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Uncertainty homework help

  1. Sep 14, 2013 #1
    Uncertainty partial derivatives

    1. The problem statement, all variables and given/known data

    1 a) Give expressions for the uncertainty in both the area and volume of a right circular cone with radius R and height H and side length S. Assume that the radius has uncertainty σR and the height has an uncertainty σH. <----First part that I got.

    b) For a radius R = 10.0 ± 0.1 cm and height H = 15.0 ± 0.2 cm, calculator the absolute uncertainty on both the area and the volume.

    2. Relevant equations
    Surface area A = ∏*R^2 + (PI)*S*R
    Volume V = (1/3)*∏*R^2*H
    (S = sqrt(R^2 + H^2))
    3. The attempt at a solution

    Ok so my attempt at solving this question was as follows


    I tried to figure out the partial derivative with respect to the radius, and with respect to the height. When you get those the formula is



    I was having trouble using the latex math equations thing so I just took a picture and here it is...
    if it is unclear sigma and delta mean the same thing.

    2lw6ull.png
    qo920g.png

    now I plugged in the values R = 10.0 ± 0.1 cm and height H = 15.0 ± 0.2 cm, and I got some absurdly high values, am I right, or did I do something wrong in my partial derivatives?
     
    Last edited: Sep 14, 2013
  2. jcsd
  3. Sep 14, 2013 #2
    Your equations are correct, except that their left-hand sides must have squares of the deltas.
     
  4. Sep 14, 2013 #3
    So for the volume the uncertainty is + or - 37.8, and the uncertainty for area is + or - 13.7? Did I plug them in correctly now?
     
  5. Sep 14, 2013 #4
    I get the same for the volume, but 14.6 for the area. I ended up using a spreadsheet to compute things after a few blunders :)
     
  6. Sep 14, 2013 #5

    gneill

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    Staff: Mentor

    I'm seeing ~ 9.1 for the area (just saying...).
     
  7. Sep 14, 2013 #6
    Oh my....which one is right. I don't knowww
     
  8. Sep 14, 2013 #7
    Since we all seem to agree on the volume, consider it correct. For area, I suggest you post the value you get for each term in the equation, so that we could then cross-check.
     
  9. Sep 14, 2013 #8
    I get 187.4 for the first section until sigma r, then 27.33 for the rest. Then I add them and take the square root and get 14.6.
     
  10. Sep 14, 2013 #9
    I get the same results. Perhaps gneill could comment on his numbers.
     
  11. Sep 15, 2013 #10

    gneill

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    Staff: Mentor

    Okay, I see that the base surface area is to be included and not just the lateral area of the right circular cone. I didn't notice that before :blushing: , so that explains a source of difference. My bad.

    Designating the differentials of the Area as DAr and DAh, and dropping the units for the calculations so that r = 10.0, h = 15.0, then I find (keeping extra decimal places for intermediate calculations):

    DAr = 136.894
    DAh = 26.140

    and so
    $$δA = \sqrt{DA_r^2 δr^2 + DA_h^2 δh^2} = 14.6$$
     
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