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Homework Help: Uncertainty for the masses

  1. Jan 31, 2007 #1
    For a physics lab, we had to test newton's second law using an atwood machine. Now, as I'm graphing my data, I realized I don't have any uncertainty for the masses. I have been told to make up an uncertainty, yet my uncertainty has to make sense, and I should have a reason. For case one, my masses are as follows:
    M m
    100 100
    110 90
    120 40
    130 70
    140 60
    150 50

    ???? Any ideas?
  2. jcsd
  3. Jan 31, 2007 #2

    Tom Mattson

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    Staff Emeritus
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    Gold Member

    There's no need to make up the uncertainty, just use the last nonfluctuating digit on the measuring apparatus. That is, if your scale gives you masses out to 0.01 kg, but the last digit drifts (assuming an electronic scale) and the one before it doesn't, then you know the mass +/- 0.1 kg. This is assuming that your scale has been calibrated, of course.
  4. Jan 31, 2007 #3


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    Staff: Mentor

    I think you didn't realize that a Mentor was moving your misplaced post from the general forums to here in the homework forums. I deleted the duplicate post that you put here mistakenly while this original post was being moved.

    You need to show a little more work in order for us to help you. What are your ideas and thoughts so far?

    How were the mass values determined? Did you weigh them, or did they come with some mass value stamped on them? If you weighed them, your scale will have some uncertainty associated with it (either in its datasheet, or its calibration data, etc.). If the masses came with values stamped on them, check their supplier datasheets for their tolerances.
  5. Jan 31, 2007 #4
    Ok thank you. Firstly, we were not given an electronic balance. They came with the mass value stamped on them. That is the only information we were given. What do you believe would be a reasonable uncertainty for the masses. I was thinking 0.01 kg since the lightest weight increment on my data table "weighs" 0.1. Or, we can find the greatest deviation and divide by 1000 to convert into kilograms. Hence, we would end up with an absolute uncertainty of 0.1 g for the masses ?
  6. Jan 31, 2007 #5


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    Staff: Mentor

    I googled beam balance masses tolerance, and got some useful hits. Check out the tolerances on these standard masses:


    So that gives you a number, and a reference that you can put into your report to support your assumption. Google can be a helpful tool if you can think of a good combination of keywords that has a good chance of intersecting in a useful set of page hits.
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