Newton

1
Newtons Third Law of Motion

• For every Action, there is an equal
  and opposite Reaction.

2

• Example 1 When you sit on a chair, the chair
  pushes back up against your weight with an equal
  force.
• Notice the action-reaction pair
• Action force
• Reaction force

3

• Example 2 When you swim, you push water
  backwards and the water pushes you forwards.
• Example 3 When birds push their wings down in
  the air, the air pushes them and their wings up.

4

• Example 4 Car wheels push backward on the road,
  and the road pushes the wheels forward.

5
Sample Problems

• 1. Sams mass is 20 kg. After he began to run,
  his acceleration reached 5 m/s when he suddenly
  hit a tree. Ouch! What force did the tree exert
  on Sam?

• Givens
• m20 kg.
• a5 m/s
• Fma
• F20(5)
• Sams F100 N
• The tree exerts an equal and opposite force to
  Sam
• Answer 100 N

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• 2. The system is only the orange(in the circle).
  If the apple pulls with x Newtons, is there any
  acceleration, or do the forces cancel?

• If you remember pages 78 and 79 in your book, you
  know that forces internal to a system cancel each
  other out, but forces external to that system
  cause it to accelerate.
• Answer There is acceleration.

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Concepts to Keep in Mind

• Action and Reaction forces come in pairs
• The forces are always equal
• Sometimes different masses cause different
  accelerations though(Fma)
• Internal vs. external systems- forces cancel out
  in an internal system, but an external force
  acting on the system will cause acceleration.

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The Quiz!
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Question Oneits a toughie!

• A bug is flying at 1 m/s and truck is traveling
  in the opposite direction at 40 m/s. When the
  two collide, which is the force of the collision
  greater on?

• This is a trick question!
• We all know that action and reaction forces are
  EQUAL, and therefore, the the collision force is
  greater on neither the truck nor the bug.

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Question Two

• When a .5 kg bullet is fired from an 8 kg rifle,
  the action-reaction pair is the rifle pushing the
  bullet forward and the bullet pushing the gun
  backward. If the force from the blast causes the
  rifle to accelerate to 125 m/s2, what is the
  acceleration of the bullet ?

• Givens F?
• Bullet Rifle
• m .5 kg m 8 kg
• a ? a 125 m/s2
• Fma
• F8(125)
• F1000 N
• 1000 N.5a
• 1000 N/.5a
• Answera 2000 m/s2

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Question 3this one is extra hard!

• If the action is Mrs. B pushing on a wall, the
  reaction is
• A) The floor pushing on the wall
• B) The floor pushing on Mrs. B
• C) The wall pushing on Mrs. B
• D) Thats crazy! A wall cant push back!

• In an action-reaction pair, the reaction is
  usually just the vice-versa of the action.
• In the question, the action is a person pushing
  on a wall, so the reaction would be the wall
  pushing on the person.
• The correct answer is C.

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Question 4

• The man pushing on the ground/the ground pushing
  on the man
• The knee hitting the ball/the ball hitting the
  knee
• The barbell pushing down on the man/ the man
  pushing up on the barbell
• The balloon pulling the barbell up/the barbell
  pulling the balloon down
• The air pushing out of the balloon/the balloon
  side pushing the air in

• Identify all the action-reaction force pairs.

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Congratulations!

• You are now ready for the portion of the final
  concerning Newtons 3rd Law!
• For more review, see Chapter 6 in your book.