Re: A weighty subject

[Richard Tarara <rtarara@SAINTMARYS.EDU>]

It seems to me that the problem with Leigh's and John's definition of weight
is a conceptual one for intro students.  In most force based problems, one
of the forces is often labeled WEIGHT (OK, you _could_ label it the
Gravitational Force of the Earth on the object..but that is awkward and you
have tens of thousands of instances of the other to expunge).  The word is
(from Ludwik) part of their vocabulary.  The conceptual problem (I think) is
WHAT IS THE SOURCE OF THIS FORCE CALLED WEIGHT.   Calling it what the scale
measures is simple, is consistent, probably helps with GR, but I can't see
it working for most intro classes.  As John notes below, most of the time
the scale IS reading the gravitational force.  But....the real divergence
between viewpoints comes when an object is in free fall (say near the
earth--in orbit if you wish).  From the Newtonian viewpoint, the object is
accelerating and therefore must be experiencing a net force in the direction
of the acceleration.  For the 'scale camp' the weight is zero.  But what
force is causing the acceleration.  Ok, back to the 'Gravitational Force of
the Earth on the Object', but then why does the scale read differently at
different points on the earth surface (apart from the centrifugal effects)
if not because the R varies in GMm/R^2?  The scale camp has no real physical
source for WEIGHT...only a reaction of the scale to a variety of forces.

To be sure, there are difficulties teaching any of the three primary
definitions:

1)  W = mg (where g is dv/dt locally).
2)  W = mg (where g = GM/R^2)
3)  W = what the scale reads.

I prefer (2), but then must talk about the fact that the scale doesn't
necessarily read this value due to additional forces or motions.  However,
for me, this is very useful in describing our sensations in accelerating
frames since (the experiencing forces backwards ideas I've posted before.)
The great advantage of this definition is that it provides a well defined
SOURCE and a well defined way to calculate the force called WEIGHT.  {OK, as
the students get a bit more sophisticated then g becomes the net
gravitational field due to any and all other masses..but you get the idea.}

I can't see that it is useful to eliminate the word 'weight' from our
instruction--it is just too prevalent in student's everyday experience.
They are aware of astronauts being 'weightless' and one's weight being less
on the moon.  Again the text books are full of problems dealing with weight
and free-body diagrams with the gravitational force identified as weight.
Attempts to expunge the word in one course will only lead to confusion in
the next where the term may be widely used.

As to this list actually changing the definition or how it is used in
introductory textbook...Fat Chance!  It seems to me somewhat presumptuous of
a few dozen of us (all who regularly participate in these discussions) to
believe that we somehow know better than the bulk of the physics community,
and the collected wisdom of generations of physicists and textbook writers.
Yes it is possible for this collected wisdom to be wrong (whatever wrong
means ;-) but unless there is a strong consensus within _this_ group and a
way to convey this consensus to the larger community, we are really just
'shooting the breeze' here.  That's OK, useful for each of us to reconsider
these basic ideas, but I'm sure that every textbook author whose books or
revisions have undergone extensive peer review have had to defend themselves
against many (if not most) of the criticisms voiced on this list.  They seem
to have survived such!

Rick

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Richard W. Tarara
Department of Chemistry & Physics
Notre Dame, IN 46556
219-284-4664
rtarara@saintmarys.edu

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----- Original Message -----
From: John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>

> On Thu, 14 Oct 1999, Leigh Palmer wrote:
>
> > Perhaps I was unclear. Daniell's definition is traditional; I'm
> > advocating a different definition. Daniell says g represents the
> > gravitational force. I think it should represent the inertial
> > force which is measured in the laboratory by a scale.
>
> And, of course, that is at least partially because you (and I) think the
> inertial force measured by the scale (neglecting the effects of buoyancy)
> *is* the gravitational force.
>
> The fact that one may not want to explain that to an introductory student
> is no reason not to give them this useful (and, as they may discover
> later, more meaningful) definition of weight.
>
> John Mallinckrodt      mailto:ajm@csupomona.edu