Re: Weight and reference frame

["John S. Denker" <jsd@MONMOUTH.COM>]

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.