Newton’s 3rd LawPart II

Ch. 6.4-6.7

Review

• State Newton’s 3rd Law• Let’s discuss these 3 pictures using Newton’s 3rd law.• Show next time question 6-3, 6-4

Do you ever stand up when peddaling uphill? Why

Objectives

1. Explain why accelerations can be different between an action force and reaction force

2. Explain why an action force and reaction force do not cancel each other out

3. Explain why you can’t touch without being touched

Falling Objects

• Drop a ball on the ground.

• What is the action force?– Gravtiy

• What’s the reaction force?

• What if I ask it this way: Is the earth pulling on this ball?

• What’s the reaction?

• Ball pulling on earth!

Falling Objects• Can say that this

motocross rider is now falling toward the earth?

• What about the earth? Is the earth falling towards the rider?

• Yes! But why don’t we really notice it?

Drop a pen• Sitting here in your desk you feel the chair

pushing up? And we are pushing down . . . so it’s “even” right?

• I’m going to drop this pen, see if you can notice the chair pushing back up you a little bit . . .

• Why can’t we feel it?

• Remember the 2nd Law?• Acceleration is proportional to Force and inversely

proportional to mass

•a=f/m a=f/m– Acceleration of pen is simply force divided by it’s mass

• If the earth had the same mass as this pen, how far would the pen fall before it collided with the earth?– Halfway!

• Earth has a HUGE mass --> small acceleration– So small we don’t even notice the acceleration!

• Let’s look at this in terms of a cannon firing a cannonball

Action & Reaction on Different Masses

• What is the action and reaction in this pic?

• Cannonball goes forward, cannon “kicks” or recoils

• Are the forces equal?• Yes! Then why is the

acceleration of the cannonball so much greater than the the cannon?

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Action & Reaction on Different Masses

• The masses are different!

Ball: F/m = a Cannon: F/m = a

notice forces (F) are the same

Equal forces produce unequal accelerations when different masses are involved

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Paul Hewitt explains

QuickTime™ and aSorenson Video decompressorare needed to see this picture.

• How does this rocket work?

• The rocket “recoils” from the “molecular cannonballs” it fires and climbs upward.

One last wish . . .

• Let’s say that a person has been found guilty of rational thinking in a totalitarian society and is sentenced to death by firing squad. Their one last wish would be to be struck by a bullet that is more massive than the gun, and that the antagonist pull the trigger? Is this a good wish? Why?

Show Next Time Question 6-2

Will the horse pull the cart?

In order to understand this, we must understand systems

•One system is the cart

•What horizontal force is acting on the cart?

•The horizontal force acting on the cart is the horse

•Net force on cart --> acceleration occurs

System 1

System 2

•System is the horse

•Two external horizontal forces acting on horse. What are they?

•(1) Reaction by cart & (2) reaction by ground

•If the horse pushes the ground with a greater force than it pulls on the cart --> there will be a net force on the horse and acceleration occurs.

•System is horse & cart

•One horizontal force acts on horse and cart. What is that force?

•The push of the ground

•Divide the push (of ground) by mass (horse + cart) and you have acceleration entire system

•a = F/m

System 3

More horse and cart

•Forces can cancel if they are within the system

•Only external forces can change the motion of things

Action equals Reaction

• What about a piece of paper? How hard can you punch it? How hard can Mr. Jones punch it? With 50 lbs? 100 lbs? 200 lbs?

•What happens when you punch a wall?

•How hard does it punch you back?

•You cannot hit any object harder than it can hit you back.

• Remember, for every action between things, there is always a pair of oppositely directed forces that are equal in strength

• If you hit the water, the water is going to hit you…• Next time questions 6-4, 6-5