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I have got a lab on 'Braking distance of a wooden block'?

  1. Jan 21, 2013 #1
    1. The problem statement, all variables and given/known data

    I have got a lab on 'braking distance of a wooden block' and I have to find an equation which fits our experiment and find the uncertainty. I have an equation but I can't find the uncertainty.

    This is how, I went about my lab....

    To find the relationship between the mass of the wooden block and the braking distance, we incrementally increase the mass of the wooden block. With the help of rubber bands, we project the wooden block in a linear motion on a plane surface. Then, we record the braking distance.

    2. Relevant equations
    I have come up with these equations
    lg⁡(d)=-0.8814 lg⁡(m)+3.9708

    lg⁡(d)=-0.8814 lg⁡(m)+lg⁡(〖10〗^3.9708)

    =lg⁡(m^(-0.8814) )+lg⁡(〖10〗^3.9708)

    =lg⁡(m^(-0.8814) ) ×⁡(〖10〗^3.9708)



    d^100=〖9350〗^100 m^(-0.88)

    How do I find the uncertainity and how do I go about it? It would really helpful if you guys reply quickly, the lab is due tomorrow.
  2. jcsd
  3. Jan 21, 2013 #2


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    I assume you took multiple measurements to create that equation. The variability in those measurements should indicate the uncertainty.
  4. Jan 22, 2013 #3
    Yes, I did take multiple measurements to create that equation. But what I don't get is, what will be my next step to find the uncertainty. I asked my teacher and he said, that the (max deviation - the average value) is my uncertainty but I really don't get what he means by that.

    I also need to draw error bars on my graph and every point has a different uncertainty value but excel doesn't allow me to change the uncertainty value from point to point.

  5. Jan 22, 2013 #4
    Hey, I found out what to do with uncertainty although now I am on the graph part and I have to state what the gradient means.

    I have plotted the graph of

    On Y axis: Distance Travled by the wooden block
    On X axis: Weight of the wooden block

    I don't know what the gradient will mean or even if it means something? Thanks
  6. Jan 22, 2013 #5


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    If you've taken several measurements for each point, you can calculate the mean value (sometimes called just the average). For each of your measurements, then, you can calculate how far away from the mean value it is. Your teacher is suggesting that you use the largest value of this (ignoring minus signs) as a measure of spread.

    It's not a good measure of spread, though. Imagine that you make a thousand measurements, and 999 of them are between 11 and 12, but you really mess up one measurement and get 18. Is 6-point-something really a fair measure of the spread? A better measure is the standard error on the mean - the standard deviation (look it up if you need to) of your measurements divided by the square root of the number of measurements.

    You can do different errors for each point in Excel. One of the options lets you enter a cell range. Use that and enter the error bar values in the spreadsheet. For example, x-value in column A, y-value in column B, upper error limit in column C and lower error limit in column D.
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