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Homework Help: Percent uncertainty

  1. Jan 28, 2007 #1
    1. The problem statement, all variables and given/known data
    What is the percent uncertainty in the area of a circle whose raduis is 1.8 x 10^4 cm?

    2. Relevant equations
    I calculated A=Pi x r^2 = 1017876020
    After that, I am lost...

    3. The attempt at a solution

    Thanks!
     
    Last edited: Jan 29, 2007
  2. jcsd
  3. Jan 28, 2007 #2
    The thing is, I don't know what was used to measure the radius. Can I still know what the uncertainty is to calculate the %age?

    Thanks!
     
  4. Jan 28, 2007 #3

    jtbell

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    Staff: Mentor

    If I'm not given any direct information about the uncertainty in the initial measurement (here, the radius), I take it to be +/- 1 in the last significant figure. In this case the uncertainty in the radius would be [itex]0.1 \times 10^4[/itex] cm.
     
  5. Jan 28, 2007 #4
    Thanks a lot!

    I calculated the percent uncertainty to be 5.6%. I think it is correct...
     
  6. Jan 29, 2007 #5
    Apparently this is not the correct answer! please help me ...

    Thanks!
     
  7. Jan 29, 2007 #6
    Ok it was 11%... I don't understand why though... If you could walk me through the thinking process, I'd be grateful. I will know how to do such a problem next time... Thanks!
     
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