Remember - labs due this week

Gang,

I am out today (Monday, 12th). Your labs are due this week. If you want feedback on a draft, get it to me by Wednesday.

For C block, even though we don't meet on Friday, you may turn in your lab by then.

Okily dokily?

sean

practice problem

This is due on Friday for C block, and next Monday for E block.

Also, the formal lab is due NEXT Thursday for C block and NEXT Friday for E block. Drafts (for those who wish to submit them) should be turned in by no later than one class earlier.

In addition to finishing the lab questions, try this problem:

A convex lens has a focal length of 20 cm. Calculate di under these circumstances:

do = 60 cm

do = 40 cm

do = 30 cm

do = 20 cm

do = 10 cm

This may seem tedious, but the idea is to help you see what happens to di as the object approaches the focal length.

Lab questions

First, play around with this applet:

http://www.physics.metu.edu.tr/~bucurgat/ntnujava/Lens/lens_e.html

Here are questions to ponder for this formal lab:

1. How are convex lenses and concave mirrors similar?

2. What would happen to a real image if a mirror or lens were half-covered? Does it matter HOW you cover it?

3. What is going on with convex mirrors and concave lenses?

4. Pick a data point from your convex lens trials. Use the measured do and di to calculate an experimental focal length (f), using the equation:

1/f = 1/do + 1/di

5. Repeat 4 for a data point from your concave mirror trials.

6. How close are your calculated focal lengths to your "theoretical" (taken outside) focal lengths? Calculate percent errors:

% Error = (experimental - theoretical) / theoretical x 100

7. What does the lens equation have to say about the following cases? If helpful, make up a focal length and plug it into the lens equation.

do is much, much greater than f

do = 2f

do = f

do < f

8. Do these answers (to 7) make sense in light of your data?

9. How can you tell mathematically if an image is virtual or real?

10. How could one determine the focal length of a convex mirror or concave lens?

11. So, what can you say about how images form (in a convex lens or concave mirror) as do progresses from greater than 2f to within f?

12. Don't forget to address sources of error in your report and write a general conclusion about the lab.

Take-home quiz

Due Monday (for C block) and Tuesday (for E block):

Write out your work on a separate sheet of paper - you will be turning this in. It is due IN class at the beginning of class.

You may NOT use your notes for this, NOR may you use the blog or other online resources. Use the equations given. This is a low-stress way to give me a sense of what you know today.

n = c/v

c = 3 x 10^8 m/s

n1 sin (theta 1) = n2 sin (theta 2)

sin (critical angle) = 1/n

1. Consider a rectangular (but 3-d) piece of amber (n = 1.55). Light hits it an angle of 70 degrees (with respect to normal). The block is large enough so that the light can enter and exit without interference. Find the following:

a. a drawing that represents this problem

b. the angle of refraction inside the amber

c. the angle as the light ray leaves the amber

d. the critical angle of amber

e. speed of light inside amber

2. Consider a set of parallel lines hitting a convex lens. Draw this, showing exactly what the light rays do inside the lens and outside the lens (after they leave). Be clear and specific in your drawing.

Part 2 of homework. Play around with this applet (if time allows):

http://www.physics.metu.edu.tr/~bucurgat/ntnujava/Lens/lens_e.html

Unrelated, but fairly awesome.

http://gizmodo.com/5850643/what-the-hell-magnets-why-are-you-so-amazing?tag=science

http://andrewsullivan.thedailybeast.com/2011/11/ment-5.html

homework: C and E

Apologies to C-block for not posting this sooner. E-block, this is for Friday's class.

Before next class, find the definitions of:

focal point (or focal length)

thin lens (or mirror) equation - show the equation and tell what the variables represent.

Thanks very much. If you do not get to it, because you expected it last night, I understand.

In the lab, we will take this approach:

1. Set up optical bench, using a convex lens first. Your "object" will be a candle. Find a theoretical focal length, using the method discussed in class.

