Re: [Phys-l] Weightless

["Jeffrey Schnick" <JSchnick@Anselm.Edu>]

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
> Sent: Tuesday, November 21, 2006 5:51 PM
> To: Forum for Physics Educators
> Subject: Re: [Phys-l] Weightless
>
> On 11/21/2006 05:14 PM, Richard Tarara wrote:
>
> > YES...I think many of us would go along with Anthony, and the idea
that
> the
> > astronauts are apparently weightless, or experience a weightless
> condition.
> > I use this, and believe I am consistent with my definition and
usage.  I
> > believe there are advantages to doing this for the level of student
> being
> > taught.
>
> 1) What level of student are we talking about?  I assume the students
>  have enough background to have some chance of understanding *some*
>  explanation of weightlessness;  oherwise this whole discussion is
>  obviously a waste of time.  The only question is /which/ of the
>  available explanations is most suitable.
>
> 2) How does "experiencing a weightless condition" differ from "being
>  weightless"?
>
>  Obviously I'm not understanding something here.  Are we really going
>  to argue about what the meaning of "is" is?
>
> 3) I've been following this thread pretty closely, and I don't think
>  anybody would disagree with the proposition that the astronauts
>  "appear" weightless in the spacecraft frame.  The problem lies
>  elsewhere.  There is one camp that says the astronauts appeare
>  weightless because they _are_ weightless ... and for the life of
>  me I can't figure out what the other camp is saying.  Apparently
>  they say "the astronauts appear weightless but are not weightless
>  (in the spacecraft frame)  because ......" and I have no idea how
>  to finish that sentence in a way that is consistent with the ordinary
>  definition of weight.
>
>  I repeat:  The problem is not with zero "apparent" weight.  The
problem
>  has to do with contrasting "apparent" weight with some other kind of
>  weight.  I have tried, but have been unable to figure out what this
>  other kind of weight is.

The other point of view is that weight is GMm/r^2.  The weight of the
astronaut is the GMm/r^2 force exerted on the astronaut by the earth.
Within that viewpoint there are at least two subviewpoints:  In one, the
concept of force only applies in inertial reference frames.
Classically: choose any frame you like from among the infinite set of
inertial reference frames.  In any one of them, the weight of the
astronaut is consistent with the acceleration of the astronaut, that is,
F=ma is true, with F being the weight GMm/r^2, acting on the astronaut.
The spacecraft has the same acceleration so the relative velocity
between the astronaut and the spaceship is constant when they aren't
bumping into each other.  Viewing the astronaut from a camera attached
to the spacecraft gives the illusion that there is no force on the
astronaut.  The astronaut and the spacecraft are both satellites in one
and the same orbit.

In the other subviewpoint, the non-inertial reference frame of the
spacecraft is acceptable, but in that frame, the GMm/r^2 weight force is
canceled out by the centrifugal force acting on the astronaut.  The
astronaut seems to be weightless because the net force on the astronaut
is zero.  The centrifugal force is much like the GMm/r^2 weight force in
terms of its effect, but, the weight force has an identifiable agent,
whereas the centrifugal force does not, so they are not the same kind of
force.  I think that you have done a good job of arguing that this
viewpoint, including both subviewpoints, is unconventional and
impractical.

> 4) Students (even the most naive students) start out with some rough
>  concept of weight.  If you talk to them about "apparent" weight, they
>  naturally assume that stands in contrast to some other kind of
weight.
>  If you use such a term but don't follow through with some sort of
>  contrast, that will cause all sorts of confusion.  In the field of
>  marketing and sales, this would be called overhanging the market,
>  by which I mean ginning up demand for a product you're not prepared
>  to sell.
>
> Again:  If you're going to say that the astronauts have zero
"apparent"
> weight, why not just call it weight (in the spacecraft frame) and be
> done with it?  If there's another part to the story, I'd like to hear
> it.
>
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