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

[curtis osterhoudt <flutzpah@yahoo.com>]

Leaving aside the question of tidal forces and so on (which I think the letter may not have been addressing), there is a distinction important to those doing experiments in space (growing large crystals and so forth): microjitter. If I remember correctly, the term "microgravity" is preferred by NASA in engineering contexts, because there are always small accelerations (not caused by gravity!) on-board the Shuttle, the ISS, and so forth, which transfer to the containers holding the experiments and can disrupt them. 

   There is no difference to the physics between the path followed by a passenger "thrown" in a parabolic path by the Vomit Comet (and then followed by the airplane to simulate zero-g) and the path followed by a passenger "thrown" by a rocket around the earth. The only important distinction is that the parabolic path has to end soon, because the earth doesn't curve out of the way fast enough. 


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Down with categorical imperative!
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----- Original Message ----
From: John Denker <jsd@av8n.com>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Wednesday, May 21, 2008 6:55:59 AM
Subject: Re: [Phys-l] Zero-gravity vs. weightlessness

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.



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