Newton's Third Law

This lesson covers Newton's third law as required by AQA GCSE Combined Science Trilogy (8464), section 6.5.3. This law explains why forces always occur in pairs and is frequently tested in the exam, particularly through questions about everyday situations.

Statement of Newton's Third Law

Newton's third law: Whenever two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction.

For every action, there is an equal and opposite reaction.

Key Features of Third-Law Force Pairs

A Newton's third law pair must satisfy all of the following conditions:

Feature	Detail
Equal in magnitude	Both forces have the same size
Opposite in direction	The forces act in opposite directions
Same type	Both forces are the same kind (both gravitational, both contact, both electrostatic, etc.)
Act on different objects	One force acts on object A, the other on object B
Act simultaneously	Both forces exist at the same time

Exam Tip (AQA 8464): The most common mistake is identifying two forces acting on the same object as a Newton's third law pair. For example, the weight of a book and the normal contact force from the table act on the same object (the book), so they are NOT a third-law pair — they are simply balanced forces.

Examples of Newton's Third Law Pairs

Example 1 — Person Standing on the Ground

The person pushes down on the ground with a force (due to their weight).
The ground pushes up on the person with an equal force (normal contact force).

These forces are equal in magnitude, opposite in direction, the same type (contact), and act on different objects (person and ground).

Example 2 — Earth and Moon

The Earth pulls the Moon towards it with a gravitational force.
The Moon pulls the Earth towards it with an equal gravitational force.

Both forces are gravitational, equal in magnitude, opposite in direction, and act on different objects.

Example 3 — Swimmer

The swimmer pushes the water backwards with their hands.
The water pushes the swimmer forwards with an equal force.

This is why the swimmer moves forward.

Example 4 — Rocket

The rocket pushes exhaust gases downwards.
The exhaust gases push the rocket upwards with an equal force.

flowchart LR
    subgraph "Newton’s Third Law — Rocket"
        R["Rocket pushes gases DOWN"] <-->|"Equal and opposite"| G["Gases push rocket UP"]
    end

    style R fill:#e74c3c,color:#fff
    style G fill:#27ae60,color:#fff

Third Law Pairs vs Balanced Forces — A Common Confusion

This is the single most tested misconception in AQA exams on Newton's third law.

Feature	Newton's Third Law Pair	Balanced Forces
Number of objects	Two different objects	One object
Type of force	Same type	Can be different types
Example	Book pushes table down; table pushes book up	Weight of book (gravity) and normal force on book (contact)
Result	Neither force is "cancelled" — they act on different objects	Forces cancel out; resultant = 0

Worked Example — Identifying the Pair

A book rests on a table. The weight of the book is 10 N downwards.

(a) What is the normal contact force on the book?

The normal contact force = 10 N upwards (balanced forces on the book).

(b) Identify the Newton's third law pair for the weight of the book.

The weight of the book is the gravitational pull of the Earth on the book (10 N downwards). The third-law pair is the gravitational pull of the book on the Earth (10 N upwards). These are equal, opposite, the same type (gravitational), and act on different objects (book and Earth).

They act on the same object (the book) and are different types of force (gravitational vs contact). A true third-law pair must act on different objects and be the same type.