Albert Teen YOU ARE LEARNING:  Applying Newton's Third Law  # Applying Newton's Third Law ### Applying Newton's Third Law

We will use examples where you can apply Newton's Third Law.

Whenever two objects interact, they exert a force on each other which is... 1

The image shows a balloon which has been filled with air and then released. What does the action force act on?  2

What does the reaction force act on?  3

This explains why the balloon flies when it is released. The reaction force pushes on the balloon.

The same principle applies to rockets, aeroplanes and many other things we do in everyday life. According to Newton's third law, whenever two objects interact they exert equal and opposite forces on each other. We can apply Newton's third law to real life situations to explain the effect of interaction pairs of forces.

According to Newton's third law, for every action force there is an equal and opposite reaction force. If a bird pushes down on the air with its wings, what is the effect of the reaction force? 1

The image shows a boat pulling a person on water skis. The arrow represents the force produced from the boat's propeller, acting on the water. Which best explains how this force causes the boat to move?  If you push against a wall, and the wall exerts a force back at you, why don't you or the wall move?

1

If you exert a force of $5\space N$ on the wall, what is the magnitude of the force the wall exerts on you? 2

The wall has a lot of mass and is connected to the ground so has a lot of resistance to motion. Therefore it does not move. You have less mass - so why don't you move? 3

Now imagine that you push against the wall with the same force, but this time whilst standing on a skateboard. Will you accelerate? Answer yes or no. 4

You would accelerate if you were on a skateboard because the wall exerts a force on you, and the wheels of the board reduce the friction between you and the ground.

This demonstrates Newton's third law.

Imagine two ice skaters. The first skater pushes on the second, exerting a force. Which statement is correct? When the first ice skater pushes on the second, they both have a force exerted on them of equal magnitudes. How does this affect their motion? When the first ice skater pushes on the second, they both have a force exerted on them in opposite directions, and of equal magnitudes. They accelerate away from each other, but one accelerates more than the other. How might you explain this? Newton's third law of motion can sometimes be confusing because although the same magnitude of force is exerted on two objects that interact, the acceleration that it causes also depends on the mass of an object. An object with less mass will accelerate more than an object with more mass, when the same magnitude of force is applied.

When a cannon is fired,the ball accelerates out of the cannon at great speed. The cannon recoils backwards at a much lower rate of acceleration. Which explains why the accelerations of each object are different? Newton's third law can also be applied to non-contact forces. If the Earth's gravitational force pulls on the moon, does the moon's gravitational force pull on the Earth? Which is stronger, the Earth's pull on the moon, or the moon's pull on the Earth? 