1. Newton’s Third Law of Motion: Action and Reaction
Chapter 6

2. Forces and Interactions
A force is a push or pull
Interaction – a mutual action between objects where each object exerts an equal and opposite force on the other
Interacting things exert forces on each other

3. Newton’s Third Law 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 object.
One force is called the action force and the other is called the reaction force (it doesn’t matter which we call action and which we call reaction)
Newton’s 3rd law can also be stated as “To every action there is an equal and opposite reaction.”
In every interaction, the forces always occur in pairs
When you walk, you are interacting with the ground; you push against the ground, and the ground simultaneously pushes against you

4. Identifying Action and Reaction
Sometimes the identity of the pair of action and reaction forces in an interaction is not immediately obvious
First, identify the interaction (one object, A, interacts with another object, B)
The action and reaction forces can be stated in the form:
Action: Object A exerts a force on object B
Reaction: Object B exerts a force on object A

5. Identifying Action and Reaction

6. Action and Reaction on Different Masses
The forces are equal in strength and opposite in direction
Remember that Newton’s 2nd law states that acceleration is proportional to net force, and inversely proportional to the mass
If we have different masses interacting, the accelerations on each will be different
If you consider a cannon and a cannonball; the cannon and cannonball exert equal and opposite forces on each other, but the cannonball moves much more than the cannon
Because of Newton’s 2nd law, the mass of the cannon stops it from accelerating as much as the cannonball
a = Fnet/m

7. Cannons
F = M(cannon) a(cannon)
F = m(ball) A(ball)

8. Do Action and Reaction Forces Cancel?
If action and reaction forces are internal to a system, they cancel each other and produce no acceleration of the system
Action and reaction forces do not cancel each other when either is external to the system that is being considered

9. Do Action and Reaction Forces Cancel?
An apple pulls an orange and the orange accelerates. The orange pulls back on the apple, but this affects the apple and not the orange.
Both A and C act on B. They can cancel each other so B does not accelerate.

10. The Horse-Cart Problem
Force By On
Direction
Affects the Motion of
Comments
Horse pulls Wagon
horse
wagon
right
Action/Reaction Pair
left
Horse pushes Ground
ground
Friction

11. Action Equals Reaction
You can not hit anything harder than it can hit you back
If the object is not able to exert a reaction force equal to your action force, then you cannot produce an action force that large

12. Assignment Read Chapter 6 (pg. 74-82)
Do Ch. 6 Assessment #19-39 (pg )
Do Appendix F #1-6 (pg )
Study for the Newton’s Laws Test (Ch. 4-6)