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[Physltest] [Phys-L] Re: 'Seeing' frequencies above 30-40Hz (fwd)



Reposting after replacing "horizontal" with "vertical"

On Friday, Jan 14, 2005, at 17:23 America/New_York, Ludwik Kowalski
wrote:

I would place a laser on the same rotating platform as the tuning fork
with a mirror in the VERTICAL plane (on one of the prongs). In that
way the laser beam would be constantly reflected. If the mirror were
not vibrating then the trace on the wall (or screen) would be
horizontal. But I do not know how to form a wiggly line on the wall. At
a high frequency, such as 256 Hz, the line on the wall would become
wider, according to amplitude. At a much lower frequency a fluorescent
screen would be needed, like in an oscilloscope, to display wiggles.
Otherwise one would see a single spot of light tracing a sine curve on
the wall. Yes, I know that this is not what was in Scott's message; I
am speculating.
Ludwik Kowalski

On Friday, Jan 14, 2005, at 16:44 America/New_York, James McLean wrote:

Scott Goelzer wrote:
Ludwik reminded me of a neat demo done at Lowell Regional Physics
Alliance, but I cannot remember the name of the demonstrator or
source
of demo.

Tuning fork had a bit of mirror mounted on the top of one fork and
was
set vertically on rotating turntable. A laser aimed at the mirror
reflected a nice sine wave on the wall as the fork rotated and
vibrated.

I'm not sure I understand the configuration.

The best I can understand this, I'm imagining that the mirror is at a
45deg angle to the turntable radius, and the laser is aimed almost
tangentially to the turntable, so that the laser only reflects off the
mirror for a relatively small fraction of the rotation:
__--__
/ \ turntable from above
/ \
| /-------->
\ |/
\__ __|
-- |
|
laser

But I'm still not sure how this would work, since the primary motion
of
a tuning fork tine is towards/away from the other tine, which would
not
result in vertical deflection of the laser beam. Maybe the flexing of
the tine along with the optical lever is enough?

Is this the right configuration?
--
Dr. James McLean phone: (585) 245-5897
Dept. of Physics and Astronomy FAX: (585) 245-5288
SUNY Geneseo email: mclean@geneseo.edu
1 College Circle web:
http://www.geneseo.edu/~mclean
Geneseo, NY 14454-1401
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