I'm sorry that I have been so quite on this subject but have been
overwhelmed by commitments here at USNA as well as preparing for my 30th
wedding anniversary. Additionally, I have to go out on travel to teach a
"climbing" course for the Boy Scouts.
I am currently readying a paper on just this subject and hope to have
something to AJP or JASA by the end of June. Also, I will be presenting
this work (if accepted) as part of the Apparatus Competition at the summer
meeting of AAPT. Shortly thereafter and once the AJP/JASA paper is
submitted, I will have a complete description up on my web site.
But here is what I can say with certainty:
1. Don't try a beaker! While they are generally of a cylindrical nature
which makes the calculation of their Helmholtz resonance modes quite easy,
BUT they have this horrible anisotropy in the lip for pouring --- also,
the lip curves somewhat outward. If that wasn't enough, they are made of
Pyrex --- a material that has a great deal of resistance to fracture (and
therefore, a great deal of internal loss).
2: "Fine" crystal glasses are NOT NEEDED and are JUST A WASTE OF MONEY.
I have done this experiment/demonstration many times in the past for the
students, faculty, and staff here at the Naval Academy, and I can assure
you that it is quite easy when done properly. It works so well that I
usually have someone randomly chosen from the audience do the actual
breaking. It is all but fool-proof!
The "trick" is found in a review of the equation of motion for a "damped
driven harmonic oscillator". Notice what happens to the response
frequency when there is any damping --- it is lower than the natural or
undamped frequency. Done properly, this demonstration is beautifully
simple and produces startling "Bohr resonance" fractures*. The simplicity
of the experiment and the visual feedback are compelling to the student
who wishes to understand resonant behavior.
*The name "Bohr resonance" is my own choice. I just didn't know what else
to call it. The fracture line around the perimeter of a glass almost
always appears to be an integer number of sine waves. What else could I
call it to convey this information?
Anyway, come to San Antonio for the meeting and workshops and see this as
well as many more great demonstrations!
+=================================+=================================+
ERTEL SENDS.
/^\-/^\
/ \
| * |
|#########|
===========================
/ Prof. John P. Ertel \
/ USNA-NESA Faculty Rep \
/ veteran Eagle Scout \
+==================================================+
| jpe@nadn.navy.mil or jpertel@BigFoot.com |
| Physics Department, 9C Office : 410-293-6657 |
| Michelson Rm-338 DSN : 281-6657 |
| 572 Holloway RD FAX : 410-293-3729 |
| U. S. Naval Academy Research : 410-293-2759 |
| Annapolis, MD 21402-5026 Home : 410-757-6618 |
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