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Homework Help: A question on percentage uncertainty

  1. May 24, 2016 #1
    1. The problem statement, all variables and given/known data

    Say we have the equation D2 = C x N where C = A2.
    Using a graphical method to find the % uncertainty of C (from graph of D2 against N) which involves working out the gradient which equals C and so forth. This value is 25%.
    The question is "what is the percentage uncertainty of A?"

    This is actually a question that appeared in an A-level paper i did today and it was the only one that really bamboozled me :confused:.
    2. Relevant equations

    3. The attempt at a solution
    I realize you don't divide by two;my reasoning being that when asked to calculate the % uncertainty of r in say the equation V = 4/3 x π x r3 the question doesnt explicitly state r3.

    So i think that it is between doubling 25% or just leaving it. Now i think it was the latter but in the exam I did indeed double it to 50%.
  2. jcsd
  3. May 24, 2016 #2

    Simon Bridge

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    You are propagating errors of form ##z=x^a## knowing ##x\pm xp_x/100## where ##p_x## is the known percentage error.
    So what is the rule for propagating the error on a power?
  4. May 24, 2016 #3
    Multiply the percentage uncertainty by whatever the variable is to the order of?
  5. May 24, 2016 #4
    Anyone else?
  6. May 25, 2016 #5

    Simon Bridge

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    What's wrong with the reply you got so far?
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