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Absolute Uncertainty - Enough Information?

  1. Sep 25, 2011 #1
    1. The problem statement, all variables and given/known data

    If an electronic scale reads 502.4 g, what is the absolute uncertainty in the mass reading.


    2. Relevant equations

    Not sure

    3. The attempt at a solution

    Not a clue, it seems that there isn't enough information given. How am I supposed to figure out the absolute uncertainty if this is all that is given?
     
  2. jcsd
  3. Sep 25, 2011 #2
    It should have a label on the scale.
     
  4. Sep 25, 2011 #3
    That's funny because I was thinking the same thing. Unfortunately it is straight out of a text book so nothing is given other than the question itself. Not sure how to approach this one. Any ideas?
     
  5. Sep 25, 2011 #4
    My first thought is to consider significant figures. If you were given a value of 10, would you think it's reasonable to have an uncertainty of +/- 0.1? Probably not, because the output you have is not given to any decimal places. So in this case where we have 502.4g we should ask how exact can we get before we reach the limit of the measurement.

    -Eric
     
  6. Sep 26, 2011 #5
    So then are we saying with absolute certainty that the measurement is 502.4g since no other information is given? Therefore, the uncertainty would be +/- 0.0. Obviously if we were given 502.2 - 502.6 we would know the uncertainty to be +/- 0.2.
     
  7. Sep 26, 2011 #6
    It's a lousy question and your text should be burned in an effort to the improvement the human condition.

    However

    It seems we are supposed to assume the measuring electronics deliver 100% accuracy to the display.

    If the actual mass were 502.351 g what would the meter read?
    If the actual mass were 502.449 g what would the meter read?
     
  8. Sep 26, 2011 #7

    PeterO

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    I think all figure are taken as =- 1/2 of the last digit - unless stated as bigger.

    So 502.4 =-0.05
     
  9. Sep 26, 2011 #8
    I suppose that it would read 502.4 because it rounds to the nearest tenth. I guess we assume that when only the tenths place is given. The thing is, how do I know what to assume when this isn't discussed in the text.

    Yah, fire always brings me comfort. Especially when it involves burning something that is persecuting me.
     
  10. Sep 26, 2011 #9
    Wouldn't it also be + 0.05 as well if the meter rounds to the nearest tenth? I guess we really don't know since there isn't any additional information provided.
     
  11. Sep 26, 2011 #10

    PeterO

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    Look at the keyboard ... I was too brief with the shift key so got = instead of +
     
  12. Sep 27, 2011 #11
    Given that I've correctly read between the lines of your text, what do you get for the absolute error?
     
  13. Sep 27, 2011 #12
    502.4 +/- 0.05
     
  14. Sep 27, 2011 #13
    Excellent, thank you.
     
  15. Sep 27, 2011 #14
    That's a good trick. How do you typo "+/-" into "="? The fingers must by flying.
     
  16. Sep 27, 2011 #15

    PeterO

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    Not quite, the typo was "=-" instead of "+-"
     
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