Re: Sodaplay: Tacoma Narrows resonator

[Leigh Palmer <palmer@SFU.CA>]

> PHYS-L@lists.nau.edu writes:
> > the one which is called sympathetic
> > vibrations in common parlance!
> and
> it was an aeolian oscillation driven by a steady wind
>
> Having just used this in class as an example of resonance and wanting to
> be better educated, I am puzzled by these statements. So I ask some
> questions:
> 1. If one tuning fork causes another to begin vibrating, is that
> resonance....I think so.

Yes, it certainly is.

> 2. If I sing near such a tuning fork and part of my melifluous voice (;-0)
>  causes the fork to vibrate can I cite resonance?

Yes

> 3. If a steady wind stimulates wire cables to vibrate in an assortment of
> modes, is it possible for one of those frequencies to stimulate
> sympathetic vibrations in an neighboring oscillator?

I don't understand the question. If this relates to the bridge,
then the frequencies of vibrations of the cables supporting it
are all much higher than the torsional modes of the bridge.
Vibrations of the cables, per se, had nothing to do with the
aeolian oscillation of the bridge deck, though of course their
elasticity played some role in determining those frequencies.
(I recall that at least the two lowest modes of the bridge
were seen in the dramatic amplitude motion.)

> 4. What mechanism started and supported  the oscillations of that bridge?

The bridge deck, initially more or less having the wind velocity
in-plane, got a little bit out of plane and the wind reinforced
this distortion. A vortex was shed, allowing the distortion to
relax and, it turns out, overshoot its equilibrium position, at
which time the opposite polarity distortion is driven to
increase by the wind until another vortex is shed, the deck
relaxes past equilibrium, usw. When the wind was gentler, this
oscillation did not grow in amplitude to bridge failure, which
gave the bridge its nickname of "Galloping Gertie".

That's about the best I can do in words. Take a piece of grass
between your thumbs and blow on it. If you blow skillfully you
can get a shrieking whistle. If you blow much harder you can
tear the blade of grass. Now you have kinesthetic understanding.

Leigh