Re: [Phys-l] Zero-gravity vs. weightlessness

[John Denker <jsd@av8n.com>]

On 05/21/2008 02:51 AM, Savinainen Antti wrote:

> I happened to read AAPT periodical "Interactions" (April 2008). There
> was an open forum letter stating that weightlessness should not be
> confused with zero- or microgravity. The letter made a distinction
> between the airplane flying a parabolic path (zero-gravity flight)
> and the space station following a circular orbit around the earth.
>
> It was explained how in the space station "centrifugal force, due to
> inertia, equal and opposite gravity makes people and things orbit
> weightless". On the hand, the aeroplane is flying a parabolic path
> "that does not resist the force of gravity on the passenger and they
> are weightless but accelerating toward the earth due to gravity".

Words fail to express how wrong that distinction is.

> I have thought that both cases are equivalent from the point of view
> of relativity theory: the vehicles execute essentially free motion in
> gravitational field. Hence, the gravitational field in the frame of
> the vehicles is very nearly zero in both cases.

Exactly so.

> P.S: I know that the term "weightlessness" is quite ambigious but I
> don't think that it is a problem in these examples.

I wouldn't have said "ambiguous".  I think _frame dependent_ is the
key physics idea.  For example:

> the gravitational field in the frame of the vehicles is very ....
..........................^^^^^^^^^^^^^^^^^^^^^^^^^^^^

That is the unambiguously correct way to say it.