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.