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Newton’s Third Law of Motion

Newton’s Third Law of Motion. For every Action, there is an equal and opposite Reaction. 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.

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Newton’s Third Law of Motion

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  1. Newton’s 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. 1. Sam’s 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: m=20 kg. a=5 m/s F=ma F=20(5) Sam’s F=100 N The tree exerts an equal and opposite force to Sam Answer: 100 N Sample Problems

  6. 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.

  7. Concepts to Keep in Mind • Action and Reaction forces come in pairs • The forces are always equal… • Sometimes different masses cause different accelerations though(F=ma) • Internal vs. external systems- forces cancel out in an internal system, but an external force acting on the system will cause acceleration.

  8. The Quiz!

  9. 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. Question One…it’s a toughie!

  10. 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/s^2, what is the acceleration of the bullet ? Givens F=? Bullet: Rifle: m= .5 kg m= 8 kg a= ? a= 125 m/s^2 F=ma F=8(125) F=1000 N 1000 N=.5a 1000 N/.5=a Answer:a= 2000 m/s^2 Question Two

  11. 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) That’s crazy! A wall can’t 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. Question 3…this one is extra hard!

  12. Identify all the action-reaction force pairs. 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 Question 4

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

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