Re: Sodaplay: Tacoma Narrows resonator

[Joseph Bellina <jbellina@SAINTMARYS.EDU>]

Maybe I missing lots here, but I don't really see the distinction
between the soldiers marching on the bridge, and the bow on the violin
string.  The only difference is that in the violin case, the magnitude
of the stimulation is probably large, and so many modes of vibration on
excited, and a few selected, whereas in the soldier case, the
stimulation is weak so only the one mode of vibration is excited.  I
suspect if the soldiers could stomp harder, or some such, the bridge
would behave like the string.
But if the difference between your type 1 and type 2 is just a matter of
the intensity of the stimulation, where do you draw the line of
distinction?

Or is there a difference of kind that I have failed to detect...or
perhaps read in previous messages.

joe

On Fri, 8 Dec 2000, Michael Edmiston
wrote:

> I think Leigh wrote a crucial paragraph when he wrote, "I have great
> difficulty understanding the death grip some folks have on calling this
> phenomenon a resonance. What is the practical value in doing so? All that is
> accomplished if one admits this new meaning is that the meaning of the term
> becomes less precise. The only benefit seems to be that the Tacoma Narrows
> Bridge film can then be shown as an example of that other meaning."
>
> The TNB-film is a spectacular film and teachers like to show it and students
> like to view it.  But in what sense is it physics?  In what sense does it
> fit into the curriculum?  Somehow we have to justify its connection with the
> curriculum so we can legitimately show it in class.  I think we can do this,
> but as Leigh states, some people choose the wrong physics.  Below I describe
> two types of things are often described using variations of the word
> "resonance."  I am aware that many teachers show the TNB-film within the
> context of Type-1 (below) when it is really an example of Type-2 (below).
>
> Type-1. (A) I hang a spring pendulum from an electromagnetic driving coil
> (such as Pasco-scientific SF-9324 or WA-9753) and I drive the coil with an
> oscillator and find that a certain driving frequency causes build-up of huge
> oscillations in the spring pendulum.  (B) I sing a short-duration steady
> pitch into my piano with the damper pedal depressed and upon stopping
> singing note that a particular string(s) has been excited.
>
> Type-2.  (C) I strike a tuning fork with a rubber mallet and note that the
> fork vibrates at particular frequencies.  (D) I bow a violin string and note
> the string vibrates with particular frequencies.  (E) I blow across the top
> of a soda-pop bottle and note that I produce particular frequencies of
> sound.
>
> If a river barge ran into a bridge pier, the bridge might oscillate and this
> would be like Type-2 (C).  If a steady wind blows across the bridge deck and
> oscillation begins, this is like Type-2 (D) or (E).
>
> Bridge oscillation could be like Type-1 (as others have mentioned) if we
> have soldiers marching across it and they are in-step near the natural
> frequency.  But the TNB collapse was clearly a Type-2 phenomenon.  And, as I
> mentioned above, I usually see/hear teachers using it as a Type-1
> phenomenon, which is clearly incorrect.
>
> Now, whether we use the word "resonance" with respect to both Type-1 and
> Type-2 is a completely different matter... that's word-definitions,
> semantics, etc.  I think enough people have written to convince us resonance
> is often voiced in both cases.  Rightly or wrongly, that is the situation.
> But even if we have problems with the words I hope we can keep the physics
> straight.  For Type-1 the source of energy oscillates on its own accord at a
> frequency determined by something other than the system being driven.  In
> Type-2 the source of energy does not oscillate on it's own.  It is either a
> single pulse, or (if left to itself) would be steady; and any local
> oscillation of the energy source is caused by interaction with the system
> being driven.
>
>
>
> Michael D. Edmiston, Ph.D.              Phone/voice-mail:       419-358-3270
> Professor of Chemistry & Physics        FAX:                    419-358-3323
> Chairman, Science Department            E-Mail                  edmiston@bluffton.edu
> Bluffton College
> 280 West College Avenue
> Bluffton, OH 45817