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From: rimey@ernie.BERKELEY.EDU (Ken &)
Newsgroups: net.physics,net.space,net.research
Subject: Re: Star-Wars/Space Telescopes (parabolic mirrors)
Message-ID: <11128@ucbvax.BERKELEY.EDU>
Date: Mon, 2-Dec-85 18:39:16 EST
Article-I.D.: ucbvax.11128
Posted: Mon Dec  2 18:39:16 1985
Date-Received: Thu, 5-Dec-85 04:19:20 EST
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Reply-To: rimey@ernie.UUCP (Ken Rimey)
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Summary: Catenary is wrong analogy

>> Seems to me that since air pressure is the same in all directions, this
>> would give a spherical rather than a parabolic mirror.
>By the same argument, a suspended string would form a spherical arc - 
>instead, the form is a catenary, described by the hyperbolic functions.
>	...
>Don Barry (Chemistry Dept)          CSnet: cmpbsdb%gitpyr.GTNET@gatech.CSNET

No, Saumya is certainly right.  Soap bubbles are certainly spherical,
even when they sit on circular wire frames.  The force on each piece
of the surface is equal to the pressure (times the area of the piece)
and oriented perpendicular to that piece.

If on each piece of a string you put a force (proportional to the
length of the piece) perpendicular to that piece of string, the string
would form a circle.  THAT is the correct analogy.

						Ken Rimey
						rimey@dali.berkeley.edu