2. Vary the object distance (do) and note how the resulting images (if there are images) change.

3. Watch for these things, and make notes (in a table). Is the image:

a. bigger, smaller or same size as object

b. right-side up or upside down

c. real or virtual

d. where is the image located (di)?

4. You will repeat this for a concave lens, concave mirror and convex mirror.

5. In general, you will be seeking to determine how lenses and mirrors form images, when they form images and how they form images.

6. The lens/mirror equation is very good for making predictions as to how images are formed (and what types of images are formed). You will use the lens equation to calculate an experimental focal length and compare it to your theoretical focal length (with percent error).

7. Other lab questions will be posted.

Prep for Next Formal Lab homework

In our next formal lab, we will investigate how images form. Find out the meaning of these terms:

real image
virtual image
convex lens
concave lens
convex mirror
concave mirror
plane mirror

This is homework mainly for C block class, but E block class should do it if they have time (after the 5 critical angle problems).

Applet fun

http://stwww.weizmann.ac.il/Lasers/laserweb/Java/Twoangles2.htm

http://groups.physics.northwestern.edu/vpl/optics/snell.html

homework (C and E)

Review the material on:

Critical angle
Total internal reflection
Snell's law and how to get angles using the 2nd SIN function (inverse sine) on your calculator.

Also, answer these questions:

1. Calculate the critical angle for water (n = 1.33).

2. Determine the index of refraction of a substance with critical angle 50 degrees.

3. If you were trying to hit a fish with a spear (like in the movie, "Castaway"), should you aim above, below or at the fish in the water?

4. If you were trying to hit the fish in the previous problem with a laser, would your answer change? How so?

5. If the cosine of an angle is 0.4, what is the value of that angle?

Homework (E) - same as C below

Homework (C)
Have a go at these problems:

1. What is the index of refraction of a substance that reduces the speed of light to 75% of its value in a vacuum?

2. What is the speed of light in water (n = 1.33)?

3. A light ray enters a piece of plastic at an angle of 60 degrees (with respect to the normal line). It is refracted to an angle of 40 degrees. What is the index of refraction of the plastic? Assume that the index of refraction of air is 1.

4. Can an index of refraction be less than one?

5. What are the units of index of refraction?

6*. A light ray hits a piece of quartz (index of refraction = 1.46) at an angle of 30 degrees (with respect to normal). What is the angle of refraction inside the quartz? (Hint: Solve for the sine of the angle, and then take the 'inverse sine' on your calculator. That would be the 2nd SIN key.)

7. What is atmospheric refraction?

8*. How do prisms break up white light into colors?

* tricky

Refraction applets, FYI

http://www.launc.tased.edu.au/online/sciences/physics/refrac.html

http://www.lon-capa.org/~mmp/kap25/Snell/app.htm

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/refractionsnells.htm
(But note that the wave goes into a LESS dense medium, thus refracting AWAY from normal line.)

Enjoy!

Homework (C)

Have a go at these problems:

1. What is the index of refraction of a substance that reduces the speed of light to 75% of its value in a vacuum?

2. What is the speed of light in water (n = 1.33)?

3. A light ray enters a piece of plastic at an angle of 60 degrees (with respect to the normal line). It is refracted to an angle of 40 degrees. What is the index of refraction of the plastic? Assume that the index of refraction of air is 1.

4. Can an index of refraction be less than one?

5. What are the units of index of refraction?

6*. A light ray hits a piece of quartz (index of refraction = 1.46) at an angle of 30 degrees (with respect to normal). What is the angle of refraction inside the quartz? (Hint: Solve for the sine of the angle, and then take the 'inverse sine' on your calculator. That would be the 2nd SIN key.)

7. What is atmospheric refraction?

8*. How do prisms break up white light into colors?

* tricky

Homework E block

Investigate and write meaningful definitions for these concepts -

Index of refraction

Snell's law

homework (C)

Find the meaning/definition of the following:

Snell's law

Index of refraction

Also, examine your data/graphs - particularly the graph of sin (theta 2) vs. sin (theta 1). Does it make sense in light of Snell's law?

homework

C block - none! (You just finished the test.)

