       Re: Pounds and Kilograms

• To: mathgroup at smc.vnet.net
• Subject: [mg103673] Re: Pounds and Kilograms
• From: "Kevin J. McCann" <kjm at KevinMcCann.com>
• Date: Fri, 2 Oct 2009 08:22:30 -0400 (EDT)
• References: <h9v6po\$99p\$1@smc.vnet.net> <ha210d\$n5l\$1@smc.vnet.net>

```Pressure in English units is psi (pounds per square inch), which is
Force per unit area. However,

Convert[Pound/Inch^2,Pascal]

Convert::incomp: Incompatible units in Pound/Inch^2 and Pascal.

Since, by WRI's own definition that Pound is a weight, i.e. force, this
is not correct. While imperial units cause untold problems, the
engineering world, at least in the USA, tend to use them, but I don't
want to get into all that.

Anyway, I just posted this as a notice of the inconsistency.

Kevin

Erik Max Francis wrote:
> 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:= << Units`
>
> In:= Convert[Pound, Kilogram]
>
> Out= 0.453592 Kilogram
>
> In:= Convert[Pound, Newton]
>
> During evaluation of In:= Convert::incomp: Incompatible units in
> Pound and Newton. >>
>
> Out= Pound
>
> In:= ?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.
>

```

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