Physics Photo of the Week
Vacuum Ping Pong Cannon
With funds from the Pugh Foundation the Physics Department recently
assembled a transparent vacuum ping pong cannon. The purpose of
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
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
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
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
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
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!
Photo of the
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
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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 firstname.lastname@example.org.
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