Supersonic ping pong ball blows watermelon to pieces

9:45 AM, Apr 26, 2013   |    comments
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Written by
Joel Aschbrenner, Des Moines Register

How do you get a pack of 10- and 11-year-old boys interested in science and engineering?

Exploding a watermelon with a supersonic table tennis ball cannon is a good start.

That's what Steve Lawyer, a den master with Cub Scout Pack 119 based in New Virginia south of Des Moines, had in mind. A group of 12 Webelos - fourth- and fifth-grade Cubs - built and tested an air-powered ball cannon to earn their engineering activity badge.

The results were more powerful than they imagined.

"You never actually see the ball," said Josh Sandusky, a 10-year-old from St. Charles who explained all the ins and outs of the cannon. "You just hear a loud boom, and then see whatever you shot explode."

The pack went to Applied Art and Technology, an Urbandale design studio, on Thursday afternoon to put their cannon to the test.

Here's how it works:

• A ball is placed in a 10-foot, 1.5-inch-diameter PVC pipe. The ends are covered with butcher paper using pipe fitters' sealant. A pump sucks the air out of the tube, creating a vacuum.

• A heavier PVC pipe is attached to the tail end of the 10-foot tube. It's pumped full of air, to a pressure of over 100 pounds per square inch.

• Eventually the pressure becomes too great, ripping through the butcher paper and pushing the ball through the vacuum at speeds faster than a passenger jet, the Cubs said.

During one test Thursday, the pack employed a borrowed chronograph - a piece of equipment often used to measure guns' muzzle velocity. It clocked the ball at 1,290 feet per second, or about 880 mph.

The speed of sound is roughly 768 mph.

Local freelance photographer Paul Hickey filmed the tests with a high-speed camera, shooting at 400 frames per second. The ball looked like a blur, ripping a Coke can in half and shattering into tiny white fragments.

Lawyer, a former Iowa State University physics major and now an attorney, got the idea for the cannon when he read a story about a team of Purdue University graduate students who built a similar cannon that broke the speed of sound.

"We thought we would try to take on that challenge," said Josh, the fourth-grade student from St. Charles. "We wanted to be the first in Iowa to break the sound barrier."

The boys worked on the cannon every week for two months.

They ran two dozen tests. Den leader Ron Hartfield accidentally lodged a ball in his garage door in one test.

Trevor Sims, 10, of Truro showed off a tattered notebook with all the test results, design sketches and safety procedures for the experiment.

His cousin Colton Sims, 11, also of Truro, said the experiment taught him that "there's a lot more to engineering than one step."

For the ultimate test, the Cubs set a watermelon at the business end of the cannon.

As a loud pop filled the studio, the melon burst into pieces, splattering the boys and eliciting giggles from everybody.

Des Moines Register

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