E block - study for test.

review board

Electromagnetic Spectrum

Recall that waves can be categorized into two major divisions:

Mechanical waves, which require a medium. These include sound, water and waves on a (guitar, etc.) string

Electromagnetic waves, which travel best where there is NO medium (vacuum), though they can typically travel through a medium as well. All electromagnetic waves can be represented on a chart, usually going from low frequency (radio waves) to high frequency (gamma rays). This translates to: long wavelength to short wavelength.

All of these EM waves travel at the same speed in a vacuum: the speed of light (c). Thus, the standard wave velocity equation becomes:

c = f l

where c is the speed of light (3 x 10^8 m/s), f is frequency (in Hz) and l (which should be lambda) is wavelength (in m).

HW

For C block - Go through your notes and generate a review page.

For E block - that will be your homework after Thursday's class. For tomorrow, make sure you did the Doppler problem creation already assigned.

Thanks!

Hw

Oops

I posted the homework (for E block) on the wrong blog - homework is to create and solve your own Doppler effect problem, with at least 4 parts.

This is the same hw as is due Wednesday for C block.

Thanks!

HW

C block - write and solve your own Dopper effect problem. It should have at least 4 parts, like the class example.

E block - same as earlier post. Find and interpret an equation for the Doppler effect.

Happy 3 day weekend!

Homework revisited

Doppler effect:

E block - find (and interpret) an equation for the Doppler effect

C block - define (formally) the Doppler effect and find an equation that represents it. Same as the previous blog entry.

Doppler Effect HW

Come to class with a working definition of the Doppler effect (equation optional).

Play with these applets:

http://www.lon-capa.org/~mmp/applist/doppler/d.htm

http://falstad.com/mathphysics.html

Run the Ripple tank applet -

http://falstad.com/ripple/

Homework

Practice for the upcoming quiz - which is class after next

1. Consider a string, 0.4 m long. The normal wave speed is 120 m/s. Find the wavelengths and frequencies of the first 3 harmonics. Draw the standing waves as well.

2. repeat the first problem for a tube open at both ends, if it is 0.8 m long and the speed of sound is 340 m/s.

3. Consider problem 2 - what would happen if the tube were capped on one end.

4. Consider a concert A - 440 hz. Find these frequencies:

A. The next 2 A notes
B. The note immediately after A.
c. The note 3 piano keys after A

Awesome.

http://www.youtube.com/watch?v=i31godfcZZ0&feature=relmfu

http://www.youtube.com/watch?v=XKRj-T4l-e8

Watch these!

And watch for more homework being assigned later today.

C block physics (IN CLASS) and E block physics homework

Folks - I'm out this morning. Here are notes for today's class:

We will be talking about waves in tubes/pipes (like brass instruments, woodwinds, etc.). Play around with the animations and take notes.

http://www.physics.smu.edu/~olness/www/05fall1320/applet/pipe-waves.html

http://ralphmuehleisen.com/animations.html


Ultimately, I want you to see that waves in a tube are the same (mathematically) as waves on a string - the biggest differences:

the waves are longitudinal/compressional, NOT transverse
the waves have antinodes at each end, NOT nodes

When an organ pipe/tube is open on both ends, you have antinodes (in a longitudinal wave) on both ends. This becomes (mathematically) the same as a vibrating string (though the string has nodes on both ends). The math looks like the same, however:

lambda (l) = 2L/n

The lowest harmonic (f1, where n = 1) is still found by using v = f l, and dividing v by l. Successive harmonics are 2f1, 3f1, 4f1, ....

So, a tube has a lowest possible resonant tone, but if air is pushed through it harder, higher harmonics can be heard. Think about the recorder - you may have learned how to play one in your younger years. Keeping your fingers in the same positions, but blowing a bit harder, gives higher tones.

