2/22/17 Today we will state Newton’s Third Law of Motion identify common misconceptions apply this Law to everyday situations Warm Up – What is inertia? How is it related to mass? Is it easier to move something that has more mass or something that has less mass?

Newton’s Third Law states: Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. Ex. walking, swimming, bending your fingers, pushing against a wall Walking and swimming both involve friction as part of the action and reaction forces.

Newton’s Third Law Also called: The Action – Reaction Law For every action there is an equal and opposite reaction. Or For every force there is an equal and opposite force. Ex. walking, swimming, bending your fingers, pushing against a wall Walking and swimming both involve friction as part of the action and reaction forces.

Forces and Interactions What is a force? Any push or pull A force is an interaction between two things. Example: desk and hand Take a moment and push against the table. What do you feel? Is the table pushing back on you?

Misconception #1 Inanimate objects cannot push or pull. FALSE!!! We often think of living things exerting a force. Strictly speaking to say “the desk pushes on you” is to say, “the desk exerts a force as though it were pushing on you.” The desk, the wall, the ground and other inanimate objects effectively do push back on us when we push on them!

Misconception #2 If every force has an equal and opposite force then all forces should cancel out. FALSE!!! Newton’s Third Law deals with force exerted on different objects. The force due to gravity and the normal force, acting on a book at rest on a table, is NOT a Newton Third Law force pair. Why?

Think of Newton's Third Law force pairs as Force ON and Force BY for two different objects. Force desk hand = - Force hand desk The force ON the desk BY the hand is equal to the opposing force ON the hand BY the desk.

Misconception #3 The reaction takes place after the action. FALSE!!! The action and reaction pair occur simultaneously.

Video Demonstration

Examples Live Demonstration Balloon car

Examples Jumping on Earth

Examples Solar System: Consider the Earth and Moon Is the Earth pulling more on the Moon or is the Moon pulling more on the Earth? TRICK QUESTION!

Examples Hitting a nail with a hammer

A car accelerates along a road A car accelerates along a road. Strictly speaking, what is the force that moves the car? The road pushes the car along. Interaction of tires and road Aside from air resistance the only horizontal force acting on the car comes from the interaction between the tires and the road. So the road pushes on the car as part of the pairing. Force Exerted by Ground on Wheel

Explain in terms of Newton’s Third Law Walking on floor Aside from air resistance the only horizontal force acting on the car comes from the interaction between the tires and the road. So the road pushes on the car as part of the pairing.

Explain in terms of Newton’s Third Law Fish swimming Aside from air resistance the only horizontal force acting on the car comes from the interaction between the tires and the road. So the road pushes on the car as part of the pairing.

Explain in terms of Newton’s Third Law Bird flying Why do migrating birds, such as geese, fly in a V formation? Aside from air resistance the only horizontal force acting on the car comes from the interaction between the tires and the road. So the road pushes on the car as part of the pairing.

Homework sheet Complete the Concept Development worksheet for homework.

Ticket out the door True or False: When a soccer ball hits the face of a player, the player’s face hits the soccer ball with an equal and opposite force. Because forces are always equal and opposite, they always balance out to a net force of zero. Earth pulls down on a person with a force, while the ground supplies a normal force to support the person. This normal force is the “equal and opposite force” that Newton’s third law refers to.