Re: beginner question regarding units in equations

*To*: mathgroup at smc.vnet.net*Subject*: [mg123945] Re: beginner question regarding units in equations*From*: DrMajorBob <btreat1 at austin.rr.com>*Date*: Mon, 2 Jan 2012 02:42:27 -0500 (EST)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*References*: <201112310701.CAA17278@smc.vnet.net>*Reply-to*: drmajorbob at yahoo.com

Amen. Bobby On Sun, 01 Jan 2012 01:29:47 -0600, W Craig Carter <ccarter at mit.edu> wrote: > > I feel compelled to put in my 2 cents worth on this topic. I hope that > I am barking at the choir here, but perhaps a student might be > listening. > > Every year in my set of materials science and engineering lectures, I > try to emphasize that the *first* thing a student should do when > performing a calculation is "non-dimensionalize, non-dimensionalize, > non-dimensionalize." This is a minority opinion in engineering. It is > never correct to take the Log or the Sine of a dimension. *Every* ideal > spring can be written as F/(k xo) = 1 - x/x0 = 1-xbar, the > characteristic frequency of any harmonic oscillator should be normalized > by dividing through by Sqrt[k/m] (or its equivalent) and so on. > > There is no physics in units---only relative quantities. The units > packages are useful for determining if a quantity has been > non-dimensionalized when formulas extracted from texts, etc. > > For example, this avoids frustrating errors that appear when, for > example, FindFit[data,Exp[-a (x-b)^2],{a,b},x] when data might be > student scores centered around 500: > > > > W Craig Carter > Professor of Materials Science, MIT > > > > On Dec 31, 2011, at Sat, Dec 31, 11 ---2:01 AM, Bill Rowe wrote: > >> On 12/30/11 at 7:09 AM, szhorvat at gmail.com (Szabolcs Horv=C3=A1t) wrote: >> >>> On 2011.12.29. 8:53, RDog wrote: >> >>>> Many civil engineering equations are empirically derived and >>>> therefore the units dont work out exactly. How does Mathematica >>>> handle units in equations and especially in empirical equations >>>> where there may be parameters set to weird exponetial powers. Does >>>> the program use units at all in equations or does the user need to >>>> keep track? >> >>> Mathematica does not know about units. It does not keep track of >>> units by default. So you don't need to worry about units not >>> matching. >> >>> There is the Units` package which provides some limited support for >>> units, but I have never used it seriously. I think that not using >>> units explicitly in your program will be the most productive way to >>> work. >> >> There is another package AutomaticUnits available at >> <http://library.wolfram.com/infocenter/MathSource/7655/> >> >> that significantly improves upon the Units package. Along with >> other things you can do: >> >> << AutomaticUnits` >> >> radius = r Centimeter; >> area = Pi r^2; >> Plot[area, {r, 0, 2}] >> >> and get the desired plot without worrying about the units. A >> much better solution than the Units package. >> >> > > -- DrMajorBob at yahoo.com

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