Some things to try:

Work your way through lessons on PhysicsClassroom.com

http://www.physicsclassroom.com/Class/sound/

PARTICULARLY LESSONS 5c and 2a.

Here is a problem to try:

1. Consider a tube that is 0.8-m long. The speed of sound is 345 m/s. Find the following:

a. the wavelengths of the first 4 harmonics
b. the frequencies of the first 4 harmonics
c. the wave shapes of the first 5 harmonics - see the applet, as well as my note sheet below

Note that the mathematics in this problem are IDENTICAL to those of the standing waves in the string - the speed, however, is the speed of sound.

wave addition

Homework for C block (due M0nday)
Homework for E block (due Tuesday)

Add (or subtract, as the case may be) the following waves (either on paper or on calculator):

sin x + 2 sin x

sin 2x - sin x

(Note that the 2 in sin 2x refers to the frequency.)

sin x + 2 cos x

2 sin 2x + 3 cos 3x

4 sin x - 2 cos 2x

Now, I'm no wave snob - feel free to try your own instead of mine!

Ultimately, you're generating a "superposed" wave - a third wave that is the mathematical sum of the original waves.

Of course, you can add more than just 2 waves....

2 sin x + 3 cos 2x - sin 4x

And we haven't even talked about phase differences. Consider the difference between these waves:

sin x
sin (x + 45)

Can you see the difference?

Play and comment.

One more thing to check out....

..... after you've done the homework (on the previous blog entry):

http://www.youtube.com/watch?v=kBmRNkM9saA&feature=related

(That's from my class.)


These are not from my classes:

http://www.youtube.com/watch?v=JgKzx3tZ59A&feature=related

http://www.youtube.com/watch?v=YedgubRZva8&feature=related

http://www.youtube.com/watch?v=DGSU5rcHmRQ&feature=related

Wave addition

Folks,

Please play around with the following applets. Write down some comments - anything you find interesting, anything you learn, etc. Play around with the applets. Make comments.

Two things to keep in mind - one is very important. WHAT IS SUPERPOSITION (AKA Wave addition)? Another (less important) thing to consider: What are beats?


1. Observe what happens when two (positive amplitude) wave pulses interact.

http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=18

2. Then look at this one:

http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=19.0

3. Now this one:

http://webphysics.davidson.edu/applets/superposition/GroupVelocity.html

4. Here are some more details to examine:

http://www.kettering.edu/physics/drussell/Demos/superposition/superposition.html

http://zonalandeducation.com/mstm/physics/waves/interference/waveInterference2/WaveInterference2.html

5. More complex, but cool:

http://www.colorado.edu/physics/2000/applets/fourier.html

6. Also:

http://www.walter-fendt.de/ph14e/stwaverefl.htm

7. If you have time, check this out:

http://falstad.com/mathphysics.html

Click on the first link (Ripple Tank 2-d wave applet). Play around.

Enjoy!

wave problems - try all

Some review questions from the first 3 weeks.

1. Differentiate between mechanical and electromagnetic waves. Give examples.

2. Differentiate between longitudinal and transverse waves. Give examples.

3. Draw a wave and identify (or define) the following parts: crest, trough, amplitude, frequency, period.

4. What is the frequency of a wave that travels at 25 m/s, if 3 complete waves can fit in a 10-m space?

5. Draw the first 3 harmonics for a string that is 3-m in length. Also, find the first 3 frequencies and wavelengths, if the wave speed is 100 m/s.

6. The speed of sound (in air) is approximately 345 m/s. If you stand far from a mountainside and yell at it, the echo returns to your ear in 1.8 seconds. How far is the mountain from you?

7. Approximately how much greater is the speed of light than the speed of sound in air?

8. Discuss the physics of the Chladni plate.

9. Find the wavelength of a 89.7 MHz radio wave.

Draft due

Don't forget, both C and E block physics -

The draft of Lab 1 is due next class.

