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Re: [Phys-l] definitions ... purely operational, or not



On 11/07/2010 01:10 AM, Chuck Britton wrote:

In previous go-rounds many of us had come to the conclusion that is
useful to use the term 'weight' to mean 'that which is measured by a
spring scale.'

You are as always free to define terms however you choose,
but please be aware that other may choose differently.

In particular, if you want to define some sort of _net weight_
or _effective weight_ and explain in a footnote that it includes
a buoyancy term (as Michael H. did), then I am 100% OK with
that. The rule here is simple: Say what you mean and mean
what you say.

Sometimes you care about the force on the scale (mg plus
buoyancy plus whatever else) and sometimes you care about the
plain mg. It depends on the application. You cannot make
this dependence go away by "defining" weight one way or the
other ... not if you want anybody else to understand you.

More generally I object to the extreme operational definition
that weight is "whatever the scale reads" because it leads to
nonsense if the scale is miscalibrated or misused or nonideal
in any way. In the real world, it is extreeemely common to
use an instrument that gives non-ideal "raw" readings and to
apply corrections during a later analysis stage.

This is particularly clear in the case where the scale is
calibrated in units of *mass* not weight ... which is the
overwhelmingly common case. (Recall that the pound is
defined as a unit of mass according to US law, and always
has been.) It would be insane to define mass as "whatever
the scale reads". Most people want the mass to be invariant,
independent of changes in buoyancy, changes in g, et cetera.
A balance-scale is to first-order insensitive to changes
in g, but a simple spring-scale is not ... and both types
need to be corrected for buoyancy if you want a serious
determination of mass. In air, buoyancy corrections are
quite significant relative to the resolution of an ordinary
analytical balance ... and underwater the corrections are
huuuge, no matter how crude the scale is.