Physics Photo of the Week

May 13, 2011

Vacuum Ping Pong Cannon

With funds from the Pugh Foundation the Physics Department recently assembled a transparent vacuum ping pong cannon.  The purpose of using transparent pipe is to enable fast video photography with a special camera (also purchased with funds from the Pugh Foundation). 

The vacuum cannon works by evacuating the pipe with a vacuum pump shown in the background.  A ping pong ball had been placed just inside the far end of the pipe before the ends of the pipe were sealed off with wide packing tape.  The tape at the ball's end is then punctured with a pointed knife; the outside air at atmospheric pressure rushes in with a velocity of several hundred meters/sec characteristic of molecular speeds obeying kinetic theory of gases.  The pressure accelerates the ping pong ball to high speeds; the ball exits the cannon and penetrate two of the empty soda cans placed at point-blank range.

Of course in physics class we are interested in measuring the acceleration of the ping pong ball within the cannon when the vacuum is broken as well as the exit velocity - easily facilitated with the high speed video camera capable of taking limited resolution videos at 1000 frames/second.  The slow motion video at left shows the first few video frames after the vacuum is broken.  The white tapes on the transparent pipe are placed 0.5 meters apart.  Analysis of the video in the first meter shows an acceleration of about 33,000 meter/sec2.  That is about 3,300 times larger then the acceleration due to gravity.  In the first meter the ball develops a speed of about 200 meters/sec.  Physics student Tabitha Ndung'u has conducted a special project to measure the speed of the ping pong ball at different places along the tube.  She has found that the ball acquires this maximum speed in the first meter of travel, and maintains a roughly constant speed of about 200 m/sec for the remaining 2 meters of the pipe.

This vacuum cannon, although it resembles a "shoot-em-up toy", actually presents a rich environment to study many areas of physics: kinematics, molecular kinetic theory, thermodynamics, and aerodynamics.  Students calculated that the initial acceleration would be about 42,000 meter
/sec2 - about 1.5 times larger than measured.  This disagreement indicates appreciable dissapative forces on the air rushing into the tube to replace the vacuum.  Other dissapative forces can be studied by following the trajectory of the ball after exiting the cannon.  It is not yet known how reproducible the data are from one shot to the next.  Careful observation of the high speed photography of the ball exiting the cannon shows that internal pressure builds up between the ball and the muzzle end of the cannon - the sealing tape is blown off before the ball reaches it.

Special thanks go to the physics students who participated in the firing of the cannon, Mayuri Patel for assembling the cannon, Jason Zumstein for his expertise in aerodynamics, as well as the Pugh Foundation for providing funds for the high speed camera and the supplies for the transparent cannon.  The idea of the ping pong cannon was learned from a presentation by Professor Richard Peterson of Bethel University at a physics teachers meeting several years ago.

This is the final Physics Photo of the Week as PPOW goes on summer break.  The next Physics Photo of the Week will be published August 26, 2011 after classes resume for the fall semester.  Have a great summer and please send in possible photos!




Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu. 

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