Re: A weighty subject

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

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