1. Newtons Third Law – Action and Reaction

2. Newton’s First and Second Laws – a review
First – Law of Inertia: an object at rest or in motion will remain that way until acted upon by an unbalanced force.
Second – Law of Mass & Acceleration: F = ma

3. Use the Cornell-Notetaking-Technique!

4. Newtons Third Law – Action and Reaction

5. Objetives: (please write into the cue column)
I can list Newton’s third law of motion.
I can define force.
I can identify action-reaction.
I can identify action-reaction on different masses.
I can use Newton’s third law to explain how rockets and helicopters work.
I can explain why action-reaction forces do not cancel.

6. For every force, there is an equal and opposite force.

8. Interactions Produce Forces
A force is a push or pull.
On closer inspection, it is an interaction between one thing and another.

9. Interactions Produce Forces
When hitting a nail with a hammer, the nail is driven into the wood.
The nail must also exert a force on the hammer to stop it from moving.

10. Interactions Produce Forces
This leads us to Newton’s third law, the law of ACTION and REACTION

11. Newton’s 3rd Law
Whenever one object exerts a force on a second object, the second object exerts an equal but opposite force on the first object.
For every ACTION force, there is an equal opposite REACTION force.

12. Newton’s 3rd Law
One force is called the action force and the other force is called the reaction force.
Neither force can exist with out the other.

13. Identifying Action & Reaction
Sometimes not easy to identify the Action Reaction pair.
Use this rule Object A exerts a force on Object B
then in reverse
Object B exerts a force on object A

14. Identifying Action & Reaction

15. Identifying Action & Reaction

16. Identifying Action & Reaction

17. Identifying Action & Reaction
Action: Tire pushes road
Reaction: Road pushes tire

18. Identifying Action & Reaction

19. Identifying Action & Reaction
Action: Rocket pushes gas
Reaction: Gas pushes rocket

20. Identifying Action & Reaction

21. Identifying Action & Reaction
Action: Earth pulls ball
Reaction: Ball pulls Earth

22. Identifying Action & Reaction

23. Identifying Action & Reaction
The dog wags the tail and the tail wags the dog.

24. Action and Reaction on Different Masses
In the example of the ball falling to the Earth, can we say the Earth falls towards the ball?

25. Action and Reaction on Different Masses
YES!!! But not as much.
Why?
Although the size of the force is the same, the masses are different.

26. Action and Reaction on Different Masses
Small mass on bullet, LARGE acceleration.

27. Action and Reaction on Different Masses
By considering the recoil of a gun we can understand rocket propulsion.
Instead of bullets coming out of the rocket, it’s a gas!!

28. Action and Reaction on Different Masses
The rocket recoils from the “molecular cannonballs” it fires and climbs upward.

29. Action and Reaction on Different Masses
Using Newton’s third law, we can understand how a helicopter gets its lifting force.
The whirling blades force air particles downward (action).
The air forces the blades upward (reaction).
This upward reaction force is called lift.
When lift equals the weight of the craft, the helicopter hovers in midair. When lift is greater, the helicopter climbs upward.

30. Why Action and Reaction Forces Don’t Cancel
In every interaction, one thing interacts with the other and vice versa. A on B & B on A.
Why don’t they cancel out?

31. Why Action and Reaction Forces Don’t Cancel
The answer is that the pair of forces act on different objects.
Although A acts on B and B acts on A, the individual forces themselves act on different objects. The ball accelerates away and your foot slows down when it hits the ball.

33. Action Equals Reaction