Re: [Phys-l] Weightless

["Richard Tarara" <rbtarara@sprynet.com>]

> [Original Message]
> From: John Denker <jsd@av8n.com>
>
> 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.
>
> 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.
John--Read Robert Carlson's reply.  

"Physics for Scientists and Engineers, Tipler, 3rd
Edition page 84 discusses apparent weight and
weightlessness as you have been using these terms. 
Defining weight as mg and mMG/r^2 is common practice
in high school and first year college level physics
texts. Engineering Statics and Dynamics texts also
use these definitions for weight. You are not alone
in your views, at least at the level you are teaching."

Read back through many of the posts here.  If one's definition is the
above, then the astronauts can't be weightless because mg is not zero. 
They can feel weightless because there are no forces opposing the 'weight
force'.  We have become conditioned to INTERPRET the upwards force of the
floor or chair or bed AS our weight.  When that upwards force is different
than our weight we EXPERIENCE being heavier, lighter, even weightless. 
Likewise, when that force is present in the absence of gravitational
forces, we feel as though we have weight.  This is the point of view that
has been detailed here often.  You may not agree with it, but it has been
right there for you all along.

In defense of the above over the astronauts ARE weightless (an approach I
understand but don't like for HS and general education students) then
wouldn't one have to say that the skydiver is weightless as he emerges from
the plane, then gains weight as he falls until reaching is nominal surface
weight at terminal velocity, and then if his chute fails to open he has a
moment of VERY large weight when he hits the ground?  This seems less than
simple to me.  I would prefer to say that his weight (mg) is the same
throughout but that the other forces he experiences along the way alter his
perceptions.

To Alphonsus:  Bob's post was a joke, as are the responses (mine and JM's)
are too!   <smile>

Rick