Re: Midterm Question - Sort of

[Samuel Held <Held@RHIP.PHYS.UTK.EDU>]

Leigh,
        You need to go talk to a civil engineer.  It is a driven system
from the steady wind and the bridge a big cross section.  Hold out hand
normal to the floor with your thumb on top.  This creates a "sheet" with
a big cross section.  Now imagine a finger coming in from the side and
pushing against your hand.  If strong enough and constantly driven, it
push your hand up.  But this only lasts until there is a weakness in the
driver or gravity takes over and drops your hand(the bridge) to its
original position.  This process occurs over and over.   If at the right
frequency, say of the steel beams, then you can get a resonance.
Presto!!!  (and with nothing up my sleeve).

Sam Held



-----Original Message-----
From: Leigh Palmer [mailto:palmer@SFU.CA]
Sent: Wednesday, July 07, 1999 11:20 AM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: Midterm Question - Sort of


> My gut feel is that you can't look at this as anything but impulse
> excitation, because you're not providing a sinusoidal or even a square
wave
> input at all. To do so, you'd have to pull on the swing as well.

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