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๐ฌ Think about it
Kick a ball and you feel it push back on your foot. Push a heavy table while sitting on a wheeled chair, and you roll backwards. Forces always come in pairs. Newton's third law captures this. Let us explore it.
In this Activity, we will push and pull a heavy table while sitting on a wheeled chair to feel the table pushing back.
Procedure
1. Sit on a wheeled chair with your legs raised, near a large heavy table.
2. Push the table away from you with both hands (Fig. 6.23a). What happens to your chair?
3. Now pull the table towards you (Fig. 6.23b). Which way does your chair move now?
2. Push the table away from you with both hands (Fig. 6.23a). What happens to your chair?
3. Now pull the table towards you (Fig. 6.23b). Which way does your chair move now?
Conclusion
Each time you push or pull the table, the table applies an equal force on you in the opposite direction, moving your chair the other way.
โ Summary
- Sit on a wheeled chair.
- Push the table — you roll back.
- Pull it — you roll forward.
- The table pushes back on you.
How do walking and cycling use this idea?
Why Walking and Cycling Work
Your foot pushes the ground
backwards
the action force
↓
The ground pushes your foot
forwards
the equal and opposite reaction — it is the force of friction
↓
You move forward
the same idea makes a cycle move when you push the ground
↓
Here friction
helps
you move — which is why wet floors and ice are slippery
- While walking, you push the ground backwards with your feet.
- The ground pushes your feet forward with an equal force (as friction).
- This forward force moves you ahead — so here friction helps you.
Example:
On a bicycle, pushing the ground backwards with your feet moves you and the bicycle forward.
In this Activity, we will connect two spring balances and pull them to verify that the action and reaction forces are equal.
Procedure
1. Take two identical spring balances.
2. Place them horizontally and connect them by their hooks (Fig. 6.26). Fix the free end of one.
3. Predict the readings if you pull the free end of the other while both are stationary.
4. Now pull, repeating with different forces. Is your observation the same as your prediction?
2. Place them horizontally and connect them by their hooks (Fig. 6.26). Fix the free end of one.
3. Predict the readings if you pull the free end of the other while both are stationary.
4. Now pull, repeating with different forces. Is your observation the same as your prediction?
Conclusion
The two readings are the same every time. This shows the forces the balances apply on each other are equal in magnitude and opposite in direction.
โ Summary
- Join two spring balances.
- Pull and read both.
- Readings are equal.
- Forces are equal and opposite.
What is Newton's third law of motion?
Newton's third law
Action
object A exerts a force on object B
Reaction
object B exerts an equal, opposite force on A
↓
Whenever one object is exerting a force on a second object, the second object is simultaneously exerting an equal and opposite force on the first object.
The forces are equal and opposite
But they do NOT cancel
Same magnitude, opposite direction
They act on
two different objects
Two spring balances hooked together read the same
The paddle pushes water; the water pushes the paddle
Only forces on the
same
object can balance
So a third-law pair never balances itself out
- Whenever one object exerts a force on a second object, the second exerts an equal and opposite force on the first.
- The two forces always occur in pairs.
- They act on two different objects, so they do not cancel each other.
Example:
A canoeist pushes water backwards; the water pushes the paddle and canoe forward.
๐งฉ Threads of Curiosity
- How hard one paddles is not the only thing that matters.
- Drag, water currents, the canoe's mass and the rowing style also affect its speed.
๐ Note
- The forces always occur in pairs, but the two forces act on two different objects.
In this Activity, we will release air from a balloon on a thread to see how a rocket moves by Newton's third law.
Procedure
1. Collect a large balloon, a drinking straw, adhesive tape, a long thread and two nails.
2. Inflate the balloon and tie its neck with a small thread.
3. Tape the straw along the balloon, one end pointing to the neck (Fig. 6.29).
4. Pass the thread through the straw and tie its ends to the nails, keeping it taut.
5. Release the thread on the neck and watch which way the straw and balloon move.
2. Inflate the balloon and tie its neck with a small thread.
3. Tape the straw along the balloon, one end pointing to the neck (Fig. 6.29).
4. Pass the thread through the straw and tie its ends to the nails, keeping it taut.
5. Release the thread on the neck and watch which way the straw and balloon move.
Conclusion
The balloon pushes air out; the escaping air pushes the balloon the opposite way. A rocket works the same way — its engine expels gas downward, and the gas pushes the rocket up.
โ Summary
- Tape a balloon to a straw on a thread.
- Release the air.
