Re: [Axiom-developer] Unit package question

[C Y]

--- William Sit <address@hidden> wrote:
> 
> Martin Rubey wrote:
> > This would not allow for Length/Time
> 
> You can add (among the ...) "Velocity" (I already used 
> "Acceleration" in the examples). Also, we can expand Dimension
> to a fuller domain that can do arithmetic, and equating Length/Time 
> as Velocity (but notice that there are cases where the fundamental 
> units are the same, but the physical derived unit can have multiple
> meaning: Length*Force can be Work or Moment, so dimensions
> should not be confused with their component basic dimensions).

That's a good point, but the distinction is most definitely based on
circumstance.  What would we do if someone assigns or enters a
non-dimensionally-unique unit?  Or one falls out of a calculation?  How
do we do dimensional analysis?  (For that matter, how would that be
handled in the "real world" - are Work and Moment in some sense the
same thing? If not, how can the difference be expressed?)

> I don't think the use of mixed unit systems in an expression should
> be encouraged: one should not encourage the use of 3 meter + 5 feet 
> (not even 3 meter + 10 kilometer) because the answer must be either
> in feet or in meter or something else, but only one unit. 

Right.  I don't understand the difficulty here - the only place mixed
unit systems would be likely to occur is in an input, and the first
thing to do will be to convert them to standard units.  We want people
to be able to enter 3 meter + 5 feed and get x meters out - that's one
of the things a unit package is good for!

> So the way to handle mixed unit expression should be to first 
> convert say 5 feet to meter before addition. I would not
> worry about such expressions since that is not "scientific" (Axiom:
> The Scientific Computation System).

Do you mean worry about preserving them interally or as output?  If so,
I agree.  As input, they should be legal if dimensionally legal and
converted internally to useful form.

> To repeat, my view of units is that units are "decorations" to be
> handled in the domains of UnitCategory(S), using a fixed unit
> system, and by coercion to OutputForm;and one should adhere to the 
> standards as given by:
> 
> http://physics.nist.gov/Document/checklist.pdf

Absolutely agree.  In fact, we should incorporate that document and/or
other relevant documentation into our eventual coding of the output
logic for units, IMO.

Cheers,
CY


                
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