Meet Me In The Back of the Room; Bring Nothing

Yesterday, I told them too much. I made the pattern with how the wavelength, frequency, and speed change when a wave enters a new medium. So they didn't really think about those problems; they played WTF again. (What's the formula rather than thinking.)

Today, after doing some whiteboarding, I wanted to try the interference pattern with overhead transparencies. But I hadn't had space in my brain to figure out the pattern of questioning I wanted to do, so it wasn't very coherent. I figured it out more by my afternoon period, but I still didn't feel good about. More interesting stuff tomorrow.

I didn't write a blog for Friday; it was a test day.

Today, I passed back the test. I wanted to point out a particular problem that almost every student missed. The problem didn't look like a problem we haven't really done, and I just drew a simple diagram of the problem. Over half the class immediately understood the answer. The problem I have is too little meaning making with drawing and too much WTF. (WTF stands for What's the Formula.) I really like this problem, and I don't want to say much more, because I want to use the problem for a few more years, and I know at least one student (Hello from Walmart!) is reading this.

After going over the test, I wanted to show more of the power of drawing:

We are building slowly to a way of drawing constructive and destructive interference in two dimensions. 

We got out the tiny little wave tank, which is so worth it, since I use them so much more than I would ever use the old larger wave tanks. We saw just how much more difficult it would be to draw the waves. We then went out into the hallway, held hands, and shows how the waves change direction when you go from a faster medium to a slower medium. I like emphasizing that we use this more difficult model because we can predict more using this model.

In one class, I finally said why I called this unit the mechanical wave model, since mechanical waves cannot use a vacuum as its medium. (I always mention that a vacuum is not a vacuum cleaner, as I say every time, since I have had students not understand why I kept talking about rug care in my class.) So we did a few old AP problems in the few minutes we had before a spirit week assembly, and I felt it helped students solidify their understanding. 

I know these are just simple cylinders with a piston, but I really like how I can do two labs in one day with this equipment, in two different ways. 

First, we use the speaker, tune it to a frequency (they seem to like 440 Hz, which is the same as our bell, and 670 Hz, for a reason that every dang teacher knows now. If you don't, my word, I am jealous.)

Now, when you find the resonances, where the wave constructively interferes with itself, you can use that, along with the wave equation, to find the speed of sound. (Many students get an answer off by a factor of two, which makes for a great discussion.)

Then, we can replace the speaker with a microphone, and do a SNAP. You can see the initial snap, and then, milliseconds later, the echo of the snap of the sound wave that went down the tube. So then again, I ask them to find the speed of sound using distance and time. (Many students get an answer off by a factor of two, which makes for a great discussion.)

I absolutely love it when students get both answers off by a factor of two, for two wildly different reasons.