Name: Diane H.
Date: Thursday, April 18, 2002 8:36:18 PM
We did an experiment using vortex cannons made with PVC
pipe of a consistent diameter and 3 varying lengths. There was a balloon
stretched across one end and card stock with a hole on the opposite end.
We used a "marble hammer" and cannon holder so that we used the same
force to tap the cannon every time. From our research, we expected to see
the shortest pipe send rings out that went the farthest. What we saw was
that the shortest pipe indeed performed best, but that the mid-length
pipe performed worst. We tried several different things...first, each
pipe had its own balloon. We then tried using the same balloon on all 3
cannons, taking care to put it on the same way every time. There is a
slight "innie" or "outie" depending on how you put the balloon on the pipe
(we either used all "innies" or all "outies". We got the same results
every time, regardless of what balloon we used or how we put it on...the
mid-length cannon always performed worst. We do not have a good
explanation for what we saw. Can you give us some ideas?
I cannot say for sure, but I do have a suggestion. When you hit the
balloon, it vibrates at a certain frequency. The vibrations will travel
through the air at the speed of sound. These two factors yield a wavelength
for the vibrations. Since a great deal of the vibration is reflected back
and forth repeatedly, the effect may be due to wave interference. It is
possible that the standing wave that builds up in the medium pipe has very
little vibration at the output end. Perhaps more of the energy is output
through the balloon. Try a very thick balloon stretched to about the same
tension as a thin balloon. If this makes a difference, then interference is
supported: the heavier balloon will oscillate more slowly. If no
difference occurs, I expect there is another factor.
Dr. Ken Mellendorf
Illinois Central College
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