Storm in a Teacup: The Physics of Everyday Life(64)

A trebuchet is an extremely ingenious device that was developed over many centuries with the input of several civilizations: the early Chinese, the Byzantine and Islamic empires, and finally western Europe. When it came of age in the eleventh and twelfth centuries, it proved itself a monstrous lumbering brute, capable of demolishing castles that had previously been thought impregnable. A trebuchet could hurl 200-pound rocks over hundreds of yards. Siege engines like this contributed to the disappearance of motte-and-bailey castles (strategically useful but made only of wood and earth). Solid stone was the only defense, and so stone fortresses became the norm.

The benefits of the trebuchet were the same for me and my team as for the medieval warmongers: It’s mechanically simple and extremely effective. We borrowed scaffolding poles from a local building site, dug around in the college skip for stuff to make a sling from, persuaded the technicians in the Cavendish Laboratory to let me have a 16-foot—long metal beam, gathered all this stuff together at the top of the college playing fields, and set to work. Churchill College in Cambridge had been my home for nearly eight years by then, and the college staff were used both to me and to the sudden appearance of novel contraptions. Looking back, I’m still astonished at (and extremely grateful for) the cheerful acceptance that met any of the students whenever we had a new daft idea. At the opposite end of the playing field that week, someone else was testing a stratospheric balloon to send a teddy bear into space.

The basic structure of a trebuchet is very simple. You build a frame that gives you a pivot point which is perhaps 6 to 9 feet off the ground. Then you attach one long beam to that, like a giant seesaw, but you position the pivot point so that there’s much more of the beam on one side than the other. Now you’ve got an A-frame that looks as though it’s got a long stick laid across the top. The long end is the one that starts touching the ground. You attach a sling to the long end, and lay the sling on the ground underneath the frame. The first time we assembled it all, it was a beautiful sunny day, perfect for launching anything.

Then we hit a problem. The beautiful thing about a trebuchet (unless you’re the one who’s about to get a rock thrown at you) is that it uses gravity to spin the seesaw and the sling. You attach a heavy weight to the short end of the seesaw, and then as you drop the weight, it pulls the seesaw on your side down very quickly. The whole beam spins around the pivot, tracing out a vertical circle, and the sling also spins around the other end of the beam. So you’ve got lots of very fast rotation, and the projectile in the sling is spinning around the pivot because it’s being pulled inward by the sling. So far, so good. The first task was to get to this point, but we couldn’t find a weight that was heavy enough to move everything. I offered to swing from the beam myself as a human weight, but even I wasn’t quite heavy enough. We were stumped. That night, I spent a while pouring out my frustration to another set of friends, batting away their suggestions that I should just eat more cake. Then one of them offered me his scuba-diving weights. So the next day, I rigged myself up with a belt carrying 22 pounds of diving weights, and we tried again. This time, it worked perfectly. I swung under the pivot, the seesaw swung over the top, and the sling swung over the top of that. Everything was spinning. Now it was time for the next step.

The sling is only held in place by a small loop, and the trick to it all is that when the sling is almost at its highest point, the loop slips off. The sling is effectively broken. That means that the force that was pulling the projectile inward and keeping it on the circle has vanished. Now the situation has changed. At this moment, the projectile in the sling is traveling forward and upward very fast. As soon as it’s free of the inward force, it just keeps going in a straight line. Since it was traveling forward and upward before, it keeps going forward and upward. But it doesn’t go directly outward from the center of the spin. It carries on going sideways, as if following a line that sat on top of the spin circle. That was the theory. We put a shoe in the sling and lined everything up. I faced away from the playing field and swung down on the seesaw. The other end of the seesaw swung up, dragging the sling up, around, and over the pivot. At exactly the right moment (first time!) the sling released, and the shoe went flying over the top of my head, out on to the playing field. I wouldn’t ever want to do that with a rock, but the shoe proved the point perfectly. Our contraption could at least throw a wellington boot, and in the time we had, that was the best we could do. After a bit more practice, we took our frame apart ready to transport to the competition the next day.

Arriving at the Dorset Steam Fair poked a large pin in our buoyant bubble of confidence. Every other team consisted of middle-aged men who had spent months in garages building gloriously decorated contraptions for throwing wellies. Our small pile of scaffolding poles and discarded carpet, collected over just a few days, looked sparse and unloved. But we put a brave face on it and put it all back together. The competition officials (also middle-aged men) came around to look at it. “It’s silly to swing from it,” one said. “You should do what the medieval warriors did, and just pull the lever down with a piece of rope. That’ll work far better.” My protests that the counterweight was the invention that had led to the success of this device went unheard. The reason it never made it as a powerful siege weapon before the eleventh century is precisely because people tried to do it by manpower. But the officials stuck their hands in their pockets, opined that pulling on a rope was a far better idea, hinted that we enthusiastic but inexperienced females should be grateful that we were getting extra help from them, and didn’t leave until my teammates had given in and agreed with them. There was no time to argue. The time of the competition had come.