Transcript Lesson 12x - MrLaFazia.com

PHY111: Summer 201253
Lesson 12: Linear Mechanics II
-
Nature of Forces
Force Vocabulary
1/18
Demo: Inertia on a Stick
• Newton’s 1st Law of Inertia (or Galileo’s Law of
Inertia!)
• Two scenarios:
– #1: Object in motion…
– #2: Object at rest…
2/18
Poem:
Newton Stole the Limelight, by D. LaFazia
The English stole the limelight
While history is clear
That an Italian earned the copyright
To the Physics law presented here.
A generation before Newton
--knighted though he was-Galileo Galilei
Had already killed Newton's buzz.
Take Newton's Law of Inertia
(you can steal it like he did).
This "1st Law" as they call it
Was discovered before Newton was a kid.
Or even of his 3rd,
But when you hear of his blatant copying
You'll mark him as a lying, stealing nerd.
Now my aim is not to disrespect
Or slander Newton's name.
His contributions rightly earn him
A considerable amount of fame.
All that I am saying
(and on this I must be right)
Is that the English are plainly guilty
Of stealing Italy's limelight.
Forza Italia!
Galileo knew that objects
Simply do not change their speed
Unless something else acts on them
To precipitate this need.
So objects that are moving
Will keep moving as they were
And objects standing still at first
Need a force to make them stir.
You may not know of Newton's 2nd
3/18
Discussion
• “What is a force”?
– Is it a physical thing?
– Are there many types?
– Is there a single equation that describes forces?
• “How do forces relate to our Energy
concepts”?
4/18
Discussion:
• Complete this with a partner and discuss:
“List out Newton’s 3 Laws of Motion in your own
words. Then, imagine that you are trying to
explain these laws to a child. Write out how you
would do this. How would you know that the
child understood your explanation”?
• NOTE: If you can do this without writing it down, be my
guest. The point is the quality of the discussion!
5/18
Interactive Physics
Interactive Physics
• Sim. 6.1 – Discussion: “Why does the heavier
object NOT fall faster”??
• Open and view Simulation 6.3….discuss.
6/18
Discuss With A Partner
• “Describe an experience you have had in life
where you have had the following things
happen (or have caused these things to
happen)”:
• Had to keep an object moving (how did you do this?)
• Had to keep an object FROM moving (how did you do
this?)
• Had to stop an object while it was already IN motion
(again, how did you do this?)
• Try to provide a name (of your own choosing) for each
force involved!
VOLUNTEERS TO SHARE SOME OF THESE?
7/18
Definitions (w/ Simulations)
– Mass vs. Weight
• PLEASE learn the difference.
– European scales?
– American vs. European Scales on the Moon?
• Simulation: Interactive Physics 4.1; 4.2.
– Inertia
• Note that this is NOT an actual “quantity.”
• Simulation: Interactive Physics 6.4.
– Net-Force
• Simulation: Interactive Physics 6.5; 6.6.
– the Equilibrium Rule
• possible both while moving AND while not moving
– terminal speed (or “terminal velocity”)
• Simulation: Interactive Physics 5.1; 5.2.
– support force
• also known as the “normal” or “perpendicular” force
– I primarily call it the “Normal” force…just so you know.
8/18
LAB 8: Hooke’s Law
• We have already used this LAW! Now we will
“discover” it.
• Hooke’s Law (eXcel spreadsheet)
– complete these in groups at your lab stations
– http://mrlafazia.com/labs/HookesLaw.xls
– reflection: Hooke’s Law practicum analysis
questions (#’s 1-6)
• as projected onto board
– THESE will be your graded lab write-up portion (you only need
to turn in 1 paper per group).
9/18
Friction Video
• Here is a Paul Hewitt video clip… on calculating
Kinetic (a.k.a. “sliding”) Friction:
• http://www.youtube.com/watch?v=bWabHxouJW4
• Think about how this goes with the notes on
frictional forces which follow.
10/18
Notes on Frictional Forces (1 of 2)
11/18
Notes on Frictional Forces (2 of 2)
(maximum)
12/18
Vid Clips on friction (examples!)
These 2 clips were made by Derek Owens,
an instructor of Math/Physics from GA/SC.
• Static Friction:
• http://www.youtube.com/watch?v=P0XbjUVES8Q
• Kinetic Friction:
• http://www.youtube.com/watch?v=vvtJbs179UY&feature=related
13/18
Lab 9: Frictional Forces
• Review: What are cold weld bonds?
• Review: What is the difference between kinetic
and static friction? (physically and numerically?)
• Frictional Forces Lab:
– A participation-graded lab
• Vary with objects of differing weights and surfaces of
different coefficients-of-friction.
• In Logger-Pro (or LabQuests), be sure NOT to overwrite old
data, so that we may compare each run. Otherwise, you will
want to sketch and record the rough values for your results.
• This is a participation-based lab grade!
14/18
Poem:
When Push Comes to Shove, by D. LaFazia
There comes a time in each man's life,
Where he must push against the World.
But often he finds that cold weld bonds,
Are reluctant to release their hold.
A push or pull, that is a force,
And Newton's Laws declare,
That if there's no floor or wall to push against,
You can bet it'll push on AIR.
'Though it's nice to have sometimes,
Friction still does take its toll.
But imagine if it did not happen...
Your tires wouldn't even roll!
This poem may not be so great,
As others you have read.
But with all this typing action (whew!)
My fingers are nearly dead.
And then there's this whole idea,
That whatever action we take, a reaction's there.
We may not see it, or feel it, or hear it,
But you can bet that it is here.
With each push down on the keyboard keys,
They resist me while they move.
Accelerating til they've stopped,
They pop back up (a spring-forced groove!)
If I push a box and it doesn't move,
The box is pushing back the same!
That's friction in reaction, for you, mate,
And it drives me quite insane...
I challenge you to find a case,
Where Newton's Laws ain't true.
Cuz in this world, when you start to shove,
The world pushes back at you.
15/18
Grades/Assignments:
Read Sections 5.1-5.6.
Lab 8 should be turned in before you leave.
Lab 9 was a participation-based lab grade, so no paper to turn in.
Homework 3 (Newton’s Laws Vocab. WKST) is assigned and will
be due on Wednesday the 18th. The link to this is provided on
the next slide. During a later lesson I will give you the solutions
to this for you to check your answers.
16/18
Vocabulary Worksheet:
• Complete the below vocabulary sheet:
http://dev.physicslab.org/Document.aspx?doctype=5&f
ilename=Dynamics_NewtonsLawsVocabulary.xml
We will go over this after it is collected (as we did with
the Energy vocab. sheet), so make an extra copy for
yourself for when we go over the solutions.
17/18
Looking Ahead:
Lesson 13 will be an online class. Here we will meld parts I and
II of Linear Mechanics into a 3rd part on Momentum and
Impulse.
18/18