1. 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?

2. 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.

3. 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.

4. 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?

5. 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!

6. 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?

7. 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.

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

9. Video Demonstration

10. Examples
Live Demonstration
Balloon car

11. Examples
Jumping on Earth

12. 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!

13. Examples
Hitting a nail with a hammer

14. 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

15. 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.

16. 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.

17. 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.

18. Homework sheet
Complete the Concept Development worksheet for homework.

19. 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.