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I read Wiki:

http://en.wikipedia.org/wiki/Barn_(unit)

on this and calculations under the 'Conversion from squared prefixed SI units' do not look like correct. Is 1fm^2 = 10 mb ? (or 1b = 100m^2 ) . Which one is correct and how do you calculate it? Thanks.

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... Where did you get that second equivalence from? –  Ignacio Vazquez-Abrams Jul 10 '12 at 13:07
    
what do you not understand in the table "commonly used prefix versions" ? All are refered to cm^2. en.wikipedia.org/wiki/Barn_%28unit%29 . Your link has a missing parenthesis and ends up in a "new page". –  anna v Jul 10 '12 at 13:08
    
@Ignacio: it is there, just read from the beginning :) –  Dukunocil Jul 10 '12 at 16:18
    
You mean the part where it says "100 fm^2"? –  Ignacio Vazquez-Abrams Jul 10 '12 at 16:19
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Your question says "1b = 100m^2", which is not quite the same thing as "100 fm^2". –  Ignacio Vazquez-Abrams Jul 10 '12 at 23:57
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2 Answers

A barn is $10^{-28}$m$^2$, so it's the area of a square $10^{-14}$m by $10^{-14}$m.

One femtometer is $10^{-15}$m, so the square is 10fm by 10fm, i.e. it's 100fm$^2$.

Conversely, one square femtometer is the area of a square $10^{-15}$m by $10^{-15}$m. If rb is a square root barn ($10^{-14}$m) then the square is 0.1rb by 0.1rb so it's 0.01 barns.

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I still do not get it :( –  Dukunocil Jul 10 '12 at 16:51
    
I think I got it :) First part is obvious. Second part: 10fm X 10fm = 100 fm^2 OK. So what is it expressed in barns? I calculate: 100 fm^2 = 10^2 x (10^-15)^2 = 10^2 x 10^-30 = 10^-28 = barn. They should use brackets i.e. 100 [fm]^2 –  Dukunocil Jul 10 '12 at 17:02
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You wouldn't normally use brackets. After all it's very common to talk of a square centimeter and denote it by $cm^2$, or a square kilometer, $km^2$. You wouldn't write these as $[cm]^2$ or $[km]^2$. –  John Rennie Jul 10 '12 at 17:13
    
..then if 100[fm]^2 = 1 barn, therefore 1[fm]^2 = 10^-2 barn = 10 X 10^-3 barn = 10 mb :) –  Dukunocil Jul 10 '12 at 17:13
    
I agree, but not 100%. Units should be in brackets to avoid confusion like this one, particularly when dealing with 'exotic' types of units :) Anyway, thanks for allowing me to see this. –  Dukunocil Jul 10 '12 at 17:16
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An area with a side of 1u has an area of 1 u^2.

An area with a side of 2u has an area of 4 u^2.

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An area with a side of 10u has an area of 100 u^2.

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An area with a side of 10fm has an area of 100 fm^2 and is called a "barn".

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