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From: jeff@rtech.ARPA (Jeff Lichtman)
Newsgroups: net.misc,net.physics,net.sci
Subject: Re: Perpetual Motion Machine (Honest!)
Message-ID: <270@rtech.ARPA>
Date: Wed, 27-Mar-85 04:54:52 EST
Article-I.D.: rtech.270
Posted: Wed Mar 27 04:54:52 1985
Date-Received: Mon, 1-Apr-85 03:44:15 EST
References: <261@eneevax.UUCP> <135@tekfdi.UUCP> <300@ttidcc.UUCP>
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Organization: Relational Technology, Berkeley CA
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Xref: watmath net.misc:7721 net.physics:2361 net.sci:317

> In article <135@tekfdi.UUCP> rick@tekfdi.UUCP (Rick Wilson) writes:
> >
> >One little magnet you pull off the bottom of a lady bug that's holding
> >a shopping list to your refrigerator door will pick up a hell of a lot
> >of nails (one at a time) before it wears out.
> 
> That little magnet won't pick up _any_ nails.  It will attract a  nail  and
> probably  hold  its  weight  if  brought  into direct contact.  Any lifting
> energy comes from an outside source (e.g.: your hand and arm).
> 
> The Polymath (aka: Jerry Hollombe)

Imagine suspending a magnet by a string, and then sliding a nail underneath it.
The magnet will pick up the nail, and you haven't provided any lifting power.

Here is what happens.  When the piece of metal was made into a magnet,
potential energy was stored in it.  This energy is changed to kinetic energy
when it moves nails.  It only enough energy to pick up a finite number of
nails at one time; a magnet that has a lot of nails clinging to it won't
attract any more.

When you pull the nails off the magnet, you do work.  This work is returned
to the magnet as potential energy.  Thus, the little refrigerator magnet
isn't a perpetual motion machine.  It will do a finite amount of work (the
amount of work done on it to magnetize it, assuming perfect efficiency),
and you must perform the same amount of work (thus putting energy into the
system) in order to restore the magnet's former ability.
-- 
Jeff Lichtman at rtech (Relational Technology, Inc.)
aka Swazoo Koolak