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Homework Help: Cookie Question, Uncertainty

  1. Sep 18, 2009 #1
    I saw similar types of problems like this using Google search, but none of them answered this question specifically.

    1. The problem statement, all variables and given/known data

    As you eat your way through a bag of chocolate chip cookies, you observe that each cookie is a circular disk with a diameter of 8.5 +/- 0.02 cm and a thickness of 0.050 +/- 0.005 cm.

    Find the ratio of the diameter to the thickness and the uncertainty in this ratio.

    2. Relevant equations

    3. The attempt at a solution
    So what I did was just put 8.50 +/- 0.02 cm divided by 0.050 cm. My answer was :
    170 +/- 4cm

    Their answer was : 170 +/- 20 cm.

    So I see where they got the 170 from, but I'm not sure why they 20cm is there.
  2. jcsd
  3. Sep 18, 2009 #2
    In such questions, try to get the biggest ratio possible and the smallest one.
  4. Sep 18, 2009 #3
    Okay, I did that and I got the two answers : 154.9 and 188.4, but the difference between them isn't 20.
  5. Sep 18, 2009 #4


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    Staff Emeritus
    Science Advisor
    Homework Helper

    They seem to be rounding the error to one significant figure.

    Have you been told about adding the percent error when doing multiplication or division?

    Just FYI, those numbers are a little off. For example,
    8.48/0.055 = ____? (slightly less than the 154.9 you gave)
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