Physics Photo of the Week

December 10, 2010

Violent Ping Pong
That blur in the photograph is an empty soda can being punctured by a ping pong ball shot from the pipe on the right.  The pipe is a vacuum ping pong cannon.  It shoots ping pong balls using "nothing", or "vacuum" as the fuel.

The ping pong ball was inserted into the far end of the 10 foot section of pipe; the ends were sealed off using packaging tape; and the pipe was evacuated using a vacuum pump.  A student then punctured the tape right behind the ping pong ball at the "breech" of the cannon.  The sudden in-rush of air into the evacuated pipe accelerated the ping pong ball to a tremendous velocity.  The ping pong ball gained enough momentum to completely puncture both front and rear walls of an empty soda can placed at point-blank range.  The photo above shows the blurred image of the empty aluminum can being punctured by the ping pong ball.  The animated photo below, taken by students in the Physics class, shows how quickly the soda can was destroyed.

The Physics class was interested in determining the speed of the ping pong ball.  The video camera is too slow (at 30 frames per sec) to capture the rapidly moving projectile.  We were very lucky in this video sequence in that one half of a video frame was able to capture the rapidly disintegrating can.

The class developed another indirect method to determine the projectile's speed.  A special pendulum was assembled that consisted of a Styrofoam bucket suspended from the ceiling.   The open end of the bucket faced the muzzle of the cannon in order to catch the ping pong ball.  During the "catch" the momentum of the ping pong ball was transferred to the "large" mass of the Styrofoam.  A student recorded the video shown at left as the cannon was fired at the pendulum bucket.  If the speed of the bucket can be measured the speed of the ping pong ball can be calculated by means of the law of conservation of momentum. 

In order to measure the speed of the bucket, the conservation of energy principle was used.  Immediately after catching the ping pong ball, the bucket gained kinetic energy which was then converted into potential energy as it reached its maximum height.  The height was determined easily from the video clip with the help of the ruler supported by the ladder.

The students calculated that the speed of the ping ball emerging from the cannon was over 700 miles/hr from data obtained from this video clip.  This number agrees fairly well with a calculation based on the force of the air pressure accelerating the ball into the vacuum.  The predicted value of the speed (neglecting friction) was about 25% greater than the measured speed showing a reasonable agreement.  These experiments destroy ping pong balls from any future competitive table tennis.

We wish to thank Mayuri Patel, physics department assistant for setting up the pendulum suspended from the ceiling.  Students in the physics class took the videos.
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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