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_