Re: [Phys-l] loud bag...physics?

[John Denker <jsd@av8n.com>]

On 05/29/2010 04:53 PM, Brian Blais wrote:
> Anyway, I thought there might be some interesting physics activity in  
> there somewhere, but I am not sure what.  

There is some interesting and fairly complicated physics involved.

At the phenomenological level, there are various ways something 
can respond when overstressed.  At the extremes we have:
 -- plastic deformation and creep, versus
 -- brittle fracture.

And then there are all shades of gray in between.  Famously
Silli-Putty (silly putty?) will creep if stress is applied 
slowly, but it will shatter if shocked:
  http://plc.cwru.edu/tutorial/enhanced/files/polymers/therm/therm.htm
  http://www.youtube.com/watch?v=uWYxc8xhihg

Ideal crystals tend to be brittle.  That is, once a crack is
formed, it tends to propagate like crazy.  This can be understood
in terms of the Bragg-Nye model.  Feynman has a chapter on this.

To make a loud bag, you need
  a) strength, obviously, and
  b) brittle fracture (not plastic creep so it "lets go" all 
   at once.

OK?