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Newton's Third Law

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What is Newton's third law?

Every push or pull is one half of a deal: when object A acts on object B, object B acts back on A. Newton's third law states that the two forces are equal in size and opposite in direction. This lesson builds the statement, tests it on walking, and separates it from the laws describing single objects.

What you know already

The Momentum lessons have already shown you what momentum is, how collisions conserve it, and how F = ma ties force to changing motion. One question remains: when two bodies interact, how do the forces they feel compare? Newton's third law answers with one symmetric statement about interacting objects, and this note also trains you to tell true third law pairs apart from merely balanced forces.

Key points of understanding

Newton's third law describes the forces that appear when two different objects interact with each other.
Newton's first and second laws instead cover forces acting on just one object.
When any two objects interact, each exerts on the other a force equal in size and opposite in direction.
Walking works because your foot pushes backwards on the ground while the ground pushes forwards on your foot.
The two interacting objects here are the foot and the ground, pushing on each other with equal strength.

Newton Third law in action, downloadable IGCSE & GCSE Physics revision notes — The foot pushes the ground backwards, and the ground pushes the foot forwards. Newton's third law explains the forces that enable people to walk

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1.What is Newton's third law?

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Key points

Newton's third law describes the forces that appear when two different objects interact with each other.
Newton's first and second laws instead cover forces acting on just one object.
When any two objects interact, each exerts on the other a force equal in size and opposite in direction.
Walking works because your foot pushes backwards on the ground while the ground pushes forwards on your foot.
The two interacting objects here are the foot and the ground, pushing on each other with equal strength.

First pass

Push on something and it pushes back on you just as hard, the other way.

Analogy — It is like leaning on a wall: your lean is matched, newton for newton, by the wall pushing back on you.

Exam pointer — Name both objects when describing a third law pair — 'A pushes B, B pushes A' earns the marking points.

Going deeper

The law pairs forces, not objects: each body feels one force of the pair, equal in size, opposite in direction.

Analogy — Picture a mirrored handshake: whatever force one hand delivers, the other hand delivers straight back, flipped.

Exam pointer — The pair acts on two different objects — claiming both forces act on one object is the classic marking trap.

Checks

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1.Newton's third law of motion describes:
2.Two objects interact. Compared with the force the first exerts on the second, the force the second exerts on the first is:
3.During walking, the force that pushes you forwards is:
4.Newton's first law differs from his third because the first concerns:
5.A foot pushes backwards on the ground. The ground's push on the foot acts on:
6.State Newton's third law of motion.
Written response · recall
Your answer
7.Which two objects interact to make walking possible, and what does each one do?
Written response · recall
Your answer
8.Explain why pushing backwards on the ground moves you forwards.
Written response · explain why
Your answer
9.A swimmer pushes the water backwards. Describe the partner force and its effect on the swimmer.
Written response · application
Your answer

2.Recognising Newton's third law

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Equal and opposite forces appear in two very different situations: balanced forces on one object, and third law pairs across two objects. Confusing them is one of the most common exam errors. This lesson gives you a three-part test, then works a full example with a book resting on a table.

WE Newton Third law Question image, downloadable IGCSE & GCSE Physics revision notes

WE Newton Third law Answer image, downloadable IGCSE & GCSE Physics revision notes

Key points

A third law pair acts on two different objects, one force on each body.
The pair's two forces are always equal in size and opposite in direction.
Both forces in the pair must be the same type, such as two weights or two contact pushes.
If naming a force proves difficult, describe it simply as a push or a pull.
A book resting on a table feels its weight and the table's contact push, and both act on the book alone.
Those two book forces are balanced, so they illustrate Newton's first law rather than the third.

First pass

Run the three checks: different objects, equal-and-opposite, and the same force type.

Analogy — Matching a third law pair is like pairing socks: same kind, equal size, opposite feet.

Exam pointer — Always ask which two objects feel the forces — one body only means balanced forces, not a pair.

Going deeper

Balanced forces mimic a third law pair because both are equal and opposite; the object count and the force type separate them.

Analogy — Two bookends bracing one book versus two people leaning back-to-back: one body shares the forces, the other splits them.

Exam pointer — The book-on-table diagram is Newton's first law in disguise — expect an exam question asking exactly that.

Checks

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1.For two forces to qualify as a third law pair, they must act on:
2.Besides being equal and opposite, the two forces of a third law pair must also be:
3.A book rests on a table. Its weight and the table's upward push on it:
4.Which force is the true third law partner of the book's weight?
5.Equal, opposite forces that both act on one object demonstrate:
6.List the three checks that identify a third law pair.
Written response · recall
Your answer
7.In the book-on-table example, Student A claims the diagram shows Newton's third law. Who is correct, and why?
Written response · recall
Your answer
8.Explain why the weight and the table's push on a resting book are not a third law pair.
Written response · explain why
Your answer
9.A pupil argues: 'The Earth pulls you downwards, so the ground pushing you upwards is Newton's third law.' Give the true partner of your weight to correct them.
Written response · application
Your answer

3.Newton's third law in collisions

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Collisions make Newton's third law vivid: during the crash, each trolley pushes the other with an equal, opposite force. Yet the two vehicles do not, in general, accelerate equally, because F = ma brings mass into the outcome. This lesson follows a collision from force pair to momentum change to acceleration.

Newton’s-Third-Law-of-Motion, IGCSE & GCSE Physics revision notes — The force of Trolley A on Trolley B is equal and opposite to the force of Trolley B on Trolley A

Worked example - Newton's third law pairs, downloadable AS & A Level Physics revision notes

Key points

When two objects collide, each exerts on the other an equal force in the opposite direction.
In a collision one object generally gains momentum and speeds up while the other loses momentum and slows down.
For trolleys A and B the pair is written F(B–A) = −F(A–B): same size, opposite signs.
Equal forces do not ensure equal accelerations, because F = ma makes acceleration depend on mass as well.
Equal masses give equal accelerations; unequal masses give unequal accelerations.

First pass

During any collision, each body pushes the other equally hard in the opposite direction.

Analogy — Two skaters shoving off each other feel the same shove, yet the lighter one flies away faster.

Exam pointer — Equal forces never mean equal accelerations — the lighter body accelerates more.

Going deeper

The third law guarantees the force pair in every collision; F = ma then sets each body's own acceleration from its own mass.

Analogy — Same punch, two boxers: the force pair is symmetric, but how each mass responds to its copy of the force is not.

Exam pointer — Pair 'equal and opposite forces' with 'unequal accelerations' and justify with F = ma — a favourite exam combination.

Checks

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1.Trolleys A and B collide. The force of A on B, compared with the force of B on A, is:
2.When two trolleys collide, one gains momentum and speeds up; the other usually:
3.The relationship between the two collision forces is written:
4.Two trolleys of equal mass collide. During the impact their accelerations are:
5.A heavy trolley and a light trolley collide. During the impact:
6.State what Newton's third law says about the forces when two trolleys collide.
Written response · recall
Your answer
7.What decides each trolley's acceleration in a collision, and when are two accelerations equal?
Written response · recall
Your answer
8.Explain why colliding trolleys feel equal forces yet can accelerate unequally.
Written response · explain why
Your answer
9.Trolley A (2 kg) collides with trolley B (1 kg). At one instant the force of A on B is 6 N. State the force of B on A, and calculate each trolley's acceleration at that instant.
Written response · application
Your answer

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What is Newton's third law?

1.What is Newton's third law?

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3.Newton's third law in collisions

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