From: Henry Spencer <henry@zoo.toronto.edu> Newsgroups: sci.space.tech,sci.space.policy Subject: units (was Re: Space Station Construction) Date: Tue, 19 May 1998 14:33:49 GMT In article <35610D87.6C00DC33@elec.canterbury.ac.nz>, Robert Lynn <lynnrg@elec.canterbury.ac.nz> wrote: >kilo k, mega M, giga G, tera T, peta P, exa E prefixs only! (quick >aside: is the prefix for tera T or t? It's T -- there is now a consistent pattern that the big prefixes are uppercase letters and the small ones lowercase. By the way, you forgot zetta Z and yotta Y, and their counterparts at the other end, zepto z and yocto y. (If it gets hard to remember these, the key thing to know is that starting with peta, they're twisted variants of the familiar numeric prefixes, e.g., peta comes from penta. They're twisted a bit to avoid confusion and give them unique initial letters.) They sound a bit silly, but they do have uses, in principle -- the local intergalactic distances are conveniently expressed in Zm and the size of the universe in Ym. Of course, it may be a little while before the astronomers can be convinced to switch from parsecs and light-years... >N/mm^2 is of course equivalent to >MPa which every engineer is familiar with, so why not just say MPa? It >is quicker and easier and reveals that the author is at least slightly >aware of technical conventions. Remember, the paper I was summarizing is 20 years old. I didn't convert partly because I was feeling lazy and partly because it's usually better to avoid gratuitous unit conversions when quoting somebody else -- there is less possibility of error if conversions (even just of unit names) are delayed until calculation is actually necessary. >[Mod note: in case anyone seriously does not know what KSI are, >it's "Thousands of pounds force per square inch". 100 KSI yield strength >is 100,000 lbf/in^2, or 689 MPa. There is no group policy on the >use of SI or english units, but the moderator is stubborn. -gwh] Next he'll be using stones/furlong-fortnight^2. :-) -- Being the last man on the Moon | Henry Spencer is a very dubious honor. -- Gene Cernan | henry@zoo.toronto.edu

From: henry@spsystems.net (Henry Spencer) Newsgroups: sci.space.tech,sci.space.policy Subject: Re: units (was Re: Space Station Construction) Date: Tue, 2 Jun 1998 13:07:31 GMT In article <6kvmld$n2i@lace.colorado.edu>, Frank Crary <fcrary@rintintin.Colorado.EDU> wrote: >>I agree. I find yottagrams to be a natural unit for remembering >>planetary masses... > >Personally, I don't like it, but I may have some biases because I'm >a theorist. For many purposes (e.g. orbital dynamics) it is easier >if you use nondimensional units... While it makes things easier at theory time, it can make life harder later on. Bob Forward, in his JBIS paper (Nov 1989) on spacewarps, comments: Determining the correct equations for the physical magnitudes of the quantities being discussed was found to be difficult. All the papers by mathematicians about [general relativity] were written in units where h=c=G=1, while papers by nuclear physicists about models of the electron and nucleus were written in mixed cgs units where the constants for the permeability and capacitivity of space are suppressed. The proper procedure for the reinsertion of the various constants to form consistent equations for the numerical calculation of the magnitude of any expected experimental result is not obvious, and there exist two papers (deliberately not referenced) with drastically different versions of the same conversion... >added advantage, you don't get any problems or confusion from planets >having masses that are 10 to the twenty-somethingth power. (E.g. >Jupiter is roughly 300 Earth masses; Mars is roughly 0.1; or Jupiter >is about 0.001 solar masses.) So you get units of mass that are convenient >(i.e. within a factor of 1000 of one) ... There is also a more subtle issue here, which Peter didn't raise when talking about yottagrams: planetary masses are another one of those troublesome areas where we've got a poorly-known calibration constant standing between the measurable values and the standard units. We measure planetary masses using gravitational effects. The effects can be measured to six or seven decimal places without great difficulty... but to convert them to masses we have to use G, the constant of gravitation, and G is known to only about four places. (Gravity is such a weak force that G is quite difficult to measure precisely.) High-precision work, e.g. for interplanetary navigation, uses values of GM (aka mu) -- the measured quantity, and also the one of interest for calculating gravitational effects -- usually quoted in km^3/s^2. The use of km instead of m, and the rather small value of G, give the numbers halfway convenient sizes, e.g. the GM of Mars is about 42828 km^2/s^2. -- Being the last man on the Moon is a | Henry Spencer henry@spsystems.net very dubious honor. -- Gene Cernan | (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer) Newsgroups: sci.space.tech,sci.space.policy Subject: Re: units (was Re: Space Station Construction) Date: Fri, 5 Jun 1998 14:27:10 GMT In article <pgISDTAk$wd1EwjD@branta.demon.co.uk>, Del Cotter <del@branta.demon.co.uk> wrote: >>Why yotta_grams? Use tons (tonnes? I know English-speakers also have >>'short' tons and 'long' tons, and the spelling is mysteriously connected >>with that, but since we are talking metric...). > >It is not. If you're talking metric, you *must* say tonnes, since a >tonne is 1000kg and a ton (of any variety) isn't. There is long precedent for calling 1000kg a metric ton or just a ton; even my 20-year-old dictionary lists both of these usages. -- Being the last man on the Moon is a | Henry Spencer henry@spsystems.net very dubious honor. -- Gene Cernan | (aka henry@zoo.toronto.edu)

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