Re: First Law????

["Donald E. Simanek" <dsimanek@EAGLE.LHUP.EDU>]

On Sat, 23 Jan 1999, Chuck Britton wrote:

> > On Sat, 23 Jan 1999, Donald E. Simanek wrote:
> >
> > > Here's a first quick stab at a gravity shield motor.
> > >
> > > Load up a Ferris Wheel with as much mass as feasable.
> > > Position the wheel so that one side of the wheel is shielded and the other
> > > half is not.
> > >
> > > 'Clearly' the shielded side will rise because is has less 'weight' than the
> > > unshielded side.
> > >
> > > Put a generator on the axis and away we GO!
> >
> > If your partially gravity-shielded Ferris wheel *did* rotate, through
> > angle theta, what would be its change in potential energy during this
> > process? Zero. So why do you think it *would* rotate?
> >
> > There's always as much mass moving into the shielded side as is leaving
> > the shielded side.
>
> But we're IGNORING any sideways forces.

I was not speaking of any sideways forces. But they are there and you
can't ignore them if you choose to do a force or torque analysis (see
below). I just remarked that *if* the wheel rotates (or if you rotate it
by driving it) the amount and distribution of mass on the shielded side
remains the same, and the amount and distribution of mass on the shielded
side remains the same, so the gravitational potential energy of the total
mass *has not changed*. In any imaginable "cycle" there's no change in the
wheel's energy. So why should it rotate?

If you wish, consider a uniform wheel. If it is rotated, its state is
physically indistinguishable from that of its previous position, no matter
whether there's a gravity shield under part of it or not.

The principle underlying this is that if a mechanism has two positions,
and the two positions have identical "state", then the mechanism won't
move from one state to the other by itself.

> Lifting a loaded bucket in the shielded area gives it LESS GPE than the
> loaded bucket OUTSIDE the shielding loooses in coming down. If the
> shielding is 50% effective then fully HALF of the GPE lost by the outside
> bucket could be used to drive the generator.
>
> nicht wahr?

I'm not convinced, though a detailed force analysis of this explanation
would be interesting and instructive.

Let's say the shield reduces g by 50% as you say. Then the potential
energy of a bucket of mass m in the unshielded region changes by mgh as it
moves from the top of the wheel to the bottom, a vertical distance of h.
Suppose our reference for potential energy is at the bottom of the wheel.
This mass increases its potential by m(g/2)h = mgh/2 in moving from bottom
to top in the shielded region. Then it gains potential energy of
mgh - mgh/2 = mgh/2 as it moves across into the unshielded region. No
energy excess to tap!

This change in potential at the top (and bottom, depending on your choice
of zero of potential) does imply a horizontal force. Indeed the torques
due to these forces at the boundary of the shielded and unshielded
regions, along with the torques due to gravitational forces on the
separate parts of the wheel, add to zero. There's something profound
underlying all of this.

What say you?

I'm reminded of an 18th century proposal for a buoyant motor. A wheel in
the form of a sphere or cylinder on frictionless bearings is immersed with
its left half (to the left of the shaft as we look along the shaft) in
liquid, using a frictionless seal surrounding the wheel (so no liquid
leaks out). The right half is in air. The supposition is that the left
half experiences a buoyant upward force which makes it lighter and
therefore the wheel will turn. Some books (Ord-Hume's "Perpetual Motion, a
history of an obsession") say "Obviously it won't work because you cant
have a frictionless seal". Wrong. It wouldn't work even if you *had* a
frictionless seal.

The gravity shield machine is mentioned in Ord-Hume's book, implying that
it was a proposal of the 19th century. Ord-Hume was an expert on antique
clockwork mechanisms, not a physicist, so his book is very deficient in
physics, but is an interesting account of the broad history of perpetual
motion attempts, with lots of interesting pictures. Avoid the shoddy
Barnes and Noble reprint now in stores.

> now maybe the spoke's lengths are effected a bit as they pass through the
> shield, so it might not be QUITE that effective.

I'll be happy to ignore those details, as I'll happily ignore friction.
Any perpetual motion machine proposal I've seen still wouldn't work even
if you reduced friction to zero, and made everything of "perfect"
materials and components. To say, as Ord-Hume often does, that "It
won't work because of friction" is a cop-out. I'm not suggesting that
*you* or anyone else here said anything like that, however.

> It's no accident that stressed          Chuck Britton
> spelled backwards is desserts.          britton@odie.ncssm.edu

        -- Donald

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Donald E. Simanek
dsimanek@eagle.lhup.edu                 http://www.lhup.edu/~dsimanek
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