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Re: Weight and reference frame



However, the 'weight is the gravitational force due to the nearest planet'
camp doesn't agree that the astronauts are weightless. They are in
free-fall and therefore being robbed of their usual experience of an upwards
force to balance their 'weight', they experience a _sensation_ of
weightlessness. We've had this go around many times and I think both camps
get to the same place in the end. Both can be consistent with nomenclature
and both can explain the measured and perceived forces. There are
advantages and disadvantages to both approaches, and both are prevalent in
textbooks. If ALL books refrained from identifying the gravitational force
on an object (mg) as weight, then John's camp would have fewer problems. I
would also suggest that the more 'Newtonian' one wants to remain, the more
complications arise out of the 'weight is what the scale measures' camp.
The 'weight is the gravitational force camp' has different complications,
such as dealing with the rotational component and in having to talk about
the 'apparent' or 'perceived' weight. Once again, for terminal, intro
courses, it probably doesn't matter much which approach to take. For
physics majors, they should probably be exposed to both approaches, but
maybe the frame independent approach (scale measurement) is somewhat better.

My $.02 (once again)

Rick

**********************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556
rtarara@saintmarys.edu

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www.saintmarys.edu/~rtarara/
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----- Original Message -----
From: "John S. Denker" <jsd@MONMOUTH.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, September 25, 2001 12:15 PM
Subject: Re: Weight and reference frame


At 06:58 AM 9/25/01 -0600, SSHS KPHOX wrote:

... seems nice from the frame of reference of the mass being weighed.

Yes.

I am happy calling ...... the gravitational force but I will
also call it weight because I like to communicate with my students.

Communicating with students is commendable.

However, this does not require us to capitulate to whatever misconceived
terminology the students came in with.

The problem is that the students come in with a constellation of
unphysical
and mutually-inconsistent notions about weight (not to mention other
things).

In particular, their naive notion of weight resulting from gravity is
_almost_ right, but it is inconsistent with their notion of weightless
astronauts. They haven't figured out how to deal with this
inconsistency. Indeed, they probably haven't even clearly articulated the
inconsistency.

Specifically, they have some notions that are not portable from one
reference frame to another.

So where do we go from here? The best we can hope for is a
_correspondence
principle_.

That is, we give them a new notion of weight that is portable from frame
to
frame. We then show how this notion makes contact with their previous
knowledge in the various correspondence limits:
-- weight _almost_ equal to gravity in lab frame
-- weight equal to zero in space-station frame

===========================

BTW this way of doing business is not restricted to young students. It
applies equally well to professional researchers.

People think that research consists of discovering new things. Hah! If
only it were that simple!! In reality, most research begins by
discovering
that old ideas are not quite correct; only then can forward progress be
made. Unlearning is always hard. You can't get up each morning and say
"everything I know is wrong". Instead, you have to say "some piece of the
conventional wisdom is not quite right; I wonder which piece?"

And the correspondence principle is always important. After you have
discovered a new way of understanding something, you need to show how it
reproduces the conventional results in the appropriate limits.

=========

To repeat:
*) You mustn't let yourself be enslaved by the ideas you (or your
students) had yesterday; that would prevent all progress.
*) OTOH there are technical as well as pedagogical reasons for not
blowing off the old ideas entirely; as you progress through the land of
new ideas, keep track of how they relate to the old ideas.