On Sep 28, 2009, at 5:20 PM, Gunther Schadow wrote: (01)
> ingvar_johansson wrote:
>> one more comment. You asked:
>>
>>> 1 N.m = 1 N.m : true or false?
>>
>> and I said 'true' (and so did Pat H). But this does not imply that
>> 1 N.m
>> of energy = 1 N.m of moment of force, since energy and moment of
>> force are
>> different kinds of quantities (despite having the same dimension).
>
> and that's precisely my point and why I disagree with Pat Hayes
> that this is not useful. I was asking if 1 N.m = 1 N.m and
> the answer is ambiguous. (02)
No, it is not. If you had asked whether 1 N.m moment of force = 1 N.m
energy then the answer would clearly be no. But those are two
different questions, not one ambiguous question. (03)
> The unit is newton-meter, it is not
> newton-meter-of-energy, therefore, I would argue, that the unit
> is the same even if the kinds of quantity are different. Unless
> we agree on this (by either one of us changing our mind) I don't
> see a value at looking at ontological constructs. (04)
Well, we clearly agree. But I suspect we would have agreed if you had
simply asked me this directly :-) (05)
> I don't want to discuss the N.m issue in particular at this
> time, only that it's pointless to proceed if there is
> disagreement about this matter.
>
> The question remains what we believe jointly that UoM concepts
> should do for us. (06)
That isnt necessarily a welldefined question to ask of an ontology,
but my answer would be, to support a useful connection between the
purely metrical notions of units, quantities and so forth and the
wider medium-to-high level concepts of other ontologies, such things
as 'physical object' and 'event'. (07)
> You may want them to preserve the difference
> between torque and energy, I don't. So the question remains
> open on the list. But there is no point in proceeding if we
> don't agree on this. (08)
I disagree. If we try to establish agreement on the ultimate purpose
of an ontology, we will never stop arguing. If it is any good, it will
get used for things that we havn't yet imagined. That is part of the
point of building it, in fact. (09)
> We might, however, agree if we use these
> example to be more clear about why we have the desire for the
> UoM concepts to do what we want them to do and possibly how
> else we might get our desires fulfilled.
>
> In my experience with dealing with scientific equations and
> computations, the units were incredibly useful for (a) converting
> to a unit that I needed and (b) giving assurance that I probably
> didn't make some gross error in my equations. Thus, in my
> experience with dimensioned terms it does not matter in the end
> whether the m in N.m, was the length of a lever or a distance
> of displacement, that is all in the concerns that led to my
> equations. The units function more like a check-digit at the
> end: if the unit term does not agree with the expected kind of
> quantity, something went wrong in my calculation or the formula. (010)
I largely agree. But you have to bear in mind that mechanical
inference systems may also need to be able to do the kinds of
calculation and formula manipulation that you are doing by hand, and
will use dimensional ideas to help them in this. Such systems have
been around for over a decade now. (011)
>
> This is why around UCUM implementation I use the concept of
> a "DimensionedQuantity". A Quantity is any set of values
> where at least some values have a difference operation. A
> DimensionedQuantity is essentially a number with a dimension. (012)
No, wait. Because it is *described* using a number and a dimension
does not mean it *is* a number-dimension pair. In fact, it can't be.
We have already established that we agree that 2.3 N.m = 2300
g.m2.s-2, and if that equation means what it says, then the same one
of these can be described by two different number/dimension pairs.
.
> Such a quantity for example is 16 N.m. Units are themselves
> DimensionedQuantities with a name (and the name can be complex
> such as N.m or even 16.N.m) So, my ontology behaves exactly
> like the symbols that I write on a sheet of blank paper when
> I compute my scientific equations. (013)
Again, that does not sound like good ontology engineering. It
certainly does not generalize to more complex or more general
situations. A description of a commercial transaction is not
isomorphic to the actual transaction. (014)
> It does not do more nor
> less than what the units do on paper. I.e., 1 N.m = 1 N.m
> = 1 kg.m2.s-2 = 1 J.
>
> There is nothing you can do to separate these concepts unless
> by assuming into your theory the detail of all of mechanics
> (and all of science) (015)
No, you do not need to cover all the detail, nor do you need to assume
any of the theory. But you (we) do need to make the connections to
(some of) the relevant concepts. (016)
> which you can't do.
>
> BTW, it is not true that N.m of torque and joule of energy
> are completely unrelated. Because the torque times angle
> moved is again your energy. Whether or not we maintain a
> dimension for angle in UCUM is also besides this point. Of
> course: by adding more distinct dimensions we may be able
> to preserve more distinctions and by having less dimensions
> we lose distinctions that we can make by just looking at
> number and unit. But because I do not expect much more than
> the function of a "dimensional check digit" and defined
> conversion rates from the units, I can give or take a few
> dimensions without much trouble. The only place were I really
> get into trouble is where we haven't even started to discuss,
> i.e., idiosyncratic "procedure defined units". (017)
One or two examples of these would be useful. So far, all the physical
gotchas that everyone cites seem to be connected one way or another
with angles and rotation. (018)
Pat Hayes (019)
>
> regards,
> -Gunther
>
> --
> 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
>
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