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

3/18

The English stole the limelightWhile history is clearThat an Italian earned the copyrightTo the Physics law presented here.

A generation before Newton--knighted though he was--Galileo GalileiHad already killed Newton's buzz.

Take Newton's Law of Inertia(you can steal it like he did).This "1st Law" as they call itWas discovered before Newton was a kid.

Galileo knew that objectsSimply do not change their speedUnless something else acts on themTo precipitate this need.

So objects that are movingWill keep moving as they wereAnd objects standing still at firstNeed a force to make them stir.You may not know of Newton's 2ndOr even of his 3rd,

But when you hear of his blatant copyingYou'll mark him as a lying, stealing nerd.

Now my aim is not to disrespectOr slander Newton's name.His contributions rightly earn himA considerable amount of fame.

All that I am saying(and on this I must be right)Is that the English are plainly guiltyOf stealing Italy's limelight.

Forza Italia!

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!

7/18VOLUNTEERS TO SHARE SOME OF THESE?

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)

12/18

(maximum)

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

15/18

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.

'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!

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.

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...

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.

This poem may not be so great,As others you have read.But with all this typing action (whew!)My fingers are nearly dead.

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!)

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.

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&filename=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