The Internet can teach you how to build a

gun that launches ping-pong balls at a few hundred miles per hour. Big deal—where's the fun in shooting projectiles at subsonic speed? To kick it up a notch, a team of mechanical engineers at Purdue University has reconfigured the ping-pong-ball gun into a supersonic bazooka. Click here to watch as it turns four aluminum cans into shrapnel.

For years, Mark French has been using the regular subsonic version of the gun to teach kids about physics. "I've brought it to 4-H clubs and to schools. I've gotten ridiculous mileage out of this thing. With all that use, you can't help but wonder whether you can improve it."

The typical setup uses a PVC plastic tube, with a ping-pong ball inside, that's sealed on both ends with duct tape. A pump removes the air inside the tube, creating a vacuum. Then, when the seal at one end of the tube is broken, air rushes in, and because there's no aerodynamic drag on the ping-pong ball, it can fire out the other end at 400 mph. "The [main] limitation here is how fast you can get air to go down the tube, because the ball is only going to go as fast as the air," French says.

On a hunch, French and his students modified the gun with a convergent–divergent nozzle, the type used in rocket engines and supersonic wind tunnels to accelerate air flow. The revamped gun shoots pressurized air through the hourglass-shaped nozzle. As the air travels through the nozzle's choke point, compression accelerates the air. It blasts the ping-pong ball outward at 900 mph (the speed of sound is roughly 765 mph).

French explains how it's done in a video posted here. (At around 5:50 you can see the ball blast through a wooden ping-pong paddle.) You can also read more about the setup in the published paper. But, please, DO NOT try this at home. Even a flimsy ping-pong ball traveling at 900 mph has the same kinetic energy as a bullet.

Once the engineers got the bazooka up and running, they tested it on all sorts of materials, including VHS tapes, 3/4-inch plywood, stereo speakers, and even a sheet of steel. "The ball didn't go through the steel, but it put a whopper of a dent in it," French says. "Normally what happens is the ball comes out in pieces—it's shattered but not deformed. For this one, it melted and buckled. I didn't expect that."

With the lab's new supersonic bazooka, French is now finding it even easier to captivate audiences. "My students really want to know how this works," he says. "It's been a blast."

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