Wikipedia:Reference desk/Archives/Mathematics/2013 April 5

= April 5 =

Cubic capacity conversion
I'm working on an article about airships and tried to convert 20,000 cubic feet into SI Units. I couldn't remember how to work it out myself and used an on-line converter, which came up with 566 cubic metres. Is that right? It sounds rather on the low side to me. Alansplodge (talk) 19:00, 5 April 2013 (UTC)
 * Remember 1 m = 3.28 feet. Now just use dimensional analysis: $$20 000 ft^3 \cdot (\frac{1 m}{3.28 ft})^3=566.77 m^3$$. 72.128.82.131 (talk) 19:04, 5 April 2013 (UTC)


 * Hmmmm, there are two different feet. The international one is 30.48 cm, but the US one is 30.4800609601 cm. Not much difference but still not exactly the same. Anyway, I prefer to do the conversion by substitution. So, 20,000 ft^3 = 20,000 (30.48 cm)^3 = 20,000 (0.3048)^3 m^3 = 566.34 m^3. Count Iblis (talk) 19:18, 5 April 2013 (UTC)


 * Thank you both - I must have been asleep in that lesson. Thanks Count Iblis; however I'm not sure that the figure of 20,000 was that accurate in the first place, as it describes an airship constructed in a do-it-yourself shed on the edge of a sportsfield in 1902. Probably 566 m^3 is going to be good enough. Alansplodge (talk) 01:02, 6 April 2013 (UTC)


 * Also note that the volume of an airship may not be constant. In the case of soft-sided airships, like blimps, the interior space itself changes depending on the level of inflation.  However, even in  a rigid airship, like a dirigible, the amount of gas inside isn't constant, as more gas will squeeze into the same volume under higher pressure.  So, if you wanted to know "what would the volume of gas inside the dirigible be, if it were to be removed and brought to STP", you might get different answers depending on the initial temperature and pressure inside. StuRat (talk) 05:22, 6 April 2013 (UTC)