Re: [Phys-l] Disapointed. Was: Re: different types of waves have different speeds

[Gary Karshner <gkarshner@satx.rr.com>]

There is a nice example - Earth Quake Waves
They come in two types S-waves (Shear waves) 
which are transverse and P-waves (pressure 
waves), they travel at different speeds and their 
separation is used to determine the distance to 
the quake. Interestingly the shear waves won't 
travel through the core of the earth, which is 
why we say the core of the earth is a fluid. 
Fluids won't support shear waves. The speed 
difference is because it is they depended the 
separate moduli - Shear and Youngs.
Gary

At 07:51 PM 2/28/2010, you wrote:

> On 2010, Feb 26, , at 13:34, Bernard Cleyet wrote:
>
> > A HS teacher on a HS list asked,  "Would a 
> Transverse Wave and a Longitudinal Wave in the 
> same material have the same speed?"
> >
> > One respondent replied, "So the answer is 
> that different types of waves have different 
> speeds because they are caused by slightly different phenomena."
> >
>
> And nothing more except two examples;  one below, the other is a tuning fork.
>
>
> >
> > I wish to reply with a brief description 
> appealing to simple notions for the benefit of 
> an unusually inquisitive student.
> >
> > The respondent gave, as an example, L and T 
> waves in an aluminum rod, à la Paul Doherty at the Exploratorium.
> >
> > ringing _Al_rod
> >
> >
> > My thought is to write:  (mechanical) waves 
> require inertia and a restoring force similar 
> to mechanical oscillators, e.g. pendulum and 
> spring)  In the L. (compression) case  the 
> inertia is the mass of the material and the 
> restoring force the elasticity of the material 
> characterized by the uniaxial strain/ stress in 
> a homogenous solid (Young's 
> modulus).   OTOH,  for the T. wave the inertia 
> is the moment of inertia of an element of the 
> bar's length and the restoring force the 
> reaction (shear elasticity) to the shear 
> force.  Thus the T wave is very different from the L. wave.
> >
> > I solicit an improved version.
> >
> >
> > bc
>
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