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.)

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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