GCSE Physics revision

Newton's Third Law

Forces, accelerations and Newton's Laws of motion

AQA 4.5.6.2.3

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AQA student objectives

Learning pathway

ALL 🎯

Apply Newton’s Third Law to examples of equilibrium situations.

MOST 🎯🎯

Interpret results or representations related to newton's third law.

SOME 🎯🎯🎯

Analyse an unfamiliar problem involving newton's third law and justify the method or conclusion.

Revision summary

Read on screen, then print for Cornell-style active revision.

When a hand pushes a box with a 100 N contact force, the box simultaneously pushes back on the hand with an equal 100 N force in the opposite direction.
This reaction force from the box is what stops the hand from passing through the box as it pushes against it.

When one person on roller skates pushes another, both people move apart, demonstrating that each person exerts a force on the other.
Roller skates are used in this demonstration to reduce the effects of friction, making the resulting motion clearly visible.
Both pushing forces are equal in magnitude and opposite in direction, providing a clear realworld demonstration of Newton's third law.

2. The Roller Skates DemonstrationAQA

When one person on roller skates pushes another, both people move apart, demonstrating that each person exerts a force on the other.
Roller skates are used in this demonstration to reduce the effects of friction, making the resulting motion clearly visible.
Both pushing forces are equal in magnitude and opposite in direction, providing a clear realworld demonstration of Newton's third law.

The two forces must act on separate bodies — they cannot both act on the same object.
Both forces must be of the same type, for example both contact forces or both gravitational forces.
The force vectors must act along the same line and in opposite directions to each other.
Both forces in the pair must have the same magnitude.
Do We Always Notice Both Forces?
Although equal forces act on both objects in an interaction, we may only observe a noticeable effect in one of them.
The most common example is the gravitational interaction between the Earth and a light object, such as a feather.

3. Properties of a Newton's Third Law Force PairAQA

The two forces must act on separate bodies — they cannot both act on the same object.
Both forces must be of the same type, for example both contact forces or both gravitational forces.
The force vectors must act along the same line and in opposite directions to each other.
Both forces in the pair must have the same magnitude.
Do We Always Notice Both Forces?
Although equal forces act on both objects in an interaction, we may only observe a noticeable effect in one of them.
The most common example is the gravitational interaction between the Earth and a light object, such as a feather.

By Newton's third law, just as the Earth pulls the feather towards it, the feather also pulls the Earth towards it with the same gravitational force.
Although both forces have the same magnitude, they act in opposite directions — the feather accelerates towards the Earth, while the Earth's acceleration is immeasurably small.
By Newton's second law, the enormous difference in mass between the Earth and the feather results in a huge difference in their accelerations, even though the force is the same.

4. Gravity: Earth and a FeatherAQA

By Newton's third law, just as the Earth pulls the feather towards it, the feather also pulls the Earth towards it with the same gravitational force.
Although both forces have the same magnitude, they act in opposite directions — the feather accelerates towards the Earth, while the Earth's acceleration is immeasurably small.
By Newton's second law, the enormous difference in mass between the Earth and the feather results in a huge difference in their accelerations, even though the force is the same.

To identify a valid Newton's third law force pair, check that the forces act on separate bodies, are the same type, act along the same line, are opposite in direction, and are equal in magnitude.
It is important to recognise Newton's third law force pairs even in situations where the effect on one object is not obvious, such as the Earth–feather example.

5. Identifying Newton's Third Law Force PairsAQA

To identify a valid Newton's third law force pair, check that the forces act on separate bodies, are the same type, act along the same line, are opposite in direction, and are equal in magnitude.
It is important to recognise Newton's third law force pairs even in situations where the effect on one object is not obvious, such as the Earth–feather example.