Re: Midterm Question - Sort of

[paul o johnson <pojhome@FLASH.NET>]

Leigh Palmer wrote:
> >Well, that's a bit like saying that the steady bowing
> of a violin
> string is an example of resonance. There are certainly *normal modes
> of oscillation* for the bridge (and the violin string), and these
> modes can be excited by an external source of energy in a nonresonant
> interaction. Resonance I take to refer to the excitation of a normal
> mode by an *applied* periodic forcing function, clearly not the case
> encountered here. The applied forcing function is, more or less, DC.
That's exactly what I was saying, Leigh. The bridge and the violin string
both resonate because they are excited by an applied periodic force.
The periodic force, of course, is in addition to the DC force being applied.
In the case of the bridge, the periodic force is the 3rd-law reactive force
owing to the vortex flipping. For the string, the periodic force is the
3rd-law reactive force owing to flipping of the rosin-coated bow between
static and dynamic friction against the string.
> It was not the cables of the bridge that shed vortices
> at the bridge
> normal mode oscillation frequency, but rather the bridge deck itself.
> That is, indeed, the mechanism responsible for aeolian oscillation.
> I believe the bridge actually flexed in more than one of its normal
> modes. I won't check that just now, but maybe someone else can help
> me. I seem to have no allies in this discussion. Have I slaughtered
> a sacred cow?
As usual, you have straightened out my thinking about what was actually
shedding the vortices.
As I re-read your piece, I believe that our mutual misunderstanding
stems, perhaps, from not being explicit about which applied force causes
the resonant condition. I think we agree that it's an AC force, not a DC
force.

poj
Collin County College