Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84 exptools; site ihlpg.UUCP
Path: utzoo!watmath!clyde!cbosgd!ihnp4!ihlpg!tan
From: tan@ihlpg.UUCP (Bill Tanenbaum)
Newsgroups: net.physics,net.space,net.research
Subject: Re: Star-Wars/Space Telescopes (parabolic mirrors)
Message-ID: <1468@ihlpg.UUCP>
Date: Fri, 6-Dec-85 00:02:17 EST
Article-I.D.: ihlpg.1468
Posted: Fri Dec  6 00:02:17 1985
Date-Received: Sat, 7-Dec-85 03:24:02 EST
References: <384@ukc.UUCP> <26@sbcs.UUCP> <1124@gitpyr.UUCP>
Organization: AT&T Bell Laboratories
Lines: 22
Xref: watmath net.physics:3659 net.space:4883 net.research:371

> > (Saumya Debray)
> > Seems to me that since air pressure is the same in all directions, this
> > would give a spherical rather than a parabolic mirror.
------------
> [Don Barry]
> By the same argument, a suspended string would form a spherical arc - 
> instead, the form is a catenary, described by the hyperbolic functions.
> I am unaware of the figure of equipressure deformation of an elastic disc,
> but one technique that is used to generate true parabolas in a uniform gravity
> field is that of spin-molding.  It is an easy matter to calculate the
> equipotential surface of a spinning liquid, and imposing the condition of
> stability, the figure is a paraboloid.
-----------
Don Barry is confusing two very different cases.  The suspended 
string and spin molding are cases where the force is due to gravity,
and thus has a UNIQUE DIRECTION, and is proportional to the
mass of the object.  In the case of the mirror, the force is ISOTROPIC
and independent of the mass of the mirror (assuming the mirror is
thin enough so gravity is negligible compared to vacuum pressure).
In such a case, Saumya Debray must be right.
-- 
Bill Tanenbaum - AT&T Bell Labs - Naperville IL  ihnp4!ihlpg!tan