medium difficulty4 min

Newton's Third Law

Newton's third law states that for every action, there is an equal and opposite reaction acting on a different body.

In one line: Third law pairs are equal, opposite, and on different bodies.

Understand it

Action and reaction forces are equal in magnitude and opposite in direction, but they act on two different objects. Therefore, they do not cancel each other on the same body.

Example

When a swimmer pushes water backward, the water pushes the swimmer forward with an equal opposite force.

Rule to remember

Action force and reaction force are equal in magnitude, opposite in direction, simultaneous, and act on different bodies.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1A person pushes a wall.
2The person applies force on the wall, and the wall applies an equal opposite force on the person.

Method you can reuse

Identify body A and body B, write force by A on B, then write equal opposite force by B on A.

Sketch it in your answer

Draw two interacting bodies with equal opposite arrows, each arrow labelled on the body receiving the force.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why do action and reaction forces not cancel each other?

Think first, then show answer

They do not cancel because action and reaction forces act on different bodies, not on the same body.

Common wrong answer

Writing both forces on the same object.

Why it loses marks: Third-law pairs always act on different objects.

Don't mix this up

Students often say action and reaction cancel, but they act on different bodies, so they cannot cancel on one body.

Exam tip

Always name both bodies in the action-reaction pair.