[meteorite-list] Re: Question -Sri Lankan density

From: MexicoDoug_at_aol.com <MexicoDoug_at_meteoritecentral.com>
Date: Mon Dec 6 17:36:57 2004
Message-ID: <3E1F4481.7D8F7AD7.0BFED528_at_aol.com>

Hola Herbert and others interested,

Thanks for the very kind words. I actually received a couple of emailed questions on this (one naughty and one nice) from other list members so instead of answering individually it is probably worth taking a moment to post what you already knew if you won't mind, in order to clarify why the units in specific gravity "don't matter for list purposes". That was probably a tiny overstatement on my part and I probably should have just said if one is somewhere in the neighborhood of ambient earthly conditions and comfortable using grams and cubic centimetres (=millilitres). Here is an illustrated example to save some further bandwidth:

water density is:
=1.000 grams per cubic centimetre
=1000 grams per cubic metre
=0.578 ounces per cubic inch
=62.4 pounds per cubic foot

but specific gravity for water is still a unitless 1.000 in humane ("somewhat standard")conditions.

An iron meteorite has, say, has the following equivalent densities:
=7.8 grams per cubic centimetre
=7800 grams per cubic metre
=4.5 ounces per cubic inch
=486 pounds per cubic foot

but specific gravity for the iron meteorite is still a unitless 7.8 in humane conditions.

And if you like unwieldy units of ounces and inches, you can get back to specific gravity by dividing:

(4.5 ounces/in3) / (0.578 ounces/in3)

The units cancel and you get a not so surprising 7.8...
So it is no wonder why they are always being mixed up. In outer space ice probably has a density much closer to 0.917, and the meteorite probably wouldn't change much at all, so a geologist who hypothetically was floating in a mixed meteoroid stream might do some fancy things with a bucket of creatively crushed ice to "dunk" meteoroids in it to measure their density, so the same iron now meteoroid, would come out to have a specific gravity of 8.5 using water at frozen temperatures. But that is silly, the specific gravity business is most meaningful for geologists as a quick and dirty measurement in the field or perhaps precisely, in a comfortable laboratory, so while I don't know why it might be needed below freezing reference temperatures, nor whether the "definition" excludes solid ice as silly, perhaps a case could be invented to have fun with specific gravity and g per mL being different. Like maybe something to do with research on the formation of the dirty snowball in a rotating granular comet. Hec
k that is a wild stab at it, so back to the original point, specific gravity and g/cc (=g/mL, = kg/L) are the same for list purposes with the one exception that choice of exotic measurement units will not affect the specific gravity. Now all we need is to be sure the Sri Lankan professor didn't mix up the terms as well,,,but I suppose that is the least of the questions right now about that decimal/comma deal,,,I agree with Rob on that one, but I checked the classified in the Sri Lankan newspaper and they seemed to be using the US convention at least there. Makes a little possibility that he has a nuclear powered scale to get milligrams when weighing a 47 kg object:)

Saludos, Doug


Herbert wrote in the most friendly manner:

Doug,
yes, indeed, I wrote my message carelessly. I mixed up "density" (which has a unit) with "specific gravity", which has, as you correctly stated, no units. I promise to use my brain *before* I complain next time... :-)
Greetings,
Herbert
Received on Mon 06 Dec 2004 05:36:46 PM PST