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From: dean@ruby.berkeley.edu
Newsgroups: sci.misc
Subject: Fluids and solids (was Re: H-less liquids)
Message-ID: <1454@jade.BERKELEY.EDU>
Date: Wed, 15-Oct-86 13:33:51 EDT
Article-I.D.: jade.1454
Posted: Wed Oct 15 13:33:51 1986
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Reply-To: dean@ruby.berkeley.edu (N. Dean Pentcheff)
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Organization: University of California, Berkeley   Department of Zoology
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Keywords: fluid solid viscosity deformation
Summary: Definitions


My understanding of the difference between a solid and a fluid comes from
the engineering principles used in biomechanics.  A solid is a material that
deforms under load but whose original dimensions are restored when the force
is removed (it demonstrates elasticity).  A fluid is a material that deforms
under load but whose original dimensions are _not_ restored when the force 
is removed (it demonstrates viscosity).  In fluids, but not in solids, the
particles within it translate past each other permanently under influence of
an external force.

An interesting thing about this definition is that it may be time-dependent
for real materials: a thick solution of corn starch in water will flow if
poured slowly (it's a liquid at that timescale) but shatters if hit with a
hammer (in that case it's a solid).  There is a continuum of behaviors from
a perfectly elastic solid to a perfectly Newtonian fluid.

By this definition, glass is a solid at short timescales (bend a glass rod
between your hands and it springs back to its original shape) but a liquid
at long timescales (large windows flow due to gravity).

-Dean     (dean@ruby.berkeley.edu)

[The opinions expressed herein are mine, but someone else probably believes
them, too.]