[Phys-L] Re: Singing Rod Demo....

[Brian Whatcott <betwys1@SBCGLOBAL.NET>]

At 04:48 PM 11/26/2005, John Denker, you wrote:

> /// I think Carl Preske gave the best answer to the original question:
> Pay attention to the polarization of the _excitation_.  Forget about the
> rosin, just smack the end of the rod against a hard floor or other hard
> surface ... or smack it end-wise with a hammer.  Geometrically this *has*
> to couple to the longitudinal mode(s) and not to any transverse modes.

A conceptual problem of relating cause to effect is that
exciting a bar transversely also gives rise to longitudinal vibrations.
For a skeptic, this might lead to the doubtful proposition,
"If a transverse excitation can lead to longitudinal vibrations,
why couldn't a longitudinal stimulus give rise to transverse vibrations?"

How could one possibly demonstrate the vibration visually?
With a telemicroscope possibly.
Instead, perhaps one could introduce a dimensionally similar bar of steel.
to compare with the aluminum rod. Or a brass one? Or a water one?
  (A water column in a plastic tube would presumably show a
longitudinal singing
mode, though transverse modes are not possible except via the plastic tube
container.....)
The ratios of both stiffness and density are well -known and would help decide
on the transmission mode if one offers the  scaling law root (modulus/density)
for  longitudinal speed of sound


Brian Whatcott    Altus OK    Eureka!
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