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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.
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?
- Sit on a wheeled chair.
- Push the table — you roll back.
- Pull it — you roll forward.
- The table pushes back on you.
- 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.
In this Activity, we will connect two spring balances and pull them to verify that the action and reaction forces are equal.
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?
- Join two spring balances.
- Pull and read both.
- Readings are equal.
- Forces are equal and opposite.
- 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.
- 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.
- 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.
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.
- Tape a balloon to a straw on a thread.
- Release the air.
- Balloon shoots the other way.
- A rocket works the same way.
- 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.
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 \( a = \dfrac{F}{m} \), 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.
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: \( a = \dfrac{2}{5} = 0.4\ \text{m s}^{-2} \). Bullet: \( a = \dfrac{2}{0.1} = 20\ \text{m s}^{-2} \).
The forces are equal, but the accelerations differ because the masses differ.
- 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.
- 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.
- 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.
- The forces on two interacting objects are always equal in magnitude.
- But they generally produce different accelerations, because the masses may differ.
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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.
- 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.
