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Anyone familiar with graphical analysis?

  1. Feb 6, 2007 #1
    After having typed in the absolute uncertainties for my values, I get error bars that stretch across the graph, when my uncertainties are rather small compared to the initial value. What am I doing wrong???! Should I change the error bars to percentage or keep it at fixed value? (Those who use this graphing software will understand me...hopefully). Thank you.
     
  2. jcsd
  3. Feb 6, 2007 #2

    Astronuc

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    Staff: Mentor

    What are you plotting, i.e. what are the abscissa and ordinate, or rather what are the independent and dependent variables?

    If the uncertainties are small with respect to the value, and the magnitude of the error bars are larger, then it would seem to be a miscalculation of the error bar magnitude. If the error bars stretch across the graph, but the magnitudes of the error bars are much less than the associated value, it could be that the range of the axis (axes) are too small.

    See this page - http://www.ncsu.edu/labwrite/res/gt/gt-stat-home.html

    This might be helpful - http://www.rit.edu/~uphysics/graphing/graphingpart1.html#errorbars
     
  4. Feb 6, 2007 #3
    Hmmm. I have re-calculated the uncertainties...and I obtain the same results. Since this is an Atwood lab, I divide the sum of the relative uncertainties of time and distance by the acceleration value. For example, for situation 2, where the difference of the masses is constant, the acceleration value is 2.55 +/- 1.08 (this is quite big)...is my arithmetic incorrect? Also, for the masses, they have an uncertainty of .03 each. Thus, when I find the sum of the masses, I double the uncertainty. Then, when graphing, I have 1/m+m2 representing the x axis which I convert to kilograms. So, would it be correct to divide 0.06 by 150 kilograms (for example) and then multiply the obtained value by 150 so to get the absolute uncertainty of the sum of the masses when it's in the denominator (argh, I'm sure you lost me by now...Well, I hope you understand the main parts!!) Thank you. I think my error bars are big because our time measurements are inconsistent, and there is a great deviation between the average time and the smallest time measurement! argh!
     
  5. Feb 7, 2007 #4

    AlephZero

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    Science Advisor
    Homework Helper

    er. yes....

    What do you mean by "an uncertainly of 0.03 each"? Do you mean the uncertainty is 3% of the measured mass? Or an uncertainty of 0.03Kg?

    If you show your complete calculation for one of two points, somebody might be able to help more. Using ESP to guess what you did is even harder than doing physics :smile:
     
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