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