ingvar_johansson wrote:
Joe C. extracted from SI/VIM:
>> n.b. - Units alone are not sufficient for determining "comparable
>> quantities". (01)
Ingvar responds:
> I am as aware of the contents of VIM as of the SI brochure, and your
> "n.b." makes exactly my point. What you might call "newton-meter as a unit
> alone", I prefer to call "nominal newton-meter". However, note that only
> some units can be called nominal units and tied to more than one
> kind-of-quantity; many units are unambiguously tied only to one
> kind-of-quantity. (02)
So we agree on this except that some of us still seem to want to
hold on to a notion of a N.m containing more than just the unit.
Nowhere in my practical experience of working with units in the
sciences do I ever see a qualification of units let's say I have
some lengthy computation with a term like this: (03)
N m s2 J s
------------
kg m2 C m (04)
that I would suddenly say "wait, that first 'm' in the formula is
a different length from the 'm' there in the denominator so I
can't cancel those." No one ever does that when computing with
units. You can always, under all circumstances cancel this term to (05)
kg m2 s2 kg m2 s kg2 m4 s2 kg m N
------------------ = ------------ = ------ = ---
s2 s2 kg m2 C m kg m3 s4 C s2 C C (06)
there is absolutely no limitation to this being done always. Never
do units carry the kind of property or any other qualification. (07)
Therefore, the answer should be unequivocal. This is how a unit
behaves, not just a "nominal unit" or "unit in name only". No that
is the unit. (08)
Interestingly the VIM talks about quantity value and reference, and
such a reference may well contain more information than the unit. (09)
Everything else is in the Quantity. (010)
I also must disagree to the notion that "many units are unambiguously
tied only to one kind-of-quantity" -- they are as long as you do
not distinguish multiple specialized kinds of quantity having the
same dimension, and you can always do that. You can always specialize
kind of quantity such that you may wish the two specialization to be
no longer directly comparable. (011)
regards,
-Gunther (012)
--
Gunther Schadow, M.D., Ph.D. gschadow@xxxxxxxxxxxxxxx
Associate Professor Indiana University School of Informatics
Regenstrief Institute, Inc. Indiana University School of Medicine
tel:1(317)423-5521 http://aurora.regenstrief.org (013)
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