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Re: A weighty subject



John wrote in part:

On Tue, 12 Oct 1999, Joel Rauber wrote:

... I don't understand why the buoyancy doesn't count
people are willing
to take the scale reading at face value if you put the scale in a
non-inertial reference frame, but not do so if you immerse it in a
fluid?

For me, it is because there is no satisfactory operational way to
determine whether or not you are in a so-called "inertial
frame." I feel
liberated by the modern viewpoint which holds that *all* frames are
equivalent and that any inertial force is a gravitational force.

... Immersing object in fluid doesn't change weight,
despite the scale
reading changing. (I don't understand this one yet.)

Does your weight change when you stand on the scale and lean on the
bathroom counter? What's so special about buoyant forces?


Good points, along with what I mentioned about variations between different
kinds of scales used in the fluid, with different sets of different density
standard masses, etc.

... weight equals sum of all non-gravitational forces.

This seems to be a repeat of a prior corruption of my original "weight
equals the *magnitude* of the vector sum of all
nongravitational forces."

This is because I want weight to be tied to the gravitational
(i.e., net
inertial) force as measured in a frame in which the object is not
accelerating. That force is equal in magnitude to the net
*non*gravitational force which is often easier to determine.

Thanks for bearing with me and repeating yourself, I think I'm beginning to
get your's and I guess Leigh's and David's point about omitting such items
as buoyancy forces in your definition. I must cogitate further upon it.


... But I'd object to this on operational grounds of "how
do I weigh an
immersed object?"; in particular how do I do it with out
comparing to a
similar measurement in a vacuum?

Point taken. But at least this operational difficulty is more easily
overcome or at least made negligible than the one that
prevents us from
identifying inertial frames.

John Mallinckrodt mailto:ajm@csupomona.edu
Cal Poly Pomona http://www.csupomona.edu/~ajm