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 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.
Photo of the
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
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explanation. Atmospheric phenomena are especially welcome.
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