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Uncertainty (physics)

  1. Aug 15, 2010 #1
    The density of the material of a rectangular block is determined by measuring the mass and
    linear dimensions of the block. The table shows the results obtained, together with their
    uncertainties.

    mass = (25.0 ± 0.1)g
    length = (5.00 ± 0.01) cm
    breadth = (2.00 ± 0.01) cm
    height = (1.00 ± 0.01) cm

    The density is calculated to be 2.50gcm–3
    .

    What is the uncertainty in this result?
    A ± 0.01gcm–3
    B ± 0.02 gcm–3
    C ± 0.05 gcm–3
    D ± 0.13gcm–3


    According to the book the answer is C and i am having D as answer..

    my work

    (o.01) x 3 = 0.3

    0.3 + 0.01 = 0.13

    i cannot figure out where is the mistake help
     
  2. jcsd
  3. Aug 15, 2010 #2
    you should add up the fractional uncertainties not the absolute uncertainties.
     
  4. Aug 15, 2010 #3
    aww fractional error...but the question deal with uncertainty..can you start it for be i will get a better idea
     
  5. Aug 15, 2010 #4
    ?? I don't get you.
     
  6. Aug 15, 2010 #5
    the question is about uncertainty..why are you working with fractional error...please start the number...work out the first part i will get a better idea of how to attempt the question
     
  7. Aug 15, 2010 #6
    isn't fractional uncertainty (or fractional error if you choose to go by that name) a form of uncertainty? i don't get what's the issue here :confused: and what do you mean by the "first part"?

    but anyway the full solution is just this:

    total fractional uncertainty: 0.1/25 + 0.01/5 + 0.01/2 + 0.01/1 = 0.021
    total absolute uncertainty: 0.021 x 25/(5 x 2 x 1) = 0.0525 = 0.05 gcm^-3
     
    Last edited: Aug 15, 2010
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