A completely useless day for the few juniors who showed up for class. Since it was also a "weather reserve day" which was reinstated due to the October snowstorm, attendance was down in general. At least we enjoyed bagels in the morning and pizza in the afternoon (my treat).

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Today I got my class roster for next year's AP Physics C course. It's all boys, again. Although this year I had one girl and 14 boys, the previous two years have been all male. And the year before that was just two girls.

It wasn't always this way. For a number of years, the class ran about 50/50. A significant number of changes have happened (course-wise, teacher-wise, school-wise) since then, so I'm finding it hard to pinpoint any particular variable as the culprit.

Suggestions? Thoughts?

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How long is a sound wave? What is the relationship between the
wavelength of sound from a Boomwacker and the length of the tube?
(Rather than telling students that the wavelength is double the tube
length, they are going to figure it out themselves.)

Part one: Determine the speed of sound in the tubes using a long
Boomwacker, a Vernier microphone and the snap of a finger.

Part two: Hit the Boomwacker in front of the microphone and get the
frequency spectrum. What do you notice about the spectrum (see
picture)? Find the frequency of the lowest resonance (first harmonic).
Calculate the wavelength. Repeat for various length Boomwackers. Graph
wavelength vs. tube length. What's the pattern? Repeat experiment for
Boomwackers capped at one end.

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Not satisfied with the paper strip "conveyer belt" I suggested students use, one group wanted to make a real conveyer belt for yesterday's investigation. (They actually built this a few days before when I showed the "I Love Lucy" candy factory clip.)

It took them about a half-hour. They were testing, failing, adjusting, repeating. And when it finally worked, the look on their faces was glorious.

I wish others would tinker, too.

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It's downright impossible to use slinkies to find the relationship between wave frequency, speed, and wavelength. (Well, unless you use standing waves, but that muddies rather than clarifies the physics at hand.)

But the relationship between candy frequency, conveyer belt speed, and candy spacing?

That's easy with buggies, adding machine paper, markers, and a metronome.

The beeping you hear in the background is the metronome set to 60 bpm (1 Hz). The kids easily vary their marker frequency to 2 Hz (in video), 1 Hz, 1/2 Hz, etc. by counting. And they swap fast and slow buggies to change the conveyer (i.e., wave) speed.

With some thinking, many kids can arrive at the wave equation just contemplating the "I Love Lucy" scenario. And that's what I've done in previous years. This is my first time trying it with the buggies first. A few kids were able to figure it out before doing any trials ("If I double the buggy speed, the dot spacing will double. If I double the frequency, the dot spacing will halve.") but I ask them for experimental confirmation anyway.

The other part I like is asking them what relationship exists between frequency and speed. They easily see that none exists because the frequency is controlled by the metronome (ie, the source) and the speed is controlled by the buggy (ie, the medium). Tomorrow, I hope I can help them l make the same connection to wave frequency and speed.

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Today was the AP Physics trip to Six Flags New England. It was the best year yet. Kids showed up on time, no traffic on the way there, short waits for rides, all-you-can-eat lunch buffet, the whole group stayed together the entire time, and Twisty's ice cream on the way home. And of course, physics.

While most physics trips to amusement parks focus on the rides (and I did collect some data using the Physics Gizmo app from Broken Airplane on my phone), my students were most interested in the physics behind the carnival games that were eating up their money.

The Ball Toss -- the ball must hit the board and bounce into the basket below.

Knock the Blocks Over -- all 3 blocks must leave the table

Crane Game -- pick up the prize with the crane

Ring Toss

Ladder Climb -- get both feet on the red ladder rung (2nd from top)

EXTRA CREDIT: From the video, determine the length of the bungee cord using TWO different methods.

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The AP Economics exam threw a wrench in my original plan for College-Prep Physics today. So we decided to enjoy the nice weather and go outside to determine the speed of sound.

