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From: usenet@ucbvax.ARPA (USENET News Administration)
Newsgroups: net.physics
Subject: Re: cancelling forces
Message-ID: <10492@ucbvax.ARPA>
Date: Sun, 29-Sep-85 21:32:53 EDT
Article-I.D.: ucbvax.10492
Posted: Sun Sep 29 21:32:53 1985
Date-Received: Tue, 1-Oct-85 03:22:44 EDT
References: <546@sri-arpa.ARPA> <116@netex.UUCP>
Reply-To: rimey@ucbernie.UUCP (Ken Rimey)
Organization: University of California, Berkeley
Lines: 45

>> Here's the question...  If I place them on opposite sides of the box,
>> the pushes will cancel.  Now I appear to be getting no energy out of
>> this system, at least not in the form of a moving box.  I am still
>> putting as much energy into the system. ...
>> 
>> -Ken Sloan

No, you aren't putting as much energy into the system.  If you measured
it, you would find that the power consumption of an electric motor drops
when you prevent it from turning.

The reason this is not intuitive is that our experience is mostly with
inefficient machines.  In particular,

	1. Electric motors lose power to electrical resistance, regardless
	of whether they are doing work.  I think this effect is smaller
	than you would think.  Does anyone know how efficient common
	electric motors are?

	2. Muscles are peculiar in that how tired they get is dependent
	mostly on how much force they exert.  I think this is because
	they are composed of fibers, each of which repeatedly contracts
	momentarily.  To see that force is not generally related to
	energy consumption, think of stretching a rubber band between
	two immobile points.

I don't see any obvious difficulties in building an electric motor with
superconducting magnets.  It also seems to me that such a motor would
be essentially 100% efficient.  It would use power when turning against
a load, but not when it is prevented by force from turning.

Now, I have a complaint.  This was a previous answer to the same question:

>You bet! The energy is being translated into heat, inside the drive
>devices of the robots. (Carefull, you may melt down!)  Eventually, the
>heat would have to be radiated, in some fashion, to the universe at
>large. (Entrophy wins again!)  I suggest you look up the laws of
>Thermodynamics.

This is at best misleading.  I'm sure the tone is unintentional, but I
must say it is bullyish.  "I suggest you look up ..." is too commonly
heard in this newsgroup.

						Ken Rimey
						rimey@dali.berkeley.EDU


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