2. Objectives Define force as part of an interaction. (6.1)
State Newton’s third law of motion. (6.2)
Given an action force, identify the reaction force. (6.3)
Explain why the acceleration caused by an action force and by a reaction force do not have to be equal. (6.4)
Explain why an action force is not cancelled by the reaction force. (6.5)
Describe the horse-cart problem. (6.6)
Explain why you can’t touch without being touched. (6.7)

3. 7.1 Forces and Interactions
A force is always a part of a mutual action that involves another force.
Forces always occur in pairs.

4. 7.2 Newton’s Third Law Describes the relationship between two forces.
Newton’s third law – whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the other object.
Once force is called the action force, the other, reaction force.

5. Questions What happens when a force exerts a force on another object?
Does a stick of dynamite contain force?
A car accelerates along the road. Strictly speaking, what is the force that moves the car?
Answers:
The second object exerts a force on the other object equal to an opposite in direction.
No, forces have to act in pairs. We will learn later that dynamite has something called energy.
The road pushes on the cars tires (reaction force). That’s because the cars tires push on the road (action force).

6. 7.3 Identifying Action and Reaction
A simple way of determining action-reaction forces:
First Identify the interacting objects.
Action: Object A exerts force on Object B
Reaction: Object B exerts force on Object A

8. Question
We know the Earth pulls on the moon. Does the moon also pull on the Earth? If so, which pull is stronger?
Answer:
Neither. The action-reaction pair is earth pulls on moon, moon pulls on earth, both pulls equal in magnitude but opposite in dircetion.

9. 7.4 Action and Reaction on Different Masses
The Earth and a falling boulder. We know the boulder falls to the Earth. But does the Earth also fall to the boulder?
You must draw on Newton’s second law to help answer this question.
Given a force exerted on a small mass produces a greater acceleration than the same force exerted on a larger mass.

11. How do rockets work. Newton’s third law
How do rockets work? Newton’s third law. The force of the escaping gas on the rocket receives and equal and opposite force by the rocket causing the gases to accelerate. In other words the pressure from the gas pushes the rocket, and the rocket pushes back on the gas. This process works even better in space, unlike what is a common misconception.

12. Lift Helicopter Planes
Rotating blades push down on air (action) air pushed on blades (reaction).
Planes
Wings receive a greater force from air underneath the wing (action), wing provides a force back on the air (reaction).
Action of wings is result of Bernoulli's principle which will not be covered here.

13. 7.5 Defining a System
Action and reaction forces do not cancel each other when either of the forces is external to the system being considered.

15. 7.6 The Horse-Cart Problem
If the horse in the horse-cart system pushes the ground with a greater force than it pulls on the cart, then there is an net force on the horse, and the horse-cart system accelerates.
It’s the force outside the horse-cart system that pushes the system.

16. 7.7 Action Equals Reaction
For every interaction between things, there is always a pair of oppositely directed forces that are equal in strength.