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

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.

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.

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

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.

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

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

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

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.

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