Newton’s 3rd Law

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

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 problemsand next-time question 6-1 p. 83 (#1-5)