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Re: Midterm Question - Sort of



Hi Leigh-
Fourier's theorem says that there's excitation at the resonant
frequency from a Fourier decomposition of the repeated impulses. Harmonics
& sub-harmonics are then important. I think I'm agreeing with you.
I thought that there was resonant excitation fo the bridge
by the turbulent eddies coming off the bridge. I've not thought about
how aeolian excitation works, but the scales seem wrong for turbulence
to play a role.
Regards,
Jack
*********************************************************
Fourier's theorem would say you're wrong. There is a DC term
if you only push, of course, but that doesn't matter.

On a related topic: It always bothers me when the Tacoma
Narrows Bridge collapse is cited as an example of resonance.
It's not. The wind was steady, more or less, and there was
no significant component at the bridge's resonant frequency.
It is a case of nearly pure DC excitation.

It's a great film, but it's an example of aeolian excitation,
like blowing on a taut piece of grass between your thumbs.
I have many good demonstrations of resonance. Using the
Tacoma Narrows Bridge as an "example" can only obscure the
concept in the mind of a student.

Leigh
***********************


"I scored the next great triumph for science myself,
to wit, how the milk gets into the cow. Both of us
had marveled over that mystery a long time. We had
followed the cows around for years - that is, in the
daytime - but had never caught them drinking fluid of
that color."
Mark Twain, Extract from Eve's
Autobiography