Re: wave momentum

[brian whatcott <inet@INTELLISYS.NET>]

At 17:54 3/10/99 -0600, Carl Mungan wrote:

> But what about mechanical waves? ...
> Intuitively it seems a mechanical wave doesn't carry momentum since there's
> no mass flow. A cork on the surface of the sea just bobs up and down (okay,
> the bobbing is slightly circular, but never mind that).

> Dr. Carl E. Mungan

Carl is undoubtedly teasing us.
 We would undoubtedly be more comfortable with the oscillatory motion of
individual balls in the Newton's Balls demonstration, I'd think.
 A liquid may not be able to sustain a transverse body wave, but it most
certainly propagates a longitudinal wave quite nicely.

 On this topic, there was a fascinating insight in an engineering
newsgroup which I am not convinced was entirely an urban fable.

It goes like this:
 Once upon a time in a country far far away ( I hear it was an
Arabian Peninsula country) the engineers of that place built a
pipe fit to transport large quantities of precious liquid -
perhaps oil, or it might even have been ...water!

 They felt it was important to test this pipe which stretched across
many miles of desert. And so they pressurized it to a moderate over
pressure in the customary way.

   But it happened that they used compressed air ( water being so
precious) and what is worse, there was a miscalculation concerning the
overpressure to be used for the test.

    And so, sad to tell, the pipe developed a little crack which
stretched a little way along the pipe.   Now it happens that the
speed of sound in air is about 1100 feet per sec but the speed of
sound ( at which some cracks propagate) is about 5 times faster in
steel pipe.

 In an unexpected development, the crack once started, leapt ahead,
mile on mile, followed not long after by escaping air from the
progressing crack. The crack propagated sooner than the air could
escape to relieve it, you see.

And so, alone and abandoned across the sand dunes sits a mighty pipe
line with a crack longer than any other crack on earth...

Brian

brian whatcott <inet@intellisys.net>
Altus OK