Ping Pong Balls Break The Sound Barrier

Air-powered cannon demonstrates physics concepts.
Karin Heineman, ISTV Executive Producer
(Inside Science TV) -- Getting students excited about science isn’t always easy, but having a live demonstration that uses ping pong balls traveling at supersonic speeds will get almost any kid's attention.
Mechanical engineering and technology students at Purdue University built a supersonic, air-powered cannon that shoots ping pong balls at speeds so fast they break the sound barrier.
“We figured out it was coming out at about 919 miles an hour, and it was just mind-blowing,” said Craig Zehrung, a Ph.D. student at Purdue.
That is faster than an F-16 fighter jet at low speeds. Drawing upon his experience in the air force, mechanical engineer Mark French designed the cannon and turned a mundane class in to a fun learning experience.
“You shoot the gun, it makes this great big bang and the ball is in the barrel for, you know, milliseconds, and nobody thinks of ping pong balls as going very fast,” said French.
A vacuum pump sucks the air out of a sealed tube and  air rushes into a special hourglass-shaped nozzle, just like those in F-16 engines. The nozzle accelerates the ball to supersonic speeds, propelling it with incredible momentum through wood, soda cans, and even denting steel.  
“You can get really, really high accelerations, the ball comes out of the barrel intact and doesn’t break until it actually hits something,” said French.
Researchers were surprised by the supersonic speeds because the lightweight balls have poor aerodynamics. Despite their measly 2.3 grams, the balls delivered a startling amount of energy to targets: the equivalent of a 125 mile-per-hour fastball or a brick falling several stories. 
“Obviously the main thing is how do we make it go faster, how we get more boom out of any device, right?” commented Zehrung.
The engineers do caution that this should not be done at home.  It should only be done in a safe lab environment.
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Karin Heineman is the executive producer of Inside Science TV.