ThinkThankThunk

Dealing with the fear of being a boring teacher.

6

# Inquiry Stylee: Pressure Cannon

Building catapults/trebuchets/ballistas is not really a new thing for physics, but I try to come about things in a different way:

In a usual physics classroom, you do a bunch of math and then build some apparatus to see if your math works. The math  rarely works out, and what’s worse is that the students get a horribly skewed idea of how most ideas were discovered. They seem to believe that everything exists to be “verified” and this really is a tragedy. In short, the scientific method is boiled-down silliness developed solely for the purpose of making sure “thought process” gets checked off on the list of state-required curricula. <BLACKGOOFROMEYES> This, like grading responsibility, is not only ludicrous, but counter-productive.

After sending out the call for shoe boxes my class began to run wild with what weapons they could create. Mind you, they didn’t run wild with questions like “how do we minimize energetic losses?” or “how much kinematics can I do before building this thing?” They pretty much just found something interesting and ran with it. This is dangerous, stay tuned.

What this gains me is a classroom full of students who are building and testing. They are actually doing the science, so that afterward, we can have some motivation for asking the more mathematical questions. We can analyze our videos (taken for fun, not data) to see how far the trebuchet launched. We can watch the slow motion capture of the pressure cannon firing. Oh, yeah, did I forget to mention THE PRESSURE CANNON:

Designed, built, and characterized (and subsequently destroyed) by these students, not me.

Does a pressure cannon fit within the framework of medieval siege weaponry? Not really. Does it provide me a natural link to teaching about pressure and kinematics? Yes. Do the kids freaking love experimenting with it? That’s rhetorical.

After we’re done playing with our weapons, the kids just seem to eat up the idea that these two equations are the same:

$f(t) = c + bt+ at^2$

$x = x_0 + v_0t+\frac{1}{2}at^2$

Where once we were lost, now we are found. Oh, the quadratic equation is esoteric rambling to you, child? No longer: Its solutions are now where the other castle should be as we try to freakin’ destroy it. Done.

With a little unit analysis, the cryptic standard form of the parabola now has meaning. Guess how many physics students — who have also taken Algebra II — cannot identify this kinematics equation as a standard-form parabola? (A lot more than I’m comfortable with). I swear that I could see the light bulbs turning on, brightening, and then exploding like Dr. Manhattan was in the room.

I know “real world” is passe, but I think I want hyper-real. I seriously couldn’t care less what’s trendy, conservative, popular, or floating around twitter; I just want my class to be informative and awesome.

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Shawn Cornally • October 8, 2010

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