On Thu, 18 Dec 2003 23:22:10 GMT, Gene Nygaard
wrote:

On Thu, 18 Dec 2003 19:41:06 GMT, R wrote:

On Thu, 18 Dec 2003 15:30:26 GMT, Gene Nygaard
wrote:

Bad example. All forces, of whatever kind, are totally irrelevant to
the weight on that driver's license.

This is definately the case with most of the women that I know.


3.4.1.2 Considerable confusion exists in the use
of the term weight as a quantity to mean either force
or mass. In commercial and everyday use, the term
weight nearly always means mass; thus, when one
speaks of a person's weight, the quantity referred to
is mass.

Oh, I don't know. Maybe it's because I live in a country using
english-based units; when someone says they weigh 170 pounds, they
mean weight,

How can that make any difference, when those pounds are legally
defined as 0.45359237 kg, all around the world where pounds were used?
It wouldn't make any difference if the NIST example had used
pounds--the only reason it used kilograms is because SP811 is the
official NIST "Guide for the Use of the International System of Units
(SI)."

Point noted; you are correct that legally pounds are measuring lbm,
or pounds of mass.

What is this country which uses the "licence" spelling and has pounds
on the driver's licence? Does Canada, or some of its provinces, have
pounds on the licence? Anyway, if it matters to you, you shouldn't be
the mystery woman without a name and without an email address, or you
should put that country info into a signature or something.

On the assumption that you're not trying to open a trolling front,
I'll say that it doesn't really matter who I am or where I am from.
That information is not difficult to divine, anyway, if you know how
to trace header information.

Bitching about spelling is bad form, as well.

as their weight was determined by a device created to
measure weight, not mass.

Nonsense. Sure, we often accept the measurements of a cheap spring
scale as a substitute for what we want to measure. Those scales
aren't very accurate for the measuring either force or mass--if your
mother's scale shows you to be 5 lb more than your scale home showed
an hour earlier, you don't congratulate yourself on a successful
weight loss program, do you?

But what about when you get serious about your weight, and weigh
yourself on one of those platform type beam balances at the doctor's
office or the gym? Those are mass-measuring devices, as are the ones
which promise

HONEST WEIGHT
NO SPRINGS

They don't measure mass in the absence of a gravitational field,
though I agree that they will give consistent readings any time that
they are oriented against any field of any strength. That is, a scale
the likes that you are talking about (no springs involved) that
returns a value of "100 pounds" on the earth will return a value of
"100 pounds" on the moon as well. As such, you're right -- those
scales measures mass, they just fail to work in a zero-g environment.
But they still measure mass.


But that's moot anyhow; while you are
correct in that we are entangled in a difference in semantics, the
real difference is not the one that you are referring to. Peter, for
example, is referring to weight in the Newtonian sense. In this
context, weight is a force, and acceleration is proof of that force.
Peter is correct within the framework that he's talking about. I am
referring to weight in the Relativistic sense.

No. As I had already pointed out in my earlier reply to Peter,
quoting from Sears and Zemansky, your differences are at a more
fundamental level than differences in how you would define it in
Newtonian physics or using relativity. As S&Z said, "There is no
general agreement among physicists as to the precise definition of
"weight."

The differences they are talking about hinge on things like whether or
not "centrifugal force" is accounted for in your definition of weight.

Yes, but, again, centrifugal force is not a force in relativistic
physics, it is only a property of the geometry of spacetime. In the
debate between Peter and David (which I have unceremoniously butted
into), this is important. It demonstrates that both of them may be
correct, depending on which variant of physics you use to view the
problem, and explains a bit why the two of them are so far from
agreeing. In Newtonian physics, what you are saying is important, but
in relativistic physics, "centrifugal force", "gravitational force",
"acceleration" -- these are all the same thing.

In this context,
weight is not a force, it is part of the geometry of spacetime.
Philosophically, Relativity probably holds more weight in this
argument. The whole problem with the Newtonian perspective in this
case is "what perspective do you approach this from"; i.e., what
gravitational fields do you reference when calculating "weight". Do
you leave out the sun? Do you leave out the galaxy?

That applies whether you are using Newtonian physics or relativity.

No, really, it doesn't, given the problem at hand. The Newtonian
approach states that one must measure the effects of gravitational
fields to determine weight, even if one is in freefall (orbit). This
can be difficult, as one must take into account forces which one may
not even be aware of or able to measure. Relativity assumes that
anything in freefall is moving in a straight, non-accelerated line
along the geodisc, and thus has no weight. If something is not being
accelerated, it has no weight, as there is no "force" acting on it.

How does Newtonian physics in this instance account for the fact
that we are *accelerating* away from distant galaxies at near the
speed of light? Why not factor in this "repulsive" force to find
one's weight? Certainly the force pushing us away is as valid as the
force that draws us towards massive objects. But then where do you
begin to measure the force? The force is huge if you measure out to
the edge of the observable universe, but virtually nonexistent if you
measure across the solar system. Relativity doesn't have this
problem. I tend to like Relativity to answer this question of weight
because with Newtonian physics the answer to this "weight" question is
arbitrary to the frame of reference that you choose to include at
possible the exclusion of other frames of reference; be it earth, sun,
galaxy, or universe. Relativity assumes that all frames of reference
are equally valid; or, more specifically, the only frame of reference
that is of any matter to you is your own.

Heck, the galaxy
itself is accelerating. Relativity is not bothered by these
questions. Your own point of view is the only one that matters; if
you are in freefall, if you are moving along the geodisc, you are
"weightless". If you're not, you're not.

Under this definition, the Earth itself has no weight.

-R

Yet Cavendish was successful in "Weighing the Earth" (the title of his
paper, IIRC)--in the same meaning used in commerce and for body
weight, not the different force definitions you and Peter have been
arguing about.

Henry "weighed the earth" using Newtonian laws and did so long
before the discovery of Relativity. He was correct within the
Newtonian framework; however, often, when dealing with matters of
stellar physics, relativistic physics is the often only way to fly. I
don't really care what commerce and those who divine national
standards think about how "weight" should be described; they have no
use for Relativity (let alone Quantum gravity), as Newtonian physics
is almost completely accurate in measuring their widgets. I'm
interested in the philosophical question of which approach is
"correct" (so far as we can tell) and most useful in describing
reality. Given that relativistic physics is correct in every instance
that Newtonian physics is correct and is also correct in instances
where Newtonian physics breaks down, I suspect that relativistic
physics is the right course to take in answering this "weight"
question. Relativity doesn't ask one to believe in an imaginary
"weight" that can't be directly measured by someone in a closed room
with no windows or external sensors.

-R