10 min readConcept sheetSection A MCQ (1 mark) and true or false (2 marks, two parts); Section B very short answer (2 marks) or short answer (3 marks)

Newton's third law: action and reaction

To every action there is an equal and opposite reaction. The two forces act on two different bodies, so they never cancel.

Third lawTo every action there is an equal and opposite reaction

Where they actAction and reaction act on two different bodies, at the same time

No contact neededThe bodies need not touch: two magnets push or pull each other across a gap

The law

Press a wall with your palm. You feel the wall pressing back on your palm. Forces always come in pairs. Your push on the wall is the action; the wall's push on you is the reaction. Newton's third law says: to every action there is an equal and opposite reaction. Two bodies and two forces are always involved.

Three features of action and reaction

They are equal in size and opposite in direction.
They act on two different bodies. That is why they do not cancel. Forces cancel only when they act on the same body.
They are simultaneous: the reaction lasts only as long as the action. Either force can be called the action.
The bodies need not touch. Two magnets repel each other across a gap.

Action and reaction around us

Walking: your foot pushes the ground backward (action). The ground pushes your foot forward (reaction), and you move ahead.
Swimming: the hands and feet push the water backward. The water pushes the swimmer forward.
Rockets and jet planes: they throw hot gases backward at high speed. The gases push them forward.
A released balloon: air rushes out one way, and the balloon moves the other way.
Jumping from a boat: your foot pushes the boat back, and the boat pushes you towards the bank.
A fire hose: the fast jet of water pushes the hose backward, so firefighters hold it firmly.

Why don't action and reaction cancel?

Two children on skates push each other. Each child receives one force, so both roll apart in opposite directions. The two forces would cancel only if both acted on the same child. So action and reaction can make bodies move, because each force acts on a different body.

Activity (practical): verify the third law with two spring balances

You need two identical spring balances (0 to 5 N), a wooden block with a hook, a pulley, a string, a hanger and 1 N slotted weights.

1
Note the least count of both spring balances and set their pointers to zero.
2
Fix a wooden block with a hook at one edge of the table. Fix a pulley at the opposite edge, in line with it.
3
Hook spring balance B to the block. Hook spring balance A to B, so that the two pull on each other.
4
Tie a string to A, pass it over the pulley and hang the hanger from it. Keep the string level with the table.
5
Read both balances. Then add 1 N weights one at a time, up to about 4 N more, reading both balances each time.
6★
Record the readings in a table. Observe: A and B show the same reading every time.
7
Conclude: A pulls B exactly as hard as B pulls A, in the opposite direction. This verifies the third law.

★ key step

How to draw the two spring balance set-up

Draw the table top as a long horizontal line.
At the left end, draw the fixed wooden block; label it block.
Draw spring balance B hooked to the block, then spring balance A hooked to B, all in one straight line.
Draw a string from A to a pulley at the right edge, then hanging down to a hanger with weights.
Add arrows: A pulls B to the right, B pulls A to the left. Label the readings F(A) and F(B).

In real life

ISRO's rockets rise because they throw hot gases downward at very high speed, and the gases push the rocket up. You walk to the bus stop the same way, by pushing the ground backward.

Exam angle

2 marks: “State Newton's third law of motion. Name the action and the reaction when a person walks on a road.” To every action there is an equal and opposite reaction, and the two act on different bodies. Action: the foot pushes the ground backward. Reaction: the ground pushes the foot forward, which moves the person ahead.

2 marks: “If action and reaction are always equal and opposite, how can anything move? Explain.” Action and reaction act on two different bodies. Each body moves under the forces acting on it alone. Forces cancel only when they act on the same body.

1 marks: “In the two spring balance practical, why do balances A and B always show the same reading?” A pulls B and B pulls A. These pulls are an action and reaction pair, so by the third law they are equal.

Remember it

Push the world and the world pushes back: equal, opposite, on the other body.