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Monday, November 30, 1998

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Lecture 35

Results from Exam #3

Overall Grades Thus Far

Sketch of Remainder of the Semester

Chapter 13: Hooke’s Law (again!)

Simple Harmonic Motion

Monday, November 30, 1998

Physics 111

Exam 3

Generally, most of

you did WAY

better this time!

I guess it really was a pretty EASY test…

As you’ll be able to tell from the grade

distribution which sits about 14 points

higher than either of the two previous

exams!

You should have been able to really

ENJOY your turkey dinner!

D

C

B

A

If I had to

put letter

grades on

this exam...

Overall Grades

(As seen on the Physics 111 web site…)

I have compiled overall grades up through

and including Exam #3. I have dropped your

lowest homework grade (as promised) but

have NOT dropped your lowest exam score(yet). I will do the latter at the end of the

semester. You will find your current overall

average marked on your test in purple crayon.

If your average

is below 70, you

might like to stop

by and visit me.

Please, do not leave town early…you’ll

just make my life …ahem…

….miserable???...

Upcoming Exams

Our final exam period is scheduled for the

very last possible day of finals’ week:

Friday, December 18, 1998!

(Is that pathetic enough?)

Upcoming Exams

This 2-hour exam period will contain TWO exams.

The first 50 minutes will be used for an exam

on the material that we cover over the next two

weeks. It will be structured very much like

Exam #3 (three sections, one of which will be

multiple choice---you do two of the three). You

will be permitted to use the usual 3”X5” note

card.

Upcoming Exams

Final Exam:

1 hour plus

comprehensive

5 sections, you do 4 (one will be multiple choice)

Each section somewhat shorter (25 points)]

8.5”X11” crib sheet + 3”X5” notecard from

Exam #4

I will give you math & constants but not

physics formulae

Upcoming Exams

Exam #4 (during the first 50 minutes of our

final exam period) counts the same as the

previous 3 exams (the best 3 scores from

Exams #1 - 4 count 15% each).

If you are REALLY happy with your performance

on the first 3 exams, simply notify me and you

will be permitted to SKIP Exam #4. In this case

you may show up 50 minutes into the exam

period.

(I’m guessing there won’t be too many of YOU!)

Upcoming Exams

Everyone MUST take the Final Exam.

No Exceptions!

You cannot pass this class without completing the final exam.

The final exam counts for everyone and

is worth 20% of your final grade.

There will be TWO more

homework assignments:

- due Monday, December 7

- due Friday, December 11

Other

Notes

There will be two more meetings of Lab:

- Thursday, December 3 (Lab 10)

- Thursday, December 10 (Lab Final=Party?)

We are going to skip Chapter 12 -- not that the

material in Chapter 12 isn’t important, but I think

the time is better spent on waves and sound

(Chapters 13 and 14).

Any Questions?

Chapter 13

Vibrations and Waves

We’ve already studies some vibrational motion,

when we we examining the curious behavior

of springs and objects that interact with them.

We will expand our studies to objects that

behave similarly to our spring, such as the

pendulum and rotating objects.

We’re going to start by reviewing some

of the basic properties of springs.

You may recognize this stuff…These are

the same notes I used back in Chapter 5!

So let’s go through them quickly just

to refresh our memories -- especially

after that long Thanksgiving Break!

x

l

Springs!

Springs!

However, if we compress or stretch the spring

by some amount x, then the spring is observed

to exert a Force in the opposite direction.

Hook discovered this force could be modeled

by the mathematical expression

F = - kx

Notice that this force operates along a linear line!

Force

x

Slope of this line

is -k, where k is

the spring constant.

Springs!

Springs!

Which means that if we looked at the plot

of Force versus compression/stretching x...

Force

x

Springs!

Springs!

Notice that the force is always in the

OPPOSITE direction of the displacement.

We call such a force a

Restoring Force

Because the force acts

to “restore” the particle

to its original position.

Force

F1

x

-x1

Springs!

Springs!

If we look at the work done by an applied

force which compresses the spring through

a distance (-x1)...

Work done BY the

external force ON

the spring.

This energy is stored in the spring...

Springs!

Springs!

Potential Energy of a spring is

So, for spring problems, we have a new

TOTAL MECHANICAL ENERGY given by

And it is THIS quantity which will be conserved

absent other, outside forces.

Concept Quiz!

Springs & Energy

A wealthy socialite,

bored of counting

his gold coins, decides

to play with his new

spring toy.

Predict the motion

of the mass at the

bottom of this

spring as explicitly

as possible.

Assume no friction and

no air resistance.

h

Now, sketch a plot

of the height of the

block above the floor

as a function of time.

What kind of

mathematical

functions (with

which you’re

familiar) result in

such a pattern?

Equilibrium

position

Amplitude

Amplitude

period

This type of oscillatory behavior is known as

Simple Harmonic Motion

NO!

Not simple

harmonica

music!

An object in simple harmonic motion displays

an acceleration that is proportional to the

displacement and in the opposite direction.