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Re: Pounds and Kilograms

  • To: mathgroup at smc.vnet.net
  • Subject: [mg103654] Re: Pounds and Kilograms
  • From: Erik Max Francis <max at alcyone.com>
  • Date: Thu, 1 Oct 2009 06:40:33 -0400 (EDT)
  • References: <h9v6po$99p$1@smc.vnet.net>

Kevin J. McCann wrote:
> The Units Package has Pounds and Kilograms. Pound is listed as a unit of 
> weight, Kilogram as a unit of mass; however,
> 
> Convert[Kilogram,Pound]
> 
> 2.20462 Pound
> 
> Seems inconsistent. (I hate units.)

Units good.  Imperial units bad.

It looks to me like a bug, either in the definition or the 
documentation.  When the unit pound was created, people did not make the 
distinction between mass and weight, but since they did, there have been 
a couple of systems rationalizing the old Imperial units to provide both 
distinct units.  Some are slug/pound, pound/poundal, 
pound-mass/pound-force, etc.  But the fact remains that if you want to 
treat them as a real system of units, you need to choose the pound to be 
one or the other.

In[1]:= << Units`

In[2]:= Convert[Pound, Kilogram]

Out[2]= 0.453592 Kilogram

In[3]:= Convert[Pound, Newton]

During evaluation of In[3]:= Convert::incomp: Incompatible units in 
Pound and Newton. >>

Out[3]= Pound

In[4]:= ?Pound

Pound is a unit of weight. >>

A weight is a unit of force, but Mathematica happily converts a pound to 
a kilogram and complains about converting a pound to a newton.

My guess would be what happened here is that they tried to include all 
the combinations:

?Slug

Slug is a unit of mass. >>

?Poundal

Poundal is a unit of force. >>

?PoundForce

PoundForce is a unit of force. >>

(These are intermixed from several different rationalized unit systems I 
listed above.)  Looks like the pound ended up with the short end of the 
stick, and so it's treated as a unit of mass even though the 
documentation uses the term "weight."  I'll send WR a note.

-- 
Erik Max Francis && max at alcyone.com && http://www.alcyone.com/max/
  San Jose, CA, USA && 37 18 N 121 57 W && AIM/Y!M/Skype erikmaxfrancis
   Nothing is so good it lasts eternally
    -- Florence, _Chess_


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