Re: Units question

• To: mathgroup at smc.vnet.net
• Subject: [mg115571] Re: Units question
• From: DrMajorBob <btreat1 at austin.rr.com>
• Date: Sat, 15 Jan 2011 04:44:07 -0500 (EST)

```If units being positive doesn't make sense, why is the solution simply
making them positive?

Bobby

On Fri, 14 Jan 2011 05:19:43 -0600, Bob Hanlon <hanlonr at cox.net> wrote:

>
> Dimensions aren't numbers, so being positive doesn't make sense. It is a
> just convenient to say that they are positive to trick Sqrt and the
> like. In some cases using this is better than using PowerExpand since
> PowerExpand would affect the treatment of variables not just the
> dimensions. You can handle all of the dimensions at once:
>
> Needs["Units`"]
>
> simplifyUnits = ToExpression[# <> " > 0" & /@ Names["Units`*"]];
>
> Simplify[Sqrt[Meter^2/Second^2] Second/Meter^2, simplifyUnits]
>
> 1/Meter
>
>
> Bob Hanlon
>
> ---- Murray Eisenberg <murray at math.umass.edu> wrote:
>
> =============
> Even after loading the Units package, Mathematica doesn't know that
> Meter and Second are positive.
>
> My naive sense is that this is just plain stupid. Am I missing something
> here?
>
> On 1/12/2011 4:07 AM, Bob Hanlon wrote:
>> Simplify[Sqrt[Meter^2/Second^2] Second/Meter^2,
>>   {Meter>  0, Second>  0}]
>>
>> 1/Meter
>>
>>
>> Sqrt[Meter^2/Second^2] Second/Meter^2 // PowerExpand
>>
>> 1/Meter
>>
>>
>> Bob Hanlon
>>
>> ---- Ted Sariyski<tsariysk at craft-tech.com>  wrote:
>>
>> =============
>> Hi,
>> Working with units I get correct result:
>>
>> Sqrt[Meter^2/Second^2] Second)/Meter^2
>>
>>    How to tell Mathematica to simplify the result to Meter^-1?
>> Simplify and FullSimplify doesn't do the job.
>> Thanks in advance,
>> --Ted
>>
>>
>

--
DrMajorBob at yahoo.com

```

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