Dealing with the fear of being a boring teacher.


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.

Shawn Cornally • October 8, 2010

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  1. Brendan October 10, 2010 - 5:27 pm

    So what’s this pressure cannon shoot? And what’s the propellant? It looks like there’s a crossbar in the back made of significantly smaller diameter tubing. If that’s where most of pressure travels, isn’t that a safety hazard? (Imagining PVC shards everywhere.)

    • Shawn October 10, 2010 - 5:55 pm

      @Brendan: Actually, it did fail, but it failed at a threaded joint in kind of a loud pop and hiss. I forced them to keep the governor down on the air pump to well below the tolerance of the PVC. It shot tennis balls and just used compressed air! It was awesome while it lasted.


  2. Mike October 8, 2010 - 9:48 pm

    Just chiming in to say that I really enjoy your posts. I’m in the midst of my second year teaching math/physics and your ideas are very tangible and enlightening.

    Once I manage to wrestle down the beasts of curriculum and institutional bureaucracy I hope to join you on this awesome journey of education you’ve set yourself up with.

    Leave you with a question: Creating and providing time to your students to actively experiment and create, do you find enough time to cover all the curriculum mandated? Does it matter?

    As a new teacher, I always get some hesitation of “running with it” and leaving our learning outcomes and curriculum behind. Even if it is for the better of our students.

    • Shawn October 9, 2010 - 7:30 pm

      @Mike: I guess I just ignore all of the silly stuff and do what I know is fun and enlightening for my kids. I’m sure it hasn’t earned me any friends with the more teetotalling members of my staff, which I mildly regrettable.

      My solution is: If it’s so freaking important, it will show up naturally. How much teasing out the specific content may need is the job of the professional educator.


  3. Sue VanHattum October 8, 2010 - 7:08 pm

    What’s the gender make-up of this class? Do girls enjoy destroying enemy castles?

    There’s an interesting bit in The Children’s Machine, by Seymour Papert, about the girls not being into the Legos (at a school project, I think), until they figured out what they wanted to do. (page 116)

    • Shawn October 9, 2010 - 7:29 pm

      @Sue: In my little experience, I’ve found that there’s not much of a difference between girls and boys doing the castle siege as there just is between the personalities of kids in general. The most tenacious group (era-specific tech, war paint, kilts, etc…) was composed of all females this semester. But, I’m at a pretty small school, so the N for my sample is low.


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