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Retooling the intro course

January 18, 2011

My intro course METIC showed that my students do not feel like they are learning as much as they could be (or I thought they were). I’m going to write on this a bit in the future, but I first want to capture a lesson we worked through two Thursdays ago, before I forget it.

In the first semester, my students were working through the modeling curriculum. If you’re a close reader of this blog, you will remember that I’ve previously expressed the thought that my intro students, in many ways, my intro students are just as capable as my honors students. However, after my final exam postmortem, I’m beginning to see places where this isn’t the case. I’m also beginning to wonder about the degree to which we need to focus on mathematical reasoning in my introductory physics class. So enough of the background, on with the lesson.

Having just studied constant acceleration (CAPM) before the break, I posed the question to my students, “What causes acceleration?”

Trying a bit of formative assessment, I gave out the following questions on a sheet of paper.

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Which resulted in us writing up all of the following ideas on the board:

IMG_0622

If you look closely at these, you’ll see we’re having some trouble distinguishing acceleration and its synonyms (speeding up/slowing down) from the things that cause acceleration. But that’s cool, this is all formative assessment—good to get ideas out there.

Rright in the middle of our discussion, one of my students said he wanted to illustrate his understanding with a graph, and luckily, I was in the mood to allow this opportunity to wander and so he came up and drew this graph:

A graph a student drew on the board to start a conversation.

And so I just asked, what do you think of this? It took a while, but eventually someone said “I don’t think we’ve ever seen an acceleration graph before. Shouldn’t that be a velocity graph?” And so we changed it, like this.

A revised version of the graph above.

And here’s where the confusion began. I asked what’s happening in the beginning of this graph, for the first 2 or so seconds. Is it speeding up or slowing down. This launched into a heated argument where so many students offered their argument for what is going on. Then someone said “We haven’t done anything like this—it isn’t constant acceleration.” “Awesome!,” I said—”Will that stop you?”

“No”, he said, and we did a quick peer-instruction stype poll where I asked tables to collect and sum quick responses to the question. It was an even split. I asked them to work together as groups and try to reason toward the answer, and after a couple minutes of discussion, I polled the tables again—”speeding up” was the near unanimous choice, except for the student who came up with the graph in the first place, who now had the courage to say he was totally confused (even though he basically drew everything right).

This led to some great teaching moments where students explained both how you could see object is getting faster, and how you could be confused into thinking the object is slowing down (slope is getting less steep, which means acceleration is decreasing, but if you confused this with a position graph, then you’d think the slope is the velocity, and that seems to be decreasing). This was a pretty awesome diversion.

Trying to pull a bunch of random points together, I stole this lesson from Bryan Battaglia, author of the great blog, New Physics Modeler. I just stood on a chair, and allows a slinky to hang down from my hand, asking the kids to make observations while another student wrote them up on the board. My kids came to the same conclusions as Bryan’s did—the coils at the top are spaced further apart than those at the bottom, and this is because the top coils are supporting the entire weight of the slinky, and so the coils must stretch more to exert more force.

Wii physics

Again, another seemingly random jump (but I will try to link them in the future, I swear), and I pulled out by brand new wiimote. I’ll admit that when I first saw the wii, I just thought it worked by magic, and it wasn’t until I saw Shawn Cornallay’s awesome post at Think Thank Thunk on Wiimotes, that I realized these things were just begging to come into my physics classroom.

And so, I set up Darwin Remote on my mac (this takes <2 minutes, while doing the same thing on my PC required learning how to install the correct bluetooth stack and bunch of other little programs, and ate up 90 minutes of my day).

I just asked my students—what does this measure? This turns out to be a very hard question, since my kids have this very strong intuition that it must measure position, otherwise, how would you play Super Smash Brothers?

Then I ask, how could you design an experiment to test whether it measures position? After some thinking, kids put the wiimote in one place, and leave it alone, and then they move it somewhere else, and leave it alone. And it turns out that the readings are all the same. So the wiimote can’t measure position (at least the part we’re curious about right now).

So what’s next? My students leap to velocity. Can the wii know if its sitting still or rolling on a skateboard at constant velocity? Nope. Conclusion: it isn’t a speedometer, either.

So what is this thing good for? Right then, students start to realize that when you lay the thing down, one axis reads “1”, and shortly there after someone says it’s 1 “g”. This leads to lots of experimentation, and eventually, students are able to see that the wii can somehow sense the pull of gravity, and use that to label all of the axes, x, y and z. We play a nice game where I say—pretend you’re a wii, and point your hand in the direction of +z, or -y.

Still, how does having a gravity detector help you to play games? What happens when you drop it? We try it, and kids see that all of the readings zero out while it’s falling, and the spike tremendously when hitting the ground. How could this be? It isn’t just measuring gravity (and I remind them we don’t call it gravity, anyway, it’s the gravitational force of the earth). Soon enough, one of my students realizes it must be measuring acceleration—and we test this by throwing the wiimote and seeing the readings get bigger when it is thrown and caught. Bingo. So what do you call something that measures acceleration? An accelerometer! But this one is still weird, since it reads “1g” when it isn’t accelerating at all. Why is that?

My hope is to help my students figure out exactly how the will works, and discover Newton’s second law in the process. The first step is to build their own accelerometers using springs and fishing weights. And that’s where we ended.

I had hoped that this would be a cool series of explorations for my students—I’m not sure that I met my goal, and many might have come away seeing it as a bunch of random, disconnected questions. And of course, all of this took place almost two weeks ago, when we last had class, thanks to a very extended snowcation, so they’re unlikely to remember any of it.

One Comment leave one →
  1. January 23, 2011 6:10 am

    I think you accomplished goal of “cool series of explorations.” They were exploring their physical world with hypotheses, observations, tests and conclusions. They were working to figure it all out. Kudos to you.

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