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Basic relative error problem

  1. Sep 17, 2007 #1
    Assume the density of air was measured as 1.00 kg/m3 ± 5%. The room has dimensions 4 m × 5 m × 2 m, each measured with 10% relative error. What is the mass of the air in the room? What is the relative error for the mass?

    I think this is a very simple problem, and i found what the mass of the air would be, i just don't understand relative errors, so how do i find the relative error for the mass?

    D = m/V D = 1.00 kg/m3 V = (4 m *5 m *2 m) = 40 m3
    m = (1.00 kg/m3)/ (40 m3)
    m = .025 kg → m = .03 kg

    relative error: Is it something like 5%/ 10% ??
     
  2. jcsd
  3. Sep 17, 2007 #2

    learningphysics

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    Have you studied the rule for how to get the relative error when you multiply two quantities?
     
  4. Sep 17, 2007 #3
    no, we didn't learn much about relative error at all.
     
  5. Sep 17, 2007 #4

    learningphysics

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    Have a look at this page:

    http://www.rit.edu/~uphysics/uncertainties/Uncertaintiespart2.html

    I myself don't know that much about how to treat relative errors... but it seems like when you multiply quantities you add relative errors.

    In your calculation there are 4 quantities being multiplied. density, length, width, height... so I'm guessing you just add the relative errors to get the relative error of the mass...
     
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