1. Law of Motion: Action and Reaction Ch. 6.1-6.3
Newton’s 3rd Law
Law of Motion:
Action and Reaction Ch

2. Objectives Define force as part of an interaction
Newton’s 3rd Law of Motion
Identify action & reaction forces

3. I can’t touch you without you touching me
I can’t touch the desk without the desk touching me
I can’t exert a force on a body without the body exerting a force on me
Contact requires TWO objects!

4. Why do my fingers bend back more when pushing on hand & wall?
Because the wall pushes back!
Somebody push on my fingers…
Most people don’t see this. Physics types see the wall pushing back!

5. Forces and Interactions
Further defines idea we’ve been talking about this whole time
Force
an interaction between one thing and another
The Interaction Rule:
Object A exerts a force on object B
Object B exerts a force on object A

6. Forces and Interactions
Which block pulls harder? Check your neighbor.
The more massive block didn’t move very much? Is it pulling harder?
Add mass to the less massive block
Move about same
What can you say about force? Mass?
Remove mass. Which is pulling harder?

7. Tug-of-War In a tug of war both teams pull with 1000 N.
There’s a scale in the middle of rope. What will the scale read?
2000 N, 1000 N, or 0 N? Check your neighbor
Imagine if a person was in the middle. Definitely NOT 0 N pulling on each arm!
If one team gets tired, attaches rope to wall, puts up a curtain. Other team continues to pull. What will the scale read?
Pulling seems to be exactly the same, 1000 N

8. Tug-of-War Student volunteer Who’s pulling harder?
Forces are the same!
How do you win tug-of-war?
Push the hardest against floor
Girls vs. Boys demo
Fellas, take off your shoes

9. Paul Hewitt Explains the forces during a tug of war

10. Rubber band Demo
Which is pulling harder? The thumb on the forefinger or the forefinger on the thumb?

11. Earth vs. Moon?
Is the distance from NY to SF the same distance as SF to NY?
Does the earth pull harder on the moon than the moon pulls on the earth (moon is only 1/6 mass of earth)?
No! There is a single gravitational interaction between the two
Action: A pulls on B; Reaction: B pulls on A

12. Newton’s 3rd Law
For every action, there is an equal and opposite reaction
There are two forces
Action force
Reaction force
Baseball clip
What are the action/reaction forces in these pics?
Forces always occur in pairs
The sound of ONE hand clapping is impossible

13. Kicking, punching, dropping and catching stuff
Kick/punch something (NOT another person!)
That object is pushing back on you with the same force that you kick/punch it with
Find two objects. Drop object A with one hand, catch it with the other. Do the same with object B. Which one required your hand to push back with the most force?

14. Action-Reaction in Action
Why do geese fly in a “V” formation?
Birds wings deflect air downwards--> air pushes bird upward
Creates updraft (greatest on side of bird)
Trailing bird positions itself in updraft to conserve energy
This bird creates updraft for following bird, and so on

15. Action-Reaction in Action
Interesting Facts
Explain this picture using your knowledge about Newton’s 3rd Law
Travel up to 9 miles at night to feed and return
Equivalent of human waling 37 miles for dinner each night
Known to swim at 28 mph for miles and miles

16. Action-Reaction in Action
Explain the three pictures using your knowledge of Newton’s 3rd Law.

17. Let’s review the homework problems and next-time question 6-1