3. Newton's Third Law of Motion

4. 1. FORCES AND INTERACTIONS n Force is an interaction between one thing and another. n Ex: 2 people pushing each other on skate boards, driving a nail, punching a bag n There is no single isolated force

5. n Law 3 - Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. n Action-reaction pairs never act on same object. 2. NEWTON'S THIRD LAW OF MOTION

6. Newton’s Third Law deals with acting on objects. two forces two different Newton’s Third Law pairs of forces always, sometimes, never cancel each other out. The only way for two forces to cancel each other out is for them to be equal and opposite and act on the same object!

7. Action:earth pulls you Reaction: you pull earth Action & Reaction on Different Masses Consider you and the earth

8. You F m = F m = a a Earth

9. Action: tire pushes on road Reaction: road pushes on tire

10. Action: rocket pushes on gases Reaction: gases push on rocket

11. n Hwk read Review book 82-86 do pg 100 #4,5, 8 – 14, 24 - 27, 31-35. n Separate sheet.

12. Newton’s 3 rd LawSix minutes n http://www.youtube.com/watch?v=cP0B b3WXJ_k&feature=relmfu http://www.youtube.com/watch?v=cP0B b3WXJ_k&feature=relmfu

13. The subscript notation: F n Convention: Fa,b means “the force acting on a due to b”. F n So F head,thumb means “the force on the head due to the thumb”. F F head,thumb

14. Newton's Third Law says... FF F A,B = - F B,A. F F m,w F F w,m F F m,f F F f,m

15. Hmmm…. The man leans on the box, the box moves. FF Fa n Since F m,b = -F b,m, why isn’t F net = 0 and a = 0 ? a ?? F F m,b F F b,m ice

16. Need free body diagram of box only. only the box n Consider only the box as the system! –FaF –F on box = ma box = F b,m a box F F m,b F F b,m ice

17. The forces acting on the box are: 1. The force of the man 2. The force of the ice (neg). a box

18. 3.SUMMARY OF NEWTON'S THREE LAWS n Law 1 (Law of Inertia) Every object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it. n Law 2 n Law 3 - Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

19. Newton’s 3 rd law …6 minutes n http://www.youtube.com/watch?v=cP0Bb3WXJ_k &feature=relmfu http://www.youtube.com/watch?v=cP0Bb3WXJ_k &feature=relmfu n Newton’s 3 Laws 4 min n http://www.youtube.com/watch?v=iH48Lc7wq0U http://www.youtube.com/watch?v=iH48Lc7wq0U n http://www.youtube.com/watch?v=8bTdMmNZm2 M&list=PLEA0D7C501E484B31&index=8 http://www.youtube.com/watch?v=8bTdMmNZm2 M&list=PLEA0D7C501E484B31&index=8

20. Review Questions

21. A bug and a car collide. Which experiences the greater force? (a) bug (b) car (c) neither, they both experience the same magnitude of force

22. Consider hitting a baseball with a bat. If we call the force applied to the ball by the bat the action force, identify the reaction force. (a) the force applied to the bat by the hands (b) the force applied to the bat by the ball (c) the force the ball carries with it in flight (d) the centrifugal force in the swing (b) the force applied to the bat by the ball

23. n Two blocks are stacked on the ground. n How many action-reaction pairs of forces are present in this system? (a) 2 (b) 3 (c) 4 a b

24. Solution: (c) 4 F E,a F a,E a b F E,b a b F b,E F b,a F a,b a b F g,b F b,g a b