1. Or the world treats you as you treat the world
Newton’s Third Law
Or the world treats you as you treat the world
Standard:
1d Students know that when one object exerts a force on a second object, the second object exerts a force of equal magnitude and in the opposite direction.
Schedule:
Monday: Exploration Lab
Tuesday: Lecture
Wednesday: Balloon Rocket Lab
Thursday: Homework Review
Friday: Quiz

2. Newton’s 3rd Law
For every action there is an equal and opposite reaction.
Action and reaction refer to forces
Action and reaction forces are equal and in opposite directions BUT are also on different objects
Other versions
Forces come in pairs
Cannot touch without being touched
Forces are interactions between two bodies
Question: State Newton’s 3rd law of motion. What do action and reaction refer to? How many objects due the action and reaction refer to?
Activities:
Write law on board.
State action and reaction refer to forces.
State that even though forces are equal and opposite they never cancel because they are never on same object.
State other versions of law.
Demonstrate using two scales or two force probes.

3. Identifying Action and Reaction
Action is force from object A on object B
Reaction is force of object B on object A
Simple Examples
Walking
Swimming
Pushing against wall
Difficult examples
Rocket
Moon and Earth
Question: What are the reaction forces for the following action forces: foot pushing on ground, hand pushing back on water, rocket pushing fuel, Earth on Moon, tire on road?
Activity
Write general pattern of action and reaction A on B, B on A.
Point out that two forces are on different objects
Go through examples on slide: walking, swimming, pushing on wall, tire on road, rocket on fuel, moon on Earth
Demo: Whack student with book and point out that instructor did not hit student with book, rather student hit book with head.

4. Check Question: The Apple
What is the reaction force to the weight of an apple as it sits on a table?
The table on the apple
The apple on the earth
The earth on the apple
Normal force

5. A and R Forces do not Cancel
Action force is on one object, reaction force is on different object
Differs from net force
Only external forces cause motion
Horse-Cart Example
Force eqn. for horse
Force eqn. for cart
Force eqn. for horse-cart system
Question: Why do action and reaction forces never cancel out? How does an action-reaction pair differ from a net force? For the horse-cart problem, write the force equation for the horse, the cart and the horse cart system.
Activity:
Again state that action and reaction forces are on two separate objects. Therefore they cannot cancel.
Contrast this with a net force where more than one force is on SINGLE object and balancing can therefore occur.
Show horse and cart problem and derive force equations for horse, cart and system. Do this by circling system and then asking what outside forces act on system. Write F = ma equations. Point out that force of cart on horse and horse on cart appear in separate equations. For system point out that only external forces cause motion so only forces are ground on horse and ground on cart. So system moves when force of ground on horse is greater than force of ground on cart.

6. Action and Reaction Forces may not Produce the Same Acceleration
Acceleration depends on net force AND mass of object force is upon (a= F net / m).
Examples
Different mass people on carts
Bug and windshield
Bullet and gun
Moon and Earth
Question: When do action and reaction forces not produce the same acceleration? When a gun is fired, how do the forces on the rifle and bullet compare? How do the masses compare? How do the accelerations compare?
Activities:
Write Newton’s second law and point out that a depends on both force and mass. This means that acceleration may differ depending on the mass of the two objects interacting even though the force on each is the same.
Demo above with two different people on wheeled carts.
Discuss bug and windshield example
Discuss bullet and gun example
Discuss Moon and Earth example

7. Check Question: Firing Squad
You find yourself before a firing squad when you are asked if you have any last request. Which of the following might help you escape this unpleasant predicament?
Ask for a large gun and a small bullet
Ask for a small gun and a large bullet
A Tombstone pizza

8. Tension Tensioning a string requires two forces
One to hold one end stationary
The other to stretch the other end
Tension is the same in both directions
Tension is the same along the entire length of a rope
Question: How many forces are required to tension something? How does the tension vary along the length of the object? How about the direction the tension is measured in?
Activities:
State that need two forces to tension things. One to hold string and one to stretch object out.
Demo above with a rubber band by violently swinging it back and forth (this doesn’t work)
Refer students to yesterday’s station where they found that tension does not depend on direction or where you measure it along system

9. The Extra Force Problem
If an object cannot return the entire force available to it, it will return what it can and accelerate with what is left over (net force)
Examples
Punching a paper bag
Cannon and Tree simulation
Tug of War
Atwood Machine Demo
Question: What happens when an object encounters a force that it cannot fully return? What does the scale for the following situation read?
Activities:
State the extra force rule.
Discuss punching a piece of paper example. Extra force goes into acceleration. What happens when paper placed against wall? Now bring up how no one could punch their way out of a paper bag because bag can not return enough force to allow for tearing.
Discuss cannon simulation from yesterday and how tree provides extra force to allow for extra acceleration.
Go through tug of war skit. Two sides both pulling, one side pulling against wall, tension is the smaller of two forces.
Demo of Atwoods machine with paper over scale.

10. Is this a good idea?