- Balloon shoots the other way.
- A rocket works the same way.
How does a rocket lift off?
How a Rocket Lifts Off
The engine burns fuel
hot gas is produced
↓
The rocket pushes the gas
down
the action force, out of the nozzle
↓
The gas pushes the rocket
up
an equal and opposite reaction force
↓
This upward force beats the rocket's weight
so the net force is upwards
↓
The rocket
lifts off
— the same idea slowed Chandrayaan-3's Vikram lander
- Its engine produces gas and expels it downward.
- The gas exerts an equal and opposite force on the rocket, upward.
- This upward force is larger than the rocket's weight, so the net force lifts it off.
Example:
The Vikram lander of Chandrayaan-3 fired its engine forward to slow down for a soft landing near the Moon's south pole.
โ Example 6.7 — As shown in Fig. 6.33,
The Earth and a fruit pull each other with equal and opposite gravitational forces (Fig. 6.33). Why does the fruit fall to the Earth, but the Earth doesn't visibly move?
The forces are equal, but using a = F m , the Earth's huge mass gives it an extremely small acceleration. So the Earth's movement is far too small to notice, while the light fruit accelerates clearly.
โ Example 6.8 — When a 0.1 kg bullet
A 0.1 kg bullet is fired from a 5 kg gun with a force of 2 N. Find the initial accelerations of the bullet and the gun.
By the third law, the recoil force on the gun is also 2 N.
Gun: a = 2 5 = 0.4 m s -2 . Bullet: a = 2 0.1 = 20 m s -2 .
The forces are equal, but the accelerations differ because the masses differ.
Important Points
- Newton's third law: every action has an equal and opposite reaction.
- The pair acts on two different objects, so they don't cancel.
- It applies to all forces — contact and non-contact.
๐ Bridging Science and Society
- When you push the ground backwards, friction pushes you forward.
- Without friction your foot would slip and you would fall.
- Grooves on soles and treads on tyres increase friction for grip.
- This is why it is hard to walk on wet polished floors or ice.
๐ Note
- The action-reaction pair acts on two different objects, so they do not balance.
- But two equal and opposite forces on the same object do balance.
๐ Note
- The forces on two interacting objects are always equal in magnitude.
- But they generally produce different accelerations, because the masses may differ.
โ Test Yourself
-
State Newton's third law.
View Answer
When one object exerts a force on a second, the second exerts an equal and opposite force on the first. -
Why don't action-reaction forces cancel?
View Answer
They act on two different objects. -
Why does a gun recoil less than a bullet accelerates?
View Answer
The gun has a much larger mass, so the same force gives it a smaller acceleration.
Important Definitions
- Newton's third law of motion — for every force one object exerts on another, the second exerts an equal and opposite force on the first.
๐ NCERT Question 7 — A sailor jumps out from
A sailor jumps out from a small boat to the shore (Fig. 6.38). As the sailor jumps forward, will the boat move? If yes, in which direction and why.
View Answer →
๐ NCERT Question 9 — A hand cart loaded with
A hand cart loaded with vegetables collides with an identical but empty hand cart. During the collision:
(i) the loaded cart exerts a force of larger magnitude on the empty cart.
(ii) the empty cart exerts a force of larger magnitude on the loaded cart.
(iii) neither cart exerts a force on the other.
(iv) the loaded cart and the empty cart, both exert an equal magnitude of force on each other.
View Answer →
(i) the loaded cart exerts a force of larger magnitude on the empty cart.
(ii) the empty cart exerts a force of larger magnitude on the loaded cart.
(iii) neither cart exerts a force on the other.
(iv) the loaded cart and the empty cart, both exert an equal magnitude of force on each other.
๐ NCERT Question 16 — When the pole of a
When the pole of a bar magnet is brought close to a magnetic compass, the bar magnet and the compass needle (which is also a magnet) exert a magnetic force on each other. As per Newton's third law of motion, both the forces are equal in magnitude and opposite in direction. However, the compass needle moves, whereas the bar magnet does not move (Fig. 6.42). Explain why.
View Answer →
๐งฌ Pause and Ponder
-
9. Why does a fireperson sometimes struggle when holding the pipe issuing water?
The hose pushes the water forward; by the third law the water pushes the hose backwards with an equal force, so it recoils. -
10. Suppose a spacecraft is moving in a region of space where the gravitational force acting upon it is negligible. Suggest how can it change its velocity.
Fire its engine to expel gas one way; the equal and opposite force on the craft changes its velocity.
