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Re: Pounds and Kilograms

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

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


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.


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[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.

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