In the video, Ryan is clapping wood blocks together at a rate of 2 claps per second (1/2 second per clap). Sophie is holding a digital metronome to help Ryan keep pace. As Ryan backs up, the sound of the clapping and the sight of the clapping will slowly fall out of step. When the sound of the claps is exactly out of sync with the sight of the claps (ie, we hear the sound when the blocks are furthest apart), we know that it took the sound 1/4 second to reach us. The we measure the distance between Ryan and us, and calculate the speed of sound.

Distance = 84 meters, so Speed = 336 m/s. Based on the air temperature outside, the speed of sound should be approximately 340 m/s. Not bad!

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Got the idea from this video. The water has baking soda added to increase conductivity (seems to work better than salt). However, I wasn't getting quite as much gas produced (lots of hydrogen, not much oxygen). And the kids were having a hard time balancing the cup on the battery while holding the test tubes. So after class, I just dropped the 9 V battery into the cup. Great results! I was able to completely fill small test tubes with hydrogen and oxygen. I was able to get a small "pop" by holding a flaming splint under the hydrogen test tube. I was not able to get a glowing ember to brighten by putting it inside the oxygen test tube. (The chem teachers told me they have similar troubles even when using the Hoffmann apparatus.)

It was also hard to get a 2:1 ratio of H2:O2 -- chem teachers said that O2 readily dissolves in water, so I'm not collecting all the O2 that is produced.

However, I like how simple this is...very easy for kids to recreate at home and show their siblings and parents!

UPDATE: Via @ryanabuck - Why O2 isn't produced when using salt water.

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One of the few evil tricks I do....

I tell the kids I'm going to administer a hearing test. If they can still hear the tuning fork after I've stopped it vibrating by touching it with my hand, then they have tinnitus and should stop listening to loud music in their headphones before they do any further damage.

Of course, the ringing they hear isn't tinnitus, but the sympathetic resonance of the other tuning fork nearby. They sure are relieved when I finally let them in on the joke.

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Perfect introduction to beats and beat frequency.

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My four favorite applets for showing the superposition of linear waves:

http://www2.biglobe.ne.jp/~norimari/science/JavaEd/e-wave2.html
http://www2.biglobe.ne.jp/~norimari/science/JavaEd/e-wave3.html
http://www2.biglobe.ne.jp/~norimari/science/JavaEd/e-wave4.html
http://www2.biglobe.ne.jp/~norimari/science/JavaEd/e-wave5.html

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We made this today. Kids thought the cups were totally awesome.

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Students each got a copy of the 2009 Mechanics multiple choice questions. Class split up into teams. Each team got a small whiteboard. On the computer projector, I pulled up a random number generator website and set the range from 1 to 35. Teams had 2 minutes to solve the MC problem given by the RNG. Points were awarded for correct answers, not for finishing first. If the RNG pulled a number we already had, the next question was worth double.

Nothing fancy. No prizes awarded. But since we rarely play games, the students were into it. We didn't get thought all the questions, but I gave them the answer key at the end of class. Tomorrow we'll do the same for the E&M multiple choice questions.

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Stopmotion animation we made using my Android phone and the Clay Frames app. Attempting to illustrate how adding energy to ice causes the thermal energy and phase energy of the system to increase over time.

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Eugenia Etkina and her physics education research group at Rutgers
University have put together a great resource: a collection of videos
for student investigation, rather than consumption.

You can view the collection here, sorted by topic:
http://paer.rutgers.edu/pt3/index.php

The two slinky videos above were taken from the collection, along with
the following questions:

1. What were the differences in the experiments in the first video?
What was the purpose of the second video?

2. Compare the speed of pulses of different amplitudes on the first Slinky.

3. Compare the speeds odf the pulses of the same amplitude on two
different Slinkies.

4. What do you think determines how fast a pulse will propagate on a Slinky?

5. What experiment you could preform to test your answer?

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How fast are the coils at the wave crest moving? Surprised? Why/why not?

(I can't believe I never noticed this before.)

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What effects does friction have on wave propagation?

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We've been watching some of the NOVA special on absolute zero in chemistry. It's got some great history which focuses on the experiments which drove the development of the models for cold and heat.

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