The physics of flinging

December 03, 2002 11:00 pm
Physics students at Baker High School launched medieval times into the present by way of trebuchets Tuesday. Sean Tomlinson loaded the sling to a trebuchet his group built, while one of his team members, Logan Mitchell, held the rope that would release the firing action. (Baker City Herald/S. John Collins).
Physics students at Baker High School launched medieval times into the present by way of trebuchets Tuesday. Sean Tomlinson loaded the sling to a trebuchet his group built, while one of his team members, Logan Mitchell, held the rope that would release the firing action. (Baker City Herald/S. John Collins).

By MIKE FERGUSON

Of the Baker City Herald

History may record them as castle-storming devices almost a thousand years old, but for the physics students at Baker High School, they looked more like a fun lesson in simple machines, friction, motion — and, in the end, gravity.

Vividly illustrated Newtonian gravity. With a flash of orange and a dull thud at the end.

Tuesday was try-out-your-homebuilt-trebuchet day at the high school football practice field for Glenda Staebler's physics students. Using wood, metal, pulleys, wire — everyday items lying around the garage or readily available at the lumberyard —students divided into small groups to build less-lethal replicas of the machine that replaced the catapult and was the dominant siege weapon in Europe from 1100 AD until about 1450 AD.

In those days, trebuchets were constructed near an opponent's castle and used to smash large holes in the walls. Stones weighing up to 1,000 pounds could be heaved 100 yards.

Plunderers of the era sometimes resorted to hurling dead animals and even diseased humans into the castle complex to spread disease.

But BHS students were more interested Tuesday in principle than plunder. Five trebuchet models were demonstrated; students were graded not only on gourd accuracy and trajectory, but on the ingenuity of their design and the journal they produced about their project.

While the spectacle of gazing at flying and then exploding pumpkins was clearly the draw for many of the parents and other onlookers who attended Tuesday's demonstration, there were, of course, scientific principles at work behind all that pumpkin-smashing.

A trebuchet is a lever that does work by applying force in some way to gain what engineers call a mechanical advantage — the ratio of the force which performs the useful work of the machine (the flinging of pumpkins) to the force which is applied to the machine (students holding up the counterweight until it's ready to descend and fling an arm forward and a pumpkin skyward).

The devices were as varied as the students themselves. Everett Coombes, Marlaena Kraft, Karl Jensen and Rhiannon Borisoff built the only metal entry out of an old boat trailer. They dubbed their trebuchet the PC 2002, for "Pumpkin Chucker."

"It was fun to build," Coombes said, "kind of like an episode of ‘Junkyard Wars.' But once we're done, grandpa's going to want his boat trailer back."

The other entries were made of wood. The main difference among the entries was the counterweight that students chose. One group simply stacked bags of sand to cock the trebuchet's arm. Another used a lead weight.

Matt Jager said his group —which also included Logan Mitchell, Kelly Poe and Sean Tomlinson —chose an oil drum filled with 420 pounds of water and sand because they'd read that the ideal ratio of weight to pumpkin was 100:1.

It took three strong boys to hold the rope that kept the barrel in the air, but when they let it go, the group's pumpkins sailed both longer and higher than any other group's. A piece of one pumpkin ended up about 240 feet from where it began.

"Wow!" said Jager's mother, Beth. "It's good to know he can defend himself."

Joelle Cockrum and Amy Servid managed just a 15-foot toss with their tiny trebuchet, but they weren't bothered by the modest result.

"We found materials lying around our houses, and we borrowed a clamp and a pulley from other things, which we need to put back later," Servid said. "We have the cutest entry for sure."

After she'd picked up the last bit of pumpkin from the grass, Staebler, a longtime mathematics teacher who's filling in for Laura Miller while she teaches physics for a year in Bulgaria, said she was pleased with how receptive her students were to the school's first-ever trebuchet experiment.

"I really didn't know what to expect," she said. "I thought I'd have to offer them extra credit if they showed up in costume, but fortunately, they all got pretty excited about flinging pumpkins."