Re: [Phys-L] Perpetual Motion Machine, Earth as

[Ken Caviness <caviness@southern.edu>]

Have I missed something obvious here?  I don't see what the fuss is about here.

An object moving through space at constant velocity is in "perpetual motion" in the same sense that a rotating body in empty space is in perpetual motion.  But there's no change of energy, we're basically talking about conservation of linear (*) or angular momentum.  No law against that, on the contrary!  

There's nothing at all wrong with this sort of perpetual motion.  What is impossible is perpetual motion of some object while energy is transferred from that object, either to be used elsewhere or turned into heat (such as when the object is subject to friction), without a corresponding energy input.  

I repeat:  Perpetual linear uniform motion is _expected_, in the absence of friction and other forces.  (Think Newton's 1st Law.)  Similarly, perpetual rotational motion is _expected_ in the absence of an applied external net torque:  conservation of angular momentum.

We can "build" a perpetual motion machine of these trivial types any time we want to:  Just fire a spacecraft out into space, exceeding escape velocity for Earth, and Sol, and preferably for the Milky Way galaxy and the Local Group.  Give it a little rotation to start with, and then let it coast on forever and ever.  Just don't expect to extract energy from the linear and/or rotational motion, without slowing them down.

Oh, wait: We've already sent Voyager I & II out of the solar system, right?  Barring a collision (friction!) they'll keep on forever.  Done!

KC

(* Or we might invoke relativity in the case of the uniformly moving object: it's just as valid to view it as being at rest.  It is at rest according to observers in its inertial reference frame.)

Доверя́й Го́споду от всего́ се́рдца и не полага́йся на со́бственный ра́зум; познава́й Его́ во всех свои́х путя́х, и стези́ твои́ Он сде́лает ро́вными.  Притчи 3:5-6.

-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@www.phys-l.org] On Behalf Of Bill Norwood
Sent: Thursday, 2 April, 2015 1:21 PM
To: Phys-L@phys-l.org; Brian Whatcott
Subject: Re: [Phys-L] Perpetual Motion Machine, Earth as

Yeah, but somebody had to build it.
Far as I know none of our species built the earth.

On Wed, Apr 1, 2015 at 11:02 PM, brian whatcott <betwys1@sbcglobal.net>
wrote:

> A levitated top in a vacuum chamber is spun up.
>
>  This represents a reasonably persistent rotation....
>
> https://www.youtube.com/watch?v=8nLd5q7UOyE
>
> Brian Whatcott
>
>  * /From/: Bill Norwood <bnorwood111@gmail.com
>    <mailto:bnorwood111%40gmail.com>>
>  * /Date/: Thu, 26 Mar 2015 10:28:47 -0400
>
> ----------------------------------------------------------------------
> --
>
> Might as well open this one up for critique:
>
>
> Earth is a better perpetual motion machine than anything on it could 
> ever be, say, relative to a human lifetime.
>
>
> On Wed, Mar 25, 2015 at 7:42 AM, Peter Schoch <pschoch@fandm.edu> wrote:
>
>  So, we've gotten to the point in the semester where the students are
> > finding things on the Internet they want me to explain -- in addition 
> > to covering the 'normal' material.
> >
> > One student found this:
> >
> > https://www.youtube.com/watch?v=7KqOwJKWIAw
> >
> > and wants me to explain how this is NOT a perpetual motion machine.
> >
> > I think I can explain the chain one fairly well.  I'm having trouble 
> > with the liquid motion one, explaining it in terms of freshman physics.
> >
> > Any suggestions would be welcome.
> >
> > Peter Schoch
>
>
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