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Re: Units question

• To: mathgroup at smc.vnet.net
• Subject: [mg115552] Re: Units question
• From: Bill Rowe <readnews at sbcglobal.net>
• Date: Fri, 14 Jan 2011 06:19:07 -0500 (EST)

On 1/13/11 at 3:24 AM, Kevin.McCann at umbc.edu (Kevin J. McCann) wrote:

>I have seen a number of responses that suggest the use of
>PowerExpand, which I do myself. But shouldn't Mathematica know that
>Meter and all the other units are Real? After all, these are in the
>Units package. Even more irritating is that in a Simplify,
>Mathematica does not deconstruct things like Joule or Pascal into
>base units to simplify things. I find that I have to use a rule set
>that turns Joule->Kilogram Meter^2/Second. It would be nice if this
>were automatic.

The units in the Units package are simply symbols with no
assigned value but with a usage message. So, unless you re-write
the Units package so that units are not just a symbol, it isn't
feasible for Mathematica to know these are positive values.
Essentially, the package AutomaticUnits available on the
Mathsource web site is a re-written Units package. That is

In[1]:= << Units`;
Head[Meter]

Out[2]= Symbol

but

In[1]:= << AutomaticUnits`
Head[Meter]

Out[2]= Unit

In[3]:= FullForm[Meter]

Out[3]//FullForm= Unit[1,"Meter"]

and it is the thing Unit that has the properties needed for
things to be done in a much nicer fashion.