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Re: A weighty subject



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?

... 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.

... 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