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Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!ihnp4!bbnccv!jr
From: jr@bbnccv.UUCP (John Robinson)
Newsgroups: net.physics
Subject: Re: The membrane cup PMM
Message-ID: <119@bbnccv.UUCP>
Date: Tue, 9-Apr-85 02:40:25 EST
Article-I.D.: bbnccv.119
Posted: Tue Apr  9 02:40:25 1985
Date-Received: Wed, 10-Apr-85 05:33:59 EST
References: <1501@decwrl.UUCP>
Reply-To: jr@bbnccv.UUCP (John Robinson)
Organization: Bolt Beranek and Newman, Cambridge, MA
Lines: 26
Summary: 

In article <1501@decwrl.UUCP> williams@kirk.DEC (John Williams 223-3402) writes:
>	1) The track length is useless beyond a certain value. 
>The water pressure will increase as you go down in depth, causing 
>the displacement to equalize diagonally across opposite sides.

Can't a wider track allow for many cups attached within the workable
depth?  If we can assume small enough friction in the chain/belt
system, their small additional forces should help.  Also, as long as
each pair that is at the same depth contributes a non-zero net force,
the chain as a whole is still helped.

>	2) The position of the weights show that the vector 
>addition of the gravity vectors for these weights will cancel out 
>any force supplied by the displacement.

I miss the point here.  Are you trying to say that the displacement
difference is countered by a torque resulting from movement of the
center of mass nearer to/away from the center of the system?

Like the original poster of this problem, I have never heard a
satisfactory (to me) explanation of this PMM.  I encountered it long
ago in a high school physics class; in fact, I was just describing it
to some of my coworkers in the hall, and then it appeared here!  If my
friends come up with the answer, I'll post it.

/jr