Physics Photo of the Week

December 13, 2013

Ping Pong Cannon - Discussion by Rachel Weber.  Video assistance by Justin Hale.

One fine morning, a gun-shot sounding bang rung throughout a science building, coming from the 11:00 physics class, except it was not a gun, but instead a ping pong cannon. Constructed from a 10 foot long clear pipe evacuated with a vacuum pump, this cannon was propelling the small 2.4 gram ping pong ball at speeds of up to 280 meters per second. 

First, the ping pong ball is placed just centimeters within the back of the pipes, followed by 3-inch wide packing tape sealing off each end.  After the pipe was evacuated, requiring about 2 minutes, the tape by the end with the ball is punctured,  where a rush of the outside air accelerates the ping pong ball with a "BANG!". Due to the molecular speeds bouncing off the ball in random motion obeying the kinetic theory of gases, the resulting pressure propelled the ball through the pipe with a velocity of several hundred meters/sec.  At such a high speed, a special high-speed video camera at a frame rate of 240 frames/sec was used in order for the results to be even slightly measured. The high speed ping pong ball, essentially invisible to the human eye, penetrated through two empty soda cans. 

The destruction of the empty soda cans can be seen in the high speed video clip at right, being played back at one frame/sec.  Two destroyed cans from a previos shot can be seen in the foreground of the pictures.

After seeing the simple ping pong ball decapitate two soda cans, we wanted to determine just how fast the ball was going by using a ballistic pendulum.  After firing the ping pong cannon into a suspended 550 gram cardboard box , we would measure the speed of the box after it essentially "caught" the ball. Instead of pushing the box, the ball had so much momentum that it punched right through it! The ping pong ball broke through several layers of corrugated cardboard, even after reinforcements were made.  Thankfully though, the high speed camera was able to photograp the ball emerging from the box after breaking out of the box.  This is shown in the second video clip.  The video shows the blurred ball emerging from the box (marked with the indicator in each frame) along with flying shards of cardboard box material.

Through some of the pictures, we were able to determine the
velocity and momentum of the emerging ping pong ball and the velocity and momentum of the box (seen in the third video clip at right.  Even when the ball gets crushed and the box is penetrated, we know that conservation of momentum is true because the only forces are "internal", between the box and the ping pong ball, so we know the total momentum is constant.  By equating the total momentum of the ball and box after the collision to the momentum of the ball before the collision, we found that the speed of the ball emerging from the vacuum gun was 280 m/s which means 630 miles an hour! Through this experiment, kinematics, molecular kinetic theory, conservation of momentum, thermodynamics, and aerodynamics are all demonstrated.

Both Rachel Weber and Justin Gale are physics students at Mars Hill University where this experiment was conducted.  The high speed video camera, vacuum cannon, and a vacuum pump were on loan from the Physics and Chemistry Department at Warren Wilson College.

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