It is NOT required, but is strongly recommended. Also, the final version of this lab will be due the next class. There will likely be no time in class to work on it.

Formal lab guidelines

FIRST - THIS IS NOT YOUR HOMEWORK FOR NEXT CLASS. A draft lab will be due in TWO classes (E) or THREE classes (C).

The homework for next class is the previous blog posting (for E), and the homework posting before that (for C).

Lab format.

Typically, each lab should have the following items:

Title
Your name
Lab partner name(s)
Date performed

Purpose of experiment - a line or two telling me the purpose of your work

Hypothesis (where desired, as it is for this wave lab) - what do you anticipate will be true?

Introductory remarks - "In this experiment, we blah blah blah...." These can be short - it's the place where you say anything special about your approach to the problem.

Data tables - don't forget units

SAMPLE calculation, including relevant formulas used

Graphs, where relevant - there may be no relevant graphs for the wave lab. That part is up to you. Don't forget the units and axis labels.

Conclusion - To me, this is the meat of the lab report. Here are questions to consider:

Discuss the extent to which your hypothesis was validated (or rejected).
Discuss sources of error - be specific. Saying "human error" is somewhat meaningless.
Give suggestions for improvement.
Tell me something you learned and/or liked about the experiment.

The conclusion may be a few paragraphs or a couple of pages (or more, if you write a lot). Be specific, write well, use good grammar and spelling, etc.

In every case, you will be encouraged (but not required) to submit a draft lab. I will get it back to you the same day (in your mailbox) with some comments. The formal lab is typically due the class after that.

Points are deducted for late labs, unless there are extenuating circumstances.

Homework for E block class (and for C block class after Sep 28)

For NEXT class (E), answer these questions and expect to turn them in. You may use your notes, but NOT any other web resources.

1. What exactly is simple about the simple pendulum? What assumptions are made?

2. What is the connection between an oscillator (such as a pendulum or mass on a spring) and a wave?

3. If you have a simple pendulum that is 3-m long, what is its period of oscillation?

4. How long must a pendulum be so that it has a 0.25-s period?

5. What is the theoretical shape of a graph of period vs. length (for a simple pendulum)?

THIS IS DUE NEXT CLASS, TO BE HANDED IN SEPTEMBER 30.

Board notes (from E block class)

Homework

E block - see yesterday's assignment

C block - see yesterday's assignment for the E block class

Thanks!

Homework

C block

Finish HW from last class - definitions, etc. If you've done that, investigate the meaning of: harmonics, as they relate to waves on a string. We will begin a formal lab on Monday.

E block

You began a lab last class. Prepare a data table for formal collection. It's should contain the following variables: string length, harmonic number, wavelength, frequency, speed.

Also - now that you know A little about the experiment, write a short hypothesis related to this question: what relationship exists between frequency, wavelength, distance, harmonic number and wave speed? If you cannot suggest a relationship for all the variables (and that's totally understandable), suggest some kind of expected result. Your formal lab will begin with that hypothesis.

Thanks!

Wave homework

Sorry for the delay in posting.

Please investigate and/or define the following wave words:

Wavelength
Period
Frequency
Amplitude
Reflection
Refraction
Interference
Superposition

The last 3 are tricky.

Thanks, and sorry for the wait.

Graphing

Hiya.

For next class, download and play around with LoggerPro:

https://parkscience.pbworks.com/w/page/351062/FrontPage

Scroll down to "Computing at Park". The password (which you may need to open the program) is: technology

Enjoy!

Also, use Physicsclassroom.com to investigate the connection between oscillations and waves.

http://www.physicsclassroom.com/Class/waves/

Lessons 0, 1, and 2 are worth exploring.

See ya soon, see ya on the Moon!

Pendulum revisited

Use the data for the pendulum period versus the length.

Plot a graph of period versus length. Remember unit and axis labeling.

Is there a mathematical relationship that you can see?
Can you predict what an equation (of period versus length) might resemble?

Look up the following:

Period of a simple pendulum.

