Re: weightless in orbit

[Savinainen Antti <antti.savinainen@KUOPIO.FI>]

Hi,

I wrote:
> I disagree with Leigh that the gravitational field would be zero (or very
> nearly zero) in the orbiting space shuttle. The very reason for the
> orbiting is the gravitational force exerted by the Earth and hence the
> gravitational field cannot be zero. Of course astronauts inside the
> shuttle feel apparent weightlessness because the satellite is in a free fall.

John Denker wrote:
> This position is inconsistent with modern physics.

> We are accustomed to measuring quantities that are very sensitive to what
> frame of reference is used;  this includes velocity, kinetic energy, and
> many other frame-dependent quantities.   So it is with something that Antti
> would call "apparent gravity" and modern physics calls "gravity":  inside
> the orbiting shuttle this quantity is essentially zero.  An observer on a
> tall (!) tower, in the earth's reference frame, would measure a value of
> 9.8m/s^2 for this same quantity at the same location as the shuttle flies
past.

> Leigh's statement is 100% correct in the specified orbiting frame of
> reference.  Antti's counterargument that "the gravitational field cannot be
> zero" is not true in the orbiting frame of reference, although it would be
> true in a nearby tower-based frame of reference.

Thanks for this essential correction. I was thinking of the situation from "a
nearby tower-based frame of reference" without explicitly stating it. Now I see
the point that Leigh made.

I am teaching high school physics (International Baccalaureate). So the level
is not terribly advanced. It is not an easy task for high school students to
understand that a change in direction (e.g. orbiting) implies acceleration and
a net force. That may be the reason why I (and the syllabus) address mainly
inertial reference frames. Of course it is not an excuse for careless
interpretation of Leigh's argument!

Best regards,

Antti Savinainen
Finland