Newton's Third Law

This lesson covers action–reaction pairs, how they involve the same type of force on different objects, and common examples, as required by the Edexcel GCSE Combined Science specification (1SC0). Newton's Third Law is one of the most frequently misunderstood ideas in physics, so understanding it clearly will give you an advantage in the exam.

Newton's Third Law

Newton's Third Law states: When two objects interact, the forces they exert on each other are equal in magnitude, opposite in direction, and of the same type.

This is sometimes summarised as: "Every action has an equal and opposite reaction." However, this short version can be misleading, so let us look at the full meaning.

Key Features of Third Law Pairs

Every Newton's Third Law pair has four essential features:

Feature	Meaning
Equal in magnitude	Both forces have the same size
Opposite in direction	The forces point in opposite directions
Same type of force	Both are the same kind (e.g. both gravitational, both contact)
Act on different objects	One force acts on object A, the other on object B

Exam Tip: The most common mistake is to identify two forces acting on the SAME object as a Third Law pair. For example, the weight of a book (downward) and the normal force of the table (upward) act on the same object — the book — so they are NOT a Third Law pair. They happen to be equal when forces are balanced, but that is Newton's First Law, not the Third.

Examples of Newton's Third Law

Example 1: A Person Standing on the Floor

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

These are the same type of force (contact forces), equal in size, opposite in direction, and act on different objects (person and floor).

Example 2: A Swimmer Pushing Off a Wall

The swimmer pushes the wall backward with a contact force.
The wall pushes the swimmer forward with an equal contact force.

This is why the swimmer accelerates away from the wall — the wall exerts a force on them.

Example 3: Earth and the Moon (Gravitational)

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

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

Example 4: A Rocket

The rocket pushes exhaust gases downward.
The exhaust gases push the rocket upward with an equal force.

graph TB
    R["Rocket"] -- "Force on gases (downward)" --> G["Exhaust Gases"]
    G -- "Force on rocket (upward)" --> R

Why Third Law Pairs Do Not Cancel Out

A very common question is: "If the forces are equal and opposite, why don't they cancel out?"

The answer is that Third Law pairs act on different objects. Forces can only cancel out (give zero resultant) if they act on the same object.

Example

When you push a wall:

You exert a force on the wall.
The wall exerts an equal and opposite force on you.

These forces are on different objects, so they cannot cancel. The force on you may cause you to move backward (if on a slippery floor), and the force on the wall may cause it to flex slightly.

Exam Tip: If a question asks you to identify a Newton's Third Law pair, always state: (1) the two objects involved, (2) the direction of each force, and (3) the type of force. This structured answer earns full marks.

Identifying Newton's Third Law Pairs

Worked Example

A book rests on a table. Identify one Newton's Third Law pair involving the book.

The weight of the book is the gravitational pull of the Earth on the book (downward).

The Third Law pair is: the book pulls the Earth upward with a gravitational force of the same magnitude.

Note: the normal force of the table on the book is not the Third Law partner of the weight. The normal force is a contact force, while weight is a gravitational force — they are different types.

How to Find the Third Law Partner

Identify the force and the two objects involved (e.g. "Earth pulls book").
Swap the objects and reverse the direction: "Book pulls Earth" — upward, with the same magnitude.
Confirm the force type is the same (both gravitational in this case).

Common Misconceptions

Misconception	Correction
Weight of book and normal force from table are a Third Law pair	They act on the SAME object (the book), so they are NOT a Third Law pair
Third Law forces cancel out	They act on DIFFERENT objects, so they cannot cancel
Third Law only applies when objects are stationary	It applies at all times, whether objects are stationary, accelerating or at constant velocity
The "reaction" force only exists when an object is at rest	The pair always exists whenever two objects interact

Combining with Newton's First and Second Laws

Understanding how all three laws work together is key:

First Law: tells us what happens when the resultant force on a single object is zero.
Second Law: tells us how the acceleration of an object relates to the resultant force and mass.
Third Law: tells us about the pair of forces that arise whenever two objects interact.

Worked Example

A 60 kg ice skater pushes off a 90 kg ice skater. The push force is 180 N.

By Newton's Third Law, both skaters experience a force of 180 N (in opposite directions).

For the 60 kg skater: $a = \frac{F}{m} = \frac{180}{60} = 3.0 \text{ m/s}^2$

For the 90 kg skater: $a = \frac{F}{m} = \frac{180}{90} = 2.0 \text{ m/s}^2$

The lighter skater accelerates more because they have less mass (Newton's Second Law), even though the force on each is the same (Newton's Third Law).

Newton's Third Law

Newton's Third Law