 # Chapter 7 Newton’s 3rd Law of Motion

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

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. Conceptual Physics Chapter 7

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. Conceptual Physics Chapter 7

Conceptual Physics Chapter 7
Newton’s 3rd Law Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. Conceptual Physics Chapter 7

Conceptual Physics Chapter 7
Newton’s 3rd 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. Conceptual Physics Chapter 7

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. Conceptual Physics Chapter 7

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! Conceptual Physics Chapter 7

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! Conceptual Physics Chapter 7

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. Conceptual Physics Chapter 7

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? Conceptual Physics Chapter 7

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! Conceptual Physics Chapter 7

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? Conceptual Physics Chapter 7

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. Conceptual Physics Chapter 7

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 force on the cannon or the force on the cannonball? Which is greater – the acceleration of the cannon or the acceleration of the cannonball? Conceptual Physics Chapter 7

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? Conceptual Physics Chapter 7

Action/Reaction Forces
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. The opposing forces act on different bodies and comprise an action-reaction pair. These forces can’t cancel one another! Conceptual Physics Chapter 7

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. Conceptual Physics Chapter 7

Conceptual Physics Chapter 7
Horse-Cart Problem Conceptual Physics Chapter 7

Conceptual Physics Chapter 7
Horse-Cart Problem What is the net force that acts on the cart? What is the net force that acts on the ground? 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? Conceptual Physics Chapter 7