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[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. >