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Re: What is weight? (was Re: Internal or external?)



Some people have more or less described what I think is common among
chemists. I mention chemists because they have to answer questions
about mass versus weight quite a bit, and they also have to make
buoyancy corrections, and they often do labs to show the difference
between these things (as do some physicists).

I think it is extremely common to define "weight" as mg, where g is
your local value for objects that are static with respect to your
planet (or better, inertial with respect to your planet). Then define
"apparent weight" as what the scale reads. It is important, of course,
for the chemist to get to the actual mass of the material. They take
their balance reading as an apparent weight, and then correct it to the
true weight and ultimately to the mass by applying whatever correction
factors are needed. I think the correction is most often buoyancy, but
I suppose it could be extended to chemistry labs in elevators.

BTW, I have always remembered a wise statement from an undergraduate
chemistry professor who gave an alternative definition of mass apart
from the inertial definition and the gravitational definition. The
statement is: "mass is what you measure on an equal-arm balance."
Until we get a mass standard different than some hunk of Pt-Ir, this
definition of mass is pretty good, I think. And, whenever masses are
compared to THE standard, this is how it's done.

Of course, today we have to extend the "equal-arm balance" to include
the analytical balances that have their own internal standard. Some of
our electronic analytical balances auto-calibrate every time they're
turned on using S1-class "weights" built into the balance. Hence,
these balances will provide the correct mass whether you are on the
moon, or in an elevator, as long as the apparent weight of objects
stays steady at a non-zero value during both calibration and weighing.
However, they do not automatically correct for buoyancy, so if that is
significant for you, you still have to call the reading an "apparent"
reading until buoyancy is taken into account.

Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817