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Significant Figures in Conversion Factors

  1. Dec 10, 2014 #1
    So from what I had initially understood, in a conversion problem, you go by whatever number has the smallest SF and round your answer to that number, even if that number is one.

    In the examples on my homework it seems like my teacher chooses a random number of significant figures for each problem and I cant figure out why.

    Some Examples:
    2425 fathoms x(6 ft/1 ft)x(12 in/ 1 ft)x(2.54 cm/in)x(1 m/100 cm)= 4435 m
    -Why not 1, 2 or even 3 SF?

    20.0 inx(2.54/1 in)x(1 m/ 100 cm)x(1000 mm/ 1 m)= 508 mm
    -Why not 1 SF?

    And also, not a conversion but I also wondered about this one too:
    15.4-11= 4
    -Why not 2 SF?
  2. jcsd
  3. Dec 10, 2014 #2


    User Avatar

    Staff: Mentor

    Fathom is by definition EXACTLY 6 feet, so the conversion factor has an infinite number of digits. Same about inch vs cm.

    Rules for addition and subtraction are slightly different and result depends on where the decimal point is.
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