Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Understanding Sig figs

  1. Jul 12, 2011 #1

    I've used significant figures without really thinking about them too much; and I have never really understood there use.
    I found this thread: https://www.physicsforums.com/showthread.php?t=477786&highlight=significant+figures

    Where it has been argued that they should be 'taken lightly', so to speak.

    Here's where I have always been confused, and I hope someone can help!:

    If I were to perform the multiplication, 2.09*3.52, I have three significant figures in both numbers, and so the answer would be 7.36 (from 7.3568).
    I think this example shows one issue, and that is that the 6 is not necessarily 'known'.
    Indeed, depending on the numbers in the multiplication, digits in higher powers of ten columns can be affected by 'unknown' numbers in smaller powers of ten columns. So when a number is given to x significant figures, these numbers may not be the actual numbers!, since they can be influenced by unknown numbers of smaller magnitudes of ten e.g. 1/10th column, 1/100th column, etc can affect the value in the 100 column or the 101 column, etc.
    So when I see some calculation using significant figures, I should not necessarily believe any of the numbers are absolutely certain?

    I hope that makes sense!
    Thanks in advance.
  2. jcsd
  3. Jul 13, 2011 #2


    User Avatar

    Staff: Mentor

    if we say x=1.97, then it is taken as being between 1.96 and 1.98

    consider the equation, y=1/(2-x)

    If x=1.96, then y=25
    if x=1.98, then y=50

    so, we have found, y=37 +/- 13

    Summarizing, although x was known to 3 sig figs, y is known to not much better than 1 sig fig.

    Does this illustrate what you were talking about?
  4. Jul 13, 2011 #3


    User Avatar
    Science Advisor

    Yes, but clearly the problem step was (2-x), which left you with 0.03, known to one sig fig. The OP's example had only multiplication, which tends to be less tricky.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook