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Homework Help: Area between 2 curves, Volume around X and Y, Centroid

  1. Apr 29, 2017 #1
    g(x)= √(19x) = upper curve
    f(x)= 0.2x^2 = lower curve

    Firstly, I found the point of intersection, which would later give the upper values for x and y.

    Then I found the area under g(x) and took away the area under f(x) to get the area between the curves.
    31.67 units^2

    This is where I will include the equations so it is easy to see if I have done it right.

    VolX = ∫π[(g(x)^2)-(f(x)^2)] dx
    (The outer line squared minus the inner line squared)
    Putting in my values for g(x)^2 and f(x)^2
    g(x)^2 = 19x
    f(x)^2 = 0.04x^4
    Between 0 and 7.802 this gave me a volume of 1090.133 units^3

    VolY = ∫π[(f(y)^2)-(g(y)^2)] dy
    This time the line that was the inner in relation to the x axis becomes the outer and vice versa.
    f(y)^2 = 5y
    g(y)^2 = (y^4)/361
    Solving this integral between 0 and 12.174 gave me a volume of 698.59 units^3

    Next, the task was to find the centroid of the area, specifically using the theorem of pappus. This task is given a distinction mark, however after looking at it it seems quite simple and I'm not sure if I am missing something.

    VolX = Shaded area x 2π(ybar)

    So (1090.133)/(31.67x2π) = ybar = 5.478

    Then I haven't done xbar yet but I would think that I just do the same with VolY = Shaded area x 2π(xbar)

    Is there anything that you can see that I might have missed that makes the last part more difficult?

  2. jcsd
  3. May 1, 2017 #2


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    Science Advisor
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    Gold Member

    I gather it is also bounded by x=0.
    I don't understand why you are working with volumes to find the centroid of an area. In particular, how do you get π in there?
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