Date: Wed, 15 Dec 2004 13:46:45 -0500 (EST)

Author: Richard Berg

Subject: Re: Mechanical Resonance and geezer story



I think this is a non-linear oscillator with standard hysteresis
properties. We do it with that in mind:

Dick Berg

On Wed, 15 Dec 2004, Jim McConville wrote:

> John Mocko wrote:
> >Hello Tap-l,
> >
> >I'd like to build a mechanical resonance demo using an old jigsaw (sabre
> >saw) motor to excite large metal blades of different lengths.
> When I was a youngster working as an electronic tech for a research
> group in the physics department a young faculty who was assigned the
> task of modernizing the lecture demonstration facility asked me to
> build a small mechanical resonance demonstration to be used in
> lecture and shown by shadow projection or on an overhead projector.
> I chose a clamped hacksaw blade driven by an electromagnet excited by
> a low power sine wave generator. I could certainly find resonance and
> show maximum amplitude at about the natural frequency of the undamped
> blade. There were however some difficulties. The first thing I
> noticed was the response of the blade was far from the nice resonance
> curve shown in the text books. As I approached the resonant frequency
> from below resonance the amplitude would build up as expected but
> then suddenly drop off to a very small value. Now being above
> resonance, I would have to lower the driving frequency and as I did
> so, I found I had to go far below resonance to begin to build up
> amplitude in the blade then I had to increase the frequency to get a
> larger amplitude. Again, the amplitude would drop sharply and I would
> have to backtrack to get a large amplitude.
> I varied every parameter I could think of, I did not think of
> damping. The faculty person who initiated the project checked with me
> from time to time and I repeatedly described my unexpected result. He
> had never heard of hysteresis like behavior in a resonant system and
> was sure I was overlooking something. He was very disappointed. He
> looked, more than once at the device and acknowledged that "something
> was wrong" and it was clear he thought I was doing something stupid.
> He blamed the blade mounting, the coil placement, the sine generator
> and my attitude. I had many other pressing tasks at hand and the
> demonstration got pushed to the back of the bench where it sat
> neglected as I moved on to other things.
> In retrospect, I think of the young faculty member seeing all this
> and ignoring what was so obviously more interesting than a
> mechanical resonance demonstration. While he went on to be successful
> in his career, I feel he would have been famous had he taken the time
> to look carefully at the anomalous behavior and investigate further.
> This was in the mid to late '50's before chaotic behavior became a
> hot topic.
> --
> Jim McConville
> Jackson County, Oregon

Dr. Richard E. Berg, Professor of the Practice
Director, Physics Lecture-Demonstration Facility
U.S. mail address:
Department of Physics
University of Maryland
College Park, MD 20742-4111
Phone: (301) 405-5994
FAX: (301) 314-9525
From Wed Dec 15 13:45:34 2004