Re: Centrifugal force

[Hugh Logan <hlogan@ix.netcom.com>]

Leigh Palmer wrote:
> > -----Original Message-----
> > From: Leigh Palmer <palmer@sfu.ca>
> > To: phys-l@atlantis.uwf.edu <phys-l@atlantis.uwf.edu>
> > Date: Saturday, May 02, 1998 12:59 AM
> > Subject: Re: Centrifugal force
> >
> >
> > > > As an afterthought, would it make any difference in the outcome of
> > > > Chuck's proposed experiment if the unpowered rocket were resting on the
> > > > launch pad (a launch pad one meter higher to be exact) instead of
> > > > hovering? A lot of fuel could be saved that way. I would not expect it
> > > > to make any difference whether the rocket was supported by the contact
> > > > force of the launch pad or the thrust of the rocket engine.
> > >
> > > No, it wouldn't. That is the point of Donald's example. Let's stick
> > > to the hypotheses what brung us, eh? Discussion is simpler that way.
> > > If it makes no difference, and we don't have to pay for hypothetical
> > > fuel, please let it be!
> > >
> > > Leigh
> >
> > Yes it would!  The rocket must produce an acceleration equivalent to the
> > (now removed) gravitational field of the earth!
>
> Previously I believed we were on the same page as far as the physics
> was concerned, that we only differed in outlook. Now it is clear we
> are not in agreement on the physics. I claim the results of the
> experiments performed in the rocket depend not at all on whether the
> rocket is tethered firmly to the pad and running, running and
> hovering above the pad, or resting on the pad. Of course the rocket
> must be running after the Earth is removed; we understand that.
>
> By the way, I see what the problem may have been. What was "Chuck's
> proposed experiment"? I was under the impression this was Donald's
> gedanken experiment under discussion here.
>
> Leigh

Chuck wrote:
> Next question. Let's ignore the 'slight bump' but keep in mind that the
> rocket DOES have a finite size. If our experiments are accurate enough can
> we not measure that two falling objects in the rocket will move closer to
> each other because of the non-uniformity of a 'real' gravitational field,
> than they would when the acceleration is 'caused' by the rocket blast?
>
> I'm familiar with the word 'local' used to describe the principle of
> equivalence and GR, but is there a fundemental arguement that forbids our
> experimentally distinguishing between the divergent gravitational field and
> the uniform acceleration of the rocket??

Leigh,
  I agree with everything you wrote above. I was trying to say that
detecting the convergence (not divergence) of the earth's gravitational
field by dropping two objects simultaneously and trying to determine
whether they would move closer together as they fell could just as well
have been done with the rocket resting on the launch pad. Although
detection of the convergence of the earth's gravitational field might be
beyond the capability of the instrumentation, one would expect to detect
it if sufficiently sensitive instruments were available. But there would
be no reason to expect the convergence to have any possibility of being
detected once the earth were whisked away. I think Chuck is concerned
that the rocket ship might be too large (being of "finite size")for the
gravitational field to be considered locally uniform in the cases where
the earth is in close proximity. (I referred to a positive detection of
the convergence of the earth's gravitational field as a "tidal effect"
in my previous messages).

Hugh