Usually, period is represented by a T, in seconds, and length is given as L (in meters). See if you can find an equation for this relationship.

Does this equation seem correct for your graph?

In general, this is a good place to start for class research:

http://www.physicsclassroom.com/

If you use this site, start with the physics tutorial link, and look for waves - and then Vibrations (and pendulum motion).

For next class, bring in a graph and your other findings.

Good luck!

Pendulu-mania!

You've now gathered some data for your pendulum. Your homework:

Create a graph of the data - something that pictorially represents any relationship between variables (if indeed there is any). Linear? Exponential? Nothing? Discuss. If you don't have enough data to create a decent graph, discuss trends (if any) that you see.

If you see a mathematical relationship, what does it appear to be? Can you imagine an equation type that might be happening here?

Discuss - what can be concluded so far?

Resist the urge to look up answers. Base your answers on your data.

Think about these things:

How much data is enough?
What does it take to convince someone that there is a relationship present?
What is causation versus correlation?

Anything else worth reporting?

The Whirlygig!

Physics phriends!

Welcome and thanks for an exciting first day. I don't know about you, but I'm pretty stoked for a great year.

To summarize your homework:

1. Does the whirligig spin clockwise or counterclockwise?
2. Can you reverse it (and how)?
3. Think of applications; where you've seen this behavior previously.
4. Explain! (As best you can, without research.)
5. Investigate the toilets and sinks in your home (and elsewhere, if you wish). Do they drain CW or CCW?

Thanks again, and welcome to physics!

SI Units

Howdy, and welcome to Physics!

Some comments on the first class. I speak about SI units at some length. To remind you:

Mass is measured based on a kilogram (kg) standard.
Length (or displacement or position) is based on a meter (m) standard.
Time is based on a second (s) standard.

How do we get these standards?

Length - meter (m)

- originally 1 ten-millionth the distance from north pole (of Earth) to equator
- then a distance between two fine lines engraved on a platinum-iridium bar
- (1960): 1,650,763.73 wavelengths of a particular orange-red light emitted by atoms of Kr-86 in a gas discharge tube
- (1983, current standard): the length of path traveled by light during a time interval of 1/299,792,458 seconds

That is, the speed of light is 299,792,458 m/s. This is the fastest speed that exists. Why this is is quite a subtle thing. Short answer: the only things that can travel that fast aren't "things" at all, but rather massless electromagnetic radiation. Low-mass things (particles) can travel in excess of 99% the speed of light.

Long answer: See relativity.

Time - second (s)

- Originally, the time for a pendulum (1-m long) to swing from one side of path to other
- Later, a fraction of mean solar day
- (1967): the time taken by 9,192,631,770 vibrations of a specific wavelength of light emitted by a cesium-133 atom

Mass - kilogram (kg)

- originally based on the mass of a cubic decimeter of water
- standard of mass is now the platinum-iridium cylinder kept at the International Bureau of Weights and Measures near Paris
- secondary standards are based on this
- 1 u (atomic mass unit, or AMU) = 1.6605402 x 10^-27 kg
- so, the Carbon-12 atom is 12 u in mass

Volume - liter (l)

- volume occupied by a mass of 1 kg of pure water at certain conditions
- 1.000028 decimeters cubed
- ml is approximately 1 cc

Temperature - kelvin (K)

- 1/273.16 of the thermodynamic temperature of the triple point of water (1 K = 1 degree C)
- degrees C + 273.15
- 0 K = absolute zero

For further reading:

http://en.wikipedia.org/wiki/SI_units

http://en.wikipedia.org/wiki/Metric_system#History

>

In addition, we spoke about the spherocity of the Earth and how we know its size. I've written about this previously. Please see the blog entries below:

http://howdoweknowthat.blogspot.com/2009/07/how-do-we-know-that-earth-is-spherical.html

http://howdoweknowthat.blogspot.com/2009/07/so-how-big-is-earth.html


Physics - Yeah!!!