
equal forces give different masses different accelerations.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
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Start here — have a play
Don't read anything yet. Click the force of gravity on the person and find its partner. Then do the same for the normal contact force — and check whether gravity and normal contact are a pair. Next, switch on the current for the magnet on the balance, and push the two skateboarders apart.
Click a force arrow (or a button) to find the force it is paired with
two different objects
same type of force
same size
opposite directions
Switch the current on
The balance was zeroed with the magnets on it, before the current was switched on.
Where does the force come from? This is the motor effect, from electromagnetism: a wire carrying a current in a magnetic field feels a force. You’ll meet it in Magnetism and Electromagnetism. Here, you just need Newton’s third law.
Set it up
Newton’s third law: when two objects interact, they each experience the same size and same type of force, in opposite directions. Name every force as “force on ___ due to ___” and the pair is the one with the two names swapped.
Physics With Kate · physicswithkate.com — GCSE & International GCSE Physics
Every force has a partner: same size, same type, opposite direction, on the other object.
Force on A due to B = force on B due to A, the other way.
Name the force properly and its partner is just the two names swapped. The notes show you how to write it for the marks.
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Revision notes
Newton’s third law
⭐ Learn this word for word
When two objects interact, they each experience the same size and same type of force, in opposite directions.
Forces always come in pairs. You can’t push something without it pushing you back: lean on a wall and the wall pushes on you; kick a ball and the ball pushes on your foot.
Name the force, and the pair names itself
Describe every force as “force on ___ due to ___”. Its third-law partner is the same sentence with the two objects swapped:
| Force | Its third-law partner | Type |
|---|---|---|
| force of gravity on the person due to the Earth (down) | force of gravity on the Earth due to the person (up) | gravitational |
| normal contact force on the person due to the ground (up) | normal contact force on the ground due to the person (down) | normal contact |
| force on the swimmer due to the wall (away from the wall) | force on the wall due to the swimmer (towards the wall) | normal contact |
| force on the wire due to the magnets | force on the magnets due to the wire | magnetic |
The four-point check
Two forces are a Newton’s third law pair only if all four are true:
- they act on two different objects — so they never cancel out
- they are the same type of force — both contact, both gravitational, or both magnetic
- they are the same size
- they act in opposite directions
⚠ Trap — balanced forces are not a third-law pair
A book resting on a table has its weight pulling down and the normal contact force from the table pushing up. They are equal and opposite, but they are not a third-law pair: they both act on the same object (the book) and they are different types. They are balanced forces — that’s Newton’s first law.
Measuring the pair: a magnet on a balance
A wire carrying a current sits between the poles of two magnets, and the magnets sit on a top-pan balance (zeroed before the current is switched on). When the current flows, the balance reading goes up, so the magnets are being pushed down. By Newton’s third law, the wire is pushed up with a force of the same size. You meet this again in electromagnetism (the motor effect).
The balance reads in mass, so turn it into a force with W = m g: convert to kg first (1 g = 1000 mg, 1 kg = 1000 g).
Equal forces, different accelerations
When two skateboarders push apart, the forces on them are equal and opposite: FA = −FB. Their accelerations are not equal unless their masses are, because a = F ÷ m: the smaller mass gets the larger acceleration. It’s the same reason the Earth doesn’t move noticeably when you jump: the force is the same, but the Earth’s mass is enormous.
The sentence that gets the marks
The ___ exerts a force on the ___, so the
___ exerts a force on the ___ that is
the same size and the same type, in the
opposite direction (Newton’s third law).
the two objects, swapped same size, same type opposite direction
🗣 Words to use in a written answer
Writing an “explain” answer? Tick each one off as you use it.
How to write it in the exam
A swimmer pushes on the wall of the pool with a force of 200 N. State the size and direction of the force the wall exerts on the swimmer. (2 marks)
200 N, away from the wall — the same size, in the opposite direction (Newton’s third law).
Two students stand on skateboards, facing each other. A has a mass of 45 kg and B has a mass of 60 kg. A pushes B with a force of 90 N. (a) State the size and direction of the force B exerts on A. (b) Calculate the acceleration of each student. (c) Explain why A speeds up more than B.
(a) 90 N, in the opposite direction (pushing A to the left). (b) a = F ÷ m: A: 90 ÷ 45 = 2.0 m/s²; B: 90 ÷ 60 = 1.5 m/s². (c) The two forces are equal in size, but A has the smaller mass. Since a = F ÷ m, the smaller mass gives the larger acceleration.
A stiff copper wire is clamped between the poles of two magnets. The magnets sit on a top-pan balance, zeroed before the current is switched on. With the current on, the balance reads 36 mg. (a) In which direction is the force on the wire? Explain. (b) Calculate the force on the magnets (g = 10 N/kg). (c) State the size of the force on the wire.
(a) The reading increases, so the magnets are pushed down. By Newton’s third law the wire is pushed up with an equal force. (b) 36 mg = 36 ÷ 1 000 000 = 0.000 036 kg; W = m g = 0.000 036 × 10 = 0.000 36 N (3.6 × 10⁻⁴ N). (c) 0.000 36 N — the same size, because the two forces are a Newton’s third law pair.
A book rests on a table. Explain why the weight of the book and the normal contact force from the table are not a Newton’s third law pair. (2 marks)
They both act on the same object (the book), and they are different types of force (gravitational and normal contact). They are balanced forces (Newton’s first law). The weight’s partner is the book pulling the Earth up.
✅ Check your answer
Both objects named (“on ___ due to ___”) · the size given and the direction stated as “opposite” · the type of force named · mg converted to kg before W = m g · for different accelerations: equal forces, so the smaller mass accelerates more (a = F ÷ m).
📘 Now do it in your workbook
Complete the statement, the falling ball and your own example, the magnet on the balance, the skateboard question, and the “tick every correct pair” checklist.
Edexcel iGCSE Forces and Motion Workbook · page 15 AQA GCSE Physics Workbook · FORCES · page 15✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this topic.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
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Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the notes and the free workbook page above cover every mark.
Now put it into practice
Page 15 of the Forces and Motion workbook is free: the statement, examples, the magnet on the balance and the skateboard question.
Page 15 of the Forces workbook is free: the statement, examples, the magnet on the balance and the skateboard question.
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