Objective

1. Identify the force pairs of an interaction and the objects on which they act, based on Newton’s 3rd Law. Objective

2. In the simplest sense, a force is a push or a pull. Forces are the result of an interaction between one object and another. Forces and interactions

3. When you push on the wall, the wall pushes on you. Forces and interactions

4. The interaction that pushes on the nail is the same one that stops the hammer. Forces and interactions

5. A hammer exerts a force on the nail and drives it into a board. • There must also be a force exerted on the hammer to halt it in the process. • Newton reasoned that while the hammer exerts a force on the nail, the nail exerts a force on the hammer. • In the interaction, there are a pair of forces, one acting on the nail and the other acting on the hammer. Forces and interactions

6. Force is not something an object has, like mass. Force is an interaction between one object and another. An object may possess the capability of exerting a force on another object, but it cannot possess force as a thing in itself. Forces and interactions

7. If object A exerts a force on object B, then object B exerts an equal and opposite force (of the same type) on object A. Newton’s 3rd law

8. In every interaction, the forces always occur in pairs. • You push against the floor, and the floor simultaneously pushes against you. • The tires of a car interact with the road to produce the car’s motion. The tires push back against the road, and the road simultaneously pushes forward on the tires. • When swimming, you push the water backward, and the water pushes you forward. Newton’s 3rd law

9. The interactions in many examples depend on friction. A person trying to walk on ice, where friction is minimal, may not be able to exert a force against the ice. Without the force from the person’s foot, there cannot be a force back from the ice, and thus there is no resulting forward motion. Newton’s 3rd law

10. There is a simple recipe for treating interaction forces: • First identify the interaction. Let’s say object A interacts with object B. • The forces are stated in the form: Object A exerts a force on object B. Object B exerts a force on object A. Identifying interaction pairs

11. Sometimes the identity of the pair of forces in an interaction is not immediately obvious. • For example, what are the interacting objects in the case of a falling boulder? • What are the action and reaction for the falling boulder? • If the Earth pulls down on the boulder with gravity, then the boulder pulls up on Earth with the same amount of gravity. Indentifying interaction pairs

12. What is the other half of the interaction for each of these examples? Identifying interaction pairs

13. When action is A exerts force on B, the reaction is simply B exerts force on A. Indentifying interaction pairs

14. Check for understanding

15. We know the Earth pulls on the moon. Does the moon also pull on the Earth? • Yes, with a different amount of force in the same direction. • Yes, with a different amount of force in the opposite direction. • Yes, with the same amount of force in the same direction. • Yes, with the same amount of force in the opposite direction. • No, the moon does not pull on the Earth. Question #1

16. A book sits on a table. Select the correct interaction pair. • Earth pulls down on book, table pushes up on book. • Earth pulls down on book, book pushes down on table. • Book pushes down on table, table pushes up on book. • None of the above. Question #2

17. When Joe Frazier hit Muhammed Ali in a boxing match, who was hit with the greater force? • Ali • Frazier • Each is hit with the same force. • It depends upon who is stronger. • It depends upon who has more mass. Question #3