You can tell the quality of the science by the scale of the props involved

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Back in the fall, my Bride & I went in to the Critter's 5th grade class to teach chemistry for the day. Which was rich in irony, as I struggled to find the joy in chemistry when I was a student. But the experience itself was fun - fifth graders are at that great age where they're young enough that they're not too cool to express interest in a subject, and old enough that you can make fun of them without creating a need for lasting therapy.  I told the teachers when they hit the physics & engineering segment of the curriculum, let me know. That's my wheelhouse, and I had a couple of ideas I thought might be fun. 

Here is a truckful of ideas:

I asked the teachers how many kids to expect, and if we could consolidate the classes. I enjoyed the smaller groups of individual classes for the dry ice demonstrations in the fall. But with the larger catapults, we needed to move outside (for obvious reasons), and I was a bit worried about the higher likelihood of catastrophic breakdown. Which is a lesson of a different kind, I suppose. 

Soon enough, the first horde of kids were crossing the field. 

My lesson plan was more or less as follows: 

  • Intro: Simple machines, Complex machines, and Hurling Produce
  • The basic lever in your arm - come up and throw a potato
  • Extending the lever - a lacrosse stick hurls potatoes further
  • The Slingshot - using potential energy in a device to save my rotator cuff because I am old
  • How the Romans did it - A torsion engine, an onager, and an intro to siege warfare
  • Le Trebuchet - Counterweights & hurling bigger produce with an improved design
  • The Potato Cannon - The chemical potential energy of Aquanet, and a really loud noise 

There was a lot of improvisation in this basic plan, but I showed up with a tub full of cabbages, a couple of watermelons and sacks of potatoes, and figured we could make a go of it. 

I showed up an hour early to set up. I had promised the Critter that I would do my best to embarrass her, so I brought an extra prop or two along. I did a good portion of the lecture while wearing my Roman legionnaire helmet.  The other kids seemed to dig it. 

(Someone in the school administration office asked me 'Where did you ever find that helmet!?'  To which I answered: On the internet, I am two clicks away from just about anything.)

The sling shot and lacrosse sticks served as simple ways to get the kids up and into the action: we'd hurl about half a sack of potatoes down range, and then other kids were jumping to volunteer to run down and collect them. Honestly, I probably didn't have to do anything more than let the kids play with this for an hour, but I had a few other props prepared, so we moved it along.  

I knew from the get go that I wanted to make another catapult. Bigger this time, and more durable. So I started with the same basic plan, and scaled it up a couple of notches. I scavenged some wheels off a non-functioning wagon, and added more cross-bracing. 

Soon enough, I had a working potato-onager. 

I actually ended up modifying this further. The onager is under a lot of stress, and has a bad tendency to shake itself apart after a few throws.

I switched out the arm for a metal pipe, and added a more durable & functional sling to the end to give it the arc & trajectory I wanted. 

This is me, apparently incorporating an explanation of how the Bangles had a hit song in the mid-80's with such a stupid song. 

By comparison, the trebuchet was simple to build. And a lot bigger. It only took me a couple of hours to knock it together, and scrounge about 125 lbs of weight to serve as a counterweight. (I knew those dumb bells would eventually come in handy).

I had the kids throw some cabbages by hand for comparison, and then we tried launching them from le trebuchet.  

The trebuchet won. 

Then we upped the ante, and launched a watermelon. 

It was almost 90 degrees out. The kids spontaneously chased the smashed watermelon and scooped it up off the grass as a treat. 

We ended with a potato cannon that I borrowed from one of my colleagues. I've never built one, though I've thought about it a couple of times. (there are countless versions of them on the internet). 

A little bit of PVC, some aquanet and a click-starter from a Weber, and you've got a pretty serious projectile demonstration. 

It's also loud. Which makes it even more fun. 

Once again, I owe a huge debt of gratitude to the book 'The Art of the Catapult' by William Gurstelle. There's nothing here that took more than a few hours to knock together, and provided a huge amount of fun, and hopefully some tactile examples of some of the hardcore physics concepts that the kids will remember, and maybe want to learn more about.  And of course to the teachers for inviting us in to make a fool of ourselves throwing various forms of produce around the field for an afternoon. 

Plus: It gave me an excuse to buy a legionnaire’s  helmet. 

Everybody wins. 

Do you think this will put me on the Homeland Defense watch list?

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A year or two ago, I picked up a copy of Backyard Ballistics, because I had always wanted to know how to build a potato cannon. (I know, I know: who doesn't, right?) At the same time, I found another book by the same author, The Art of the Catapult, which contains the plans and history of everything from your basic ballistae, right up through a very Frenchified trebuchet. Cool. This Sunday was a beautiful, chill morning, and I went down to my local hardware store, and bought all the necessary ingredients to make my very own onager. Just like Julius Caesar would have had.
The Critter helped me put the whole thing together, measuring and holding the pieces for me as I cut them. (With a hand saw, since all of my power tools are somewhere in storage, back in California land. Do you know when the last time I had to cut more than one thing with a hand saw? Never. That's when. Because God invented power tools for reason. But I highly recommend the saw in this picture to anyone who does have need to use a hand tool, or any of the Japanese back saws for that matter. It's quick and easy to use, thin and whippy like you never knew you always wanted your saw to be. It's the pre-skank Brintey Spears of hand saws.)
In no time, we had the thing put together. We scaled ours to be about three feet long (the one in the book is scaled for 10 inches, but I knew we could do better than that.) The difference between a catapult and an onager is the spoon on then end is replaced with a kind of sling mechanism. Either you can have a sacrificial sling (tied to the ammunition so that it launches along with your big rock), or one that opens up at the apex and hurls your ammunition with greater force. We chose to launch potatoes.
I really wasn't sure how far this thing was going to go. "Onager" translates from the Latin for "Mule". As in "Kicks-like-a-". So I had the whole thing rigged to be triggered from several feet away. The force is provided by a 'torsion engine' - a rope which is looped around the throwing arm and then twisted extremely tightly around and around to provide the potential energy, to that cocking and loading the mechanism (the Critter insisted that she be the one to load the potato) was a bit of an adventure. See? This whole thing really is a science lesson for the kid.
The first time we fired the thing, the potato flew (and I measured) not quite 3 feet. Julius Caesar would not be impressed. A few more tests, and twisting the torsion engine tighter and tighter each time, we got the potato-hurling to reach new heights at about 9 yards. If we include the distance it rolled after striking the ground. OK, so we weren't ready to go a-sieging, but we were making progress. Then the worst happened.
One twist too many, and the frame splintered. When I scaled up the design, I guess I didn't take into account the additional bracing needed, and our catapulting was over for the day just as the sun started to set. During the afternoon's construction, several of my neighbors wandered by and invariably asked what we were up to, to which I just pointed to or held up the book. My male neighbors were all keenly interested immediately, and we would exchange grunts of mutual admiration for twenty minutes while chugging beers and setting fire to large hunks of meat or other appropriately masculine signs of respect. My female neighbors all asked the same question, "Why do you want to build a catapult?" So I can hurl things, silly woman. I bet Caesar never got asked that question. The Critter never questioned the need to hurl things, fortunately. I can only conclude that there is something in the female development cycle which kicks in sometime between adolescence and getting married, and that I have a few more years of semi-destructive science projects to do with her yet. I've bought more wood at the hardware store today. The next catapult will be stronger and bigger. Incidentally, if you think I'm insane for building my own catapult, well, I'm not alone. This guy's puts our little potato-hurler to shame...
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