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It seems to me that we're all throwing out ideas based upon differentYou are free to pick one, and only one.
definitions of "weight".
From one point of view, weight is the forceThat is not a definition because it gives me no clue as to how to measure "weight".
applied to an object due to a gravitational field.
Thus weight is mg where
g can vary. Wherever there is a gravitational field, there is weight.
Based on that viewpoint, a bathroom scale doesn't necessarily measure one'sYou are saying that different definitions are incompatible. Big deal! So pick one, and stay with it.
weight at all. If you jump on the scale you get readings higher than your
weight.
If you are in freefall with the scale you get a reading of zero.If the scale defines weight, you are wightless.
Your weight didn't change,But it did, according to the scale definition. According to some other definitions, it may not have. But you are entitled to only one definition.
but the scale reading did, so the scale isn't
necessarily measuring your weight. If, otoh, you accept your weight as
being the scale reading, then your weight can vary from moment to moment
(which might make diets obsolete). Oh, and this, too, appears to me to be
an unambiguous definition. For beginning/introductory courses, I prefer
this definition for weight.
----- Original Message -----
From: "Jack Uretsky" <jlu@hep.anl.gov>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Monday, November 20, 2006 4:59 PM
Subject: Re: [Phys-l] Weightless
But I don't know what you mean by "true weight". I am strictly an
operational physicist. "Weight" is the reading on a scale. That's a
definition that has no, as far as I can see, ambiguity. It may require
extrapolation to the very large and very small, but there is a regime
where it can be straightforwardly applied.
Regards,
Jack
On Mon, 20 Nov 2006, Anthony Lapinski wrote:
People in their everyday experience think a bathroom scale measures their
downward weight. And so when the scale reads zero for the astronauts,
they
think g = 0! Lots of confusion and misconceptions with this with ALL
people.
I guess one could never have a true weight equal to zero unless you are
"very far" from all objects (so that g = 0).
We as teachers will never reach any consensus on this topic. What works
for one might not work for someone else. To me, bathroom scales are
common
devices, and very useful for illustrating ideas about free body diagrams
in elevators, N3L for objects colliding, and "apparent" weightlessness.
Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
But if an astronaut isn't "weightless," then what is? Isn't the word
of absolutely no use if we can't apply it to astronauts? Weight
implies "heaviness" and heaviness implies (to me) a tendency to stay
"pinned by gravity" to some surface. If a person (or thing?) finds
that she (or it?) has absolutely no such tendency, then it seems to
me that she has every right to say that she is "weightless" and that
we ought to agree with and abide by her assessment.
There's certainly no problem justifying any of this from a Newtonian
viewpoint. It's simply a matter of deciding what you want to mean by
"weight." I don't see any good reason to require that it mean the
same thing as "gravitational force." Indeed, it seems to me a
genuine disadvantage.
Anthony Lapinski wrote:
I agree. This is why I discuss bathroom scales and apparent weight. If
someone says there is gravity acting on the astronauts AND they are
also
weightless, it ads to more confusion...
[Rick Tarara wrote:]
In the 'g' thread, we've (yet again) opened up the definition of
weight
debate. While I see certain pedagogical advantages to either of
the two
main approaches, I would ask if the proponents of saying that one IS
weightless while in the space station can explain that from a
Newtonian
viewpoint? Seems to me that there is only one force acting on the
person.
If we call that the gravitational force due to the earth, then what
(again
in the Newtonian viewpoint) is weight? Isn't this gravitational
force a
component of your weight? But it is the only component here in the
Newtonian view and is not zero!
John Mallinckrodt
Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>
and
Lead Guitarist, Out-Laws of Physics
<http://outlawsofphysics.com>
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--
"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley
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