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Newton's Third Law of Motion

Newton's Third Law of Motion. 1. FORCES AND INTERACTIONS. Force is an interaction between one thing and another. Ex: 2 people pushing each other on skate boards, driving a nail, punching a bag There is no single isolated force. 2. NEWTON'S THIRD LAW OF MOTION.

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Newton's Third Law of Motion

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  1. Newton's Third Law of Motion

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

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

  4. 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!

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

  6. F F m m a a You = = Earth

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

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

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

  10. Newton’s 3rd Law Six minutes • http://www.youtube.com/watch?v=cP0Bb3WXJ_k&feature=relmfu

  11. The subscript notation: • Convention: Fa,b means “the force acting on a due to b”. • So Fhead,thumb means “the force on the head due to the thumb”. Fhead,thumb

  12. Fm,w Fw,m Ff,m Fm,f Newton's Third Law says... FA ,B = - FB ,A.

  13. Hmmm…. The man leans on the box, the box moves. • Since Fm,b = -Fb,m, why isn’t Fnet = 0 and a = 0 ? Fm,b Fb,m a ?? ice

  14. Need free body diagram of box only. • Consider only the box as the system! • Fon box= mabox=Fb,m Fb,m Fm,b abox ice

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

  16. 3. SUMMARY OF NEWTON'S THREE LAWS • 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. • Law 2 • Law 3 - Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

  17. Newton’s 3rd law …6 minutes • http://www.youtube.com/watch?v=cP0Bb3WXJ_k&feature=relmfu • Newton’s 3 Laws 4 min • http://www.youtube.com/watch?v=iH48Lc7wq0U • http://www.youtube.com/watch?v=8bTdMmNZm2M&list=PLEA0D7C501E484B31&index=8

  18. Review Questions

  19. 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 (c) neither, they both experience the same magnitude of force

  20. 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

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

  22. Solution: a a a a Fa,E Fb,a b Fa,b b b b Fb,E Fb,g Fg,b FE,b FE,a (c) 4

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