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In article <1992Apr15.173142.6990@cis.uab.edu> menon@ftp.phy.uab.edu (Govind Menon) writes:
>The case of the hard sphere and plane is an interesting scenario, but by no 
>means realistic.  If however, you do wish to conduct a thought experiment, 
>constrained to idealizations, then permit me if you will to make yet another
>reasonable assumption.  
>
>There is nothing preventing a perfectly spherical and a perfectly planar 
>material from being infinitely rigid.  Subject to this condition the bodies

 How fast would such a sphere transmit shock waves in it's interior?



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In article <1992Apr15.173142.6990@cis.uab.edu>, menon@ftp.phy.uab.edu (Govind Menon) writes:
|> The case of the hard sphere and plane is an interesting scenario, but by no 
|> means realistic.  If however, you do wish to conduct a thought experiment, 
|> constrained to idealizations, then permit me if you will to make yet another
|> reasonable assumption.  
|> 

Reasonable assumptions imply that you're looking at a range of phenomena for
which the assumption is adequate.  In biophysics, there might be arguments
where a comment such as 'Consider a spherical frog' makes all the sense in
the world...


|> There is nothing preventing a perfectly spherical and a perfectly planar 
|> material from being infinitely rigid.  Subject to this condition the bodies
|> do not have to deform under infinite pressure at the contact point.  

There's a problem with this in the relativistic limit -- something
infinitely rigid would transmit the signal instantaneously.  An infinitely
rigid ball only makes sense in a limit where the stiffness makes a
transmission speed much greater than any of the other dynamics in the
problem.

Dan

In article <1992Apr15.142013.8494@ms.uky.edu>, bud@ms.uky.edu (William K Glunt) writes...
>In article <1992Apr15.173142.6990@cis.uab.edu> menon@ftp.phy.uab.edu (Govind Menon) writes:
>>
>>There is nothing preventing a perfectly spherical and a perfectly planar 
>>material from being infinitely rigid.  Subject to this condition the bodies
> 
> How fast would such a sphere transmit shock waves in it's interior?
> 

Everyone knows that the finite speed of light sets limit on the rigidity of 
material bodies.  But as long as we're discussion perfectly rigid, perfectly
hard, perfectly smooth bodies, why bring physics into it? 

-Scott

platt@watson.ibm.com (Daniel E. Platt) writes:

>There's a problem with this in the relativistic limit -- something
>infinitely rigid would transmit the signal instantaneously.  An infinitely
>rigid ball only makes sense in a limit where the stiffness makes a
>transmission speed much greater than any of the other dynamics in the
>problem.

But if you are going to impose this much realism, then you must impose
the realism that we live in an atomic world.  There is no such thing as
a perfectly spherical ball (except for something like a single Helium
atom), and there is no such thing as perfectly flat surface.  The contact
"point" will involve at least 3 atoms from each object, so it will have
finite area, so the pressure won't be infinite, so you won't need infinite
rigidity.

The problem as presented is not realistic, so there is no right answer.

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