1. Newton’s 3rd Law
Action and Reaction

2. This picture shows a stake being pounded into the ground by a hammer
This picture shows a stake being pounded into the ground by a hammer. Which object is exerting the most force – the hammer or the stake? Why?

3. The stake and the hammer exert the same amount of force on each other.
How can this be?

4. Newton’s 3rd Law Sometimes referred to as the action-reaction law.
It states:
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

5. Forces
Are not a thing by themselves, but an interaction between one thing and another.
The hammer exerts a force on the stake but is itself brought to a stop in the process.
That’s because the stake exerts a force on the hammer.
Newton realized it’s neither “exerter” and “receiver” of forces, but an interaction.

6. Hammer and Stake Consider the vector arrow to the right.
The force of the hammer is downward.
The force of the stake is upward.
But both forces are equal in magnitude.
Equal and Opposite
Action Reaction

7. Have you ever… Jumped off a boat that wasn’t tied up?
The boat pushed on you, and you pushed on the boat, and because there’s no friction the boat moves in the opposite direction

8. Let’s consider some examples
Rolling in a chair
Pushing on wall
Balloon on wall
Remember: Newton’s 3rd Law refers to one interaction between two things.

9. Jet Engine Flight Jet Engines: where is the action & reaction?
Jet Engines: where is the action & reaction?
Can demonstrate by blowing up a balloon and letting it go

10. Tires & Road What are the two forces? Action: Reaction:
Tire pushes on road
Reaction:
Road pushes on tire

11. Rocket & Gas What are the two forces? Action: Reaction:
Rocket pushes on gas
Reaction:
Gas pushes on rocket
People use to think rockets were propelled by the gas pushing on the atmosphere, and therefore rockets wouldn’t work in space.

12. Gravity of falling objects
What are the two forces?
Action:
Earth pulls on ball
Reaction:
Ball pulls on Earth
No acceleration of the earth towards the ball, because the earth has so much more mass (F = MA)

13. Earth & Moon Why does the moon stay in orbit around the Earth?
Gravity – the moon is attracted to the Earth
Is the Earth attracted to the Moon?
A physicist would say there is an attractive force between the Earth and Moon.
Again it’s one interaction between 2 objects.
It’s like asking if the distance between SF and NY is the same and NY and SF.

14. Rubber Band
Stretch the rubber band between your thumb and forefinger. Q: Which is being pulled with more force: the thumb towards the forefinger or the other way around? Discuss with your neighbor.

15. Action-Reaction W.S.