So, being a science geek has its upsides sometimes. Last weekend we went up to Yankee Siege, a place where real-life geeks demonstrate some physics principles using a trebuchet and some pumpkins. How do they do this? Well, they use the trebuchet (pronounced “TREH-boo-shay”) to fling pumpkins (and not just any old pumpkins—these are regulation competition pumpkins) at a mock castle some 1,200 yards away. Yankee Siege folks are the World Champions at Punkin Chunkin, having used their trebuchet to fling a pumpkin 1,394 feet. Here are two pictures of their counterweight trebuchet, a 35-40,000-pound handmade machine. The counterweight is the big four-sided diamond of wood hanging down from the lever (which in the second picture is pointed nearly down to the ground in “launch” configuration—in fact, it has just started its launch sequence. (You can read more about its construction here.)
Where does the physics come in? From Wikipedia, I found this: “The object of a good design is to transfer as much energy as possible from the falling counterweight (the diamond-shaped box behind the lever arm in the second picture) into the projectile (in this case, a pumpkin in a net that is attached by a rope to the end of the long, pointy lever arm (pointing up in the first picture and in action in the second picture). The maximum range for a hypothetical 100% energy transfer, Rmax, of the projectile can be shown to be Rmax = 2hmc / mp, where h is the distance the counterweight falls, and mc and mp are the mass of the counterweight (the box) and projectile (the pumpkin), respectively. The efficiency of a real trebuchet is then easily determined as the ratio of the actual range achieved to the calculated maximum range.”
So, in other words, you’re taking the energy of the counterweight (which is really heavy) as it falls to the ground under the force of gravity and using it to fling the pumpkin (which might weigh 20 pounds) as far as you can.
At Yankee Siege on Saturday afternoon we saw them fling pumpkins into the woods behind their fake castle, at least 1,300 feet away. We got to see them do it three times (it takes about a half an hour to get the trebuchet ready for a launch attempt). Between times we wandered around and looked at the machinery, got some cold cider to drink, and visited the farm stand across the road. It was a major hoot, and we got to see physics in action on a warm, colorful autumn afternoon.
Wanna try your own hand at designing a virtual trebuchet? Visit the The Treb Challenge.