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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.

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

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

  2. 2

    Then show how to use it

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

  3. 3

    Add one concrete example

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

  4. 4

    Avoid this incomplete answer

    Writing both forces on the same object is wrong because third-law pairs always act on different objects.

Definition

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

Memory hook

Third law pairs are equal, opposite, and on different bodies.

Examples and method

Worked example

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

Method to apply

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

Diagram support

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

How CBSE asks it

Exams ask rocket motion, walking, swimming, gun recoil, or why equal opposite forces do not cancel.

Avoid common mistakes

Common confusion

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

Common wrong answer

Writing both forces on the same object is wrong because third-law pairs always act on different objects.

Exam tip

Always name both bodies in the action-reaction pair.

Quick check

Why do action and reaction forces not cancel each other?

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

Study the newton's third law diagram carefully

Use the labelled diagram to keep newton's third law clear in short answers and revision.

What this diagram makes clear

This diagram keeps the labels and direction of newton's third law in the right order.

Where this helps in exams

Use this for labelled diagram work and short exam answers on newton's third law.

Revision cue

Revise newton's third law through the labels before writing the answer.

Answer writing and exam use

1-mark answer

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

2-mark answer

Newton's third law states that for every action, there is an equal and opposite reaction acting on a different body. Action force and reaction force are equal in magnitude, opposite in direction, simultaneous, and act on different bodies. When a swimmer pushes water backward, the water pushes the swimmer forward with an equal opposite force.

3-mark answer

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. Action force and reaction force are equal in magnitude, opposite in direction, simultaneous, and act on different bodies. A person pushes a wall. The person applies force on the wall, and the wall applies an equal opposite force on the person. Exams ask rocket motion, walking, swimming, gun recoil, or why equal opposite forces do not cancel. Writing both forces on the same object is wrong because third-law pairs always act on different objects.
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