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

[Tom Sandin <sandint@ncat.edu>]

I just read that same letter yesterday afternoon 
and wondered why the editors would publish it in 
a magazine for physics teachers (to stir up 
interest?).

Of course, a portion of the confusion is because 
there are no completely agreed-upon definitions 
of "weight' and "weightlessness".

I don't want to reopen those definition 
controversies now. However, for those who do not 
receive Interactions, the author of the letter 
(James F. Jackson of Carlisle, Indiana) finishes 
it by writing,
	"Physicists have caused much confusion 
about the forces in rotating bodies by taking 
inertial and non-inertial frames of reference. 
They would understand the forces in seed and 
fertilizer spreaders; with rotating impellers, 
centrifuges, centrifugal pumps and roller 
coasters if they use the radius of rotation as 
the frame of reference."

Tom Sandin


At 12:51 PM +0300 5/21/08, Savinainen Antti wrote:
> Hi,
>
> 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".
>
> 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.
>
> Or am I missing something here?
>
> Regards,
>
> Antti
>
> P.S: I know that the term "weightlessness" is 
> quite ambigious but I don't think that it is a 
> problem in these examples.
>
> Antti Savinainen, Ph.D., B.Ed.
> Senior Lecturer in Physics and Mathematics
> Kuopio Lyseo High School
> Finland
> E-mail: <antti.savinainen@kuopio.fi>
> Website: <http://kotisivu.dnainternet.net/savant/>
>
>
> --
>
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