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Conceptual Physics Chapter 71 Chapter 7 Newton’s 3 rd Law of Motion.

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Presentation on theme: "Conceptual Physics Chapter 71 Chapter 7 Newton’s 3 rd Law of Motion."— Presentation transcript:

1 Conceptual Physics Chapter 71 Chapter 7 Newton’s 3 rd Law of Motion

2 Conceptual Physics Chapter 72 Forces and Interactions ¤A force is a part of an interaction between two objects. ¤There are no single forces in nature – forces can only occur in pairs.

3 Conceptual Physics Chapter 73 Forces and Interactions ¤A carpenter swings a hammer and strikes a nail – the hammer exerts a force on the nail. ¤Newton recognized that the force that drives the nail is the same as the force that halts the hammer.

4 Conceptual Physics Chapter 74 Newton’s 3 rd Law Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

5 Conceptual Physics Chapter 75 Newton’s 3 rd Law ¤One force is generally referred to as the action force; the other force is called the reaction force. ¤Neither force exists without the other. ¤They are equal in strength and opposite in direction. ¤They occur simultaneously.

6 Conceptual Physics Chapter 76 Identifying Action and Reaction ¤Identify the two bodies that are part of the interaction. ¤Object A exerts a force on object B. ¤Object B exerts an equal and opposite force on object A.

7 Conceptual Physics Chapter 77 Identifying Action and Reaction ACTION: When swimming, you exert a backward force on the water with each stroke. REACTION: The water exerts a forward force on you!

8 Conceptual Physics Chapter 78 Identifying Action and Reaction ACTION: The earth exerts a downward force on a ball released from rest. REACTION: The ball exerts an upward force on the earth!

9 Conceptual Physics Chapter 79 Identifying Action and Reaction ACTION: A rocket launched from the earth burns fuel and forces these gases back toward the earth. REACTION: The expelled gases push back on the rocket propelling the rocket forward.

10 Conceptual Physics Chapter 710 Identifying Action and Reaction The sun exerts a large force on the earth and causes the earth to orbit around the sun. Does the earth exert a force on the sun? Which is greater - the force of the sun pulling on the earth or the force of the earth pulling on the sun?

11 Conceptual Physics Chapter 711 Identifying Action and Reaction ACTION: When walking, you exert a backward force on the floor with each step that you take. REACTION: The floor exerts a forward force on you!

12 Conceptual Physics Chapter 712 Identifying Action and Reaction The force that the floor exerts on you causes you to accelerate forward. Why doesn’t the force that you exert on the floor cause the earth to accelerate in the opposite direction?

13 Conceptual Physics Chapter 713 Action/Reaction Forces ¤Newton’s second law of motion tells us that if identical forces act on two different masses, the force acting on the smaller mass will cause a larger acceleration and the force acting on the larger mass will cause a smaller acceleration.

14 Conceptual Physics Chapter 714 Action/Reaction Forces When a cannon is fired, there is an interaction between the cannon and the cannonball. The cannon exerts a force on the cannonball and the cannonball exerts a force on the cannon. Which is greater – the acceleration of the cannon or the acceleration of the cannonball? Which is greater – the force on the cannon or the force on the cannonball?

15 Conceptual Physics Chapter 715 Action/Reaction Forces A student says that you can’t move a football by kicking it because the reaction force of the kicked ball would be equal and opposite to your kicking force. The net force would be zero so no matter how hard you kick, the ball won’t move. What do you say?

16 Conceptual Physics Chapter 716 Action/Reaction Forces The opposing forces act on different bodies and comprise an action- reaction pair. These forces can’t cancel one another! Forces A and C act on the same object, not different objects, so they do not make up an action-reaction pair. If the two forces are equal, opposite and applied simultaneously, the net force will be zero.

17 Conceptual Physics Chapter 717 Action/Reaction Forces ¤An action – reaction pair will always act on two different bodies and they will therefore never cancel one another. ¤The action force on body A will cause an acceleration of A and the reaction force on body B will cause an acceleration of B.

18 Horse-Cart Problem Conceptual Physics Chapter 718

19 Conceptual Physics Chapter 719 Horse-Cart Problem If the horse exerts a force on the cart and the cart exerts an equal and opposite force on the horse, can the cart begin to move? What is the net force that acts on the horse? What is the net force that acts on the ground? What is the net force that acts on the cart?


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