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Simple Calculus Problem

  1. Jul 14, 2010 #1
    I'm trying to determine the centroid of the shape below:
    MRbfl.jpg

    The curve line is that of a circle, but the equation of for a circle, (x-a)2+(y-b)2=r2, won't work here for obvious reasons, but the equation y=r-[tex]\sqrt{2rx-x^{2}}[/tex] does.

    To determine the location of the centroid, the formula is:

    uvwMP.jpg
    EtVOC.jpg

    To ensure that the equations that I am using are correct, I am going to compare the area calculated by integrating the line with respect to it's corresponding axis against this equation:
    Wddr8.jpg
    Let's assume that we have a circle with a radius r=1. The area is therefore:

    BtCtj.jpg
    Now, I solved the integral two ways: one with Mathcad, and the other with The Integrator.

    When I solve it with Mathcad, I get:

    112B8.jpg

    which is wrong because the area calculated is negative.


    When I solve it with The Integrator, I get:

    p5yPi.jpg

    which is also wrong because if this equation where evaluated, you would obtain a number with imaginary components.

    I type this equation into my TI-84 and had it calculate the area under the curve and it got an answer that agreed with the known value, so the equation for the curve is correct.

    It's been awhile since I've used calculus like this, but this problem should be very straight forward; however, that is not turning out to be the case.

    Where have I gone wrong?

    EDIT: I'd like to point out that I've already solved for the location of the centroid using a different method. I posted this because I'm more interested in finding out why this method isn't working for me.
     

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    Last edited: Jul 14, 2010
  2. jcsd
  3. Jul 14, 2010 #2

    LCKurtz

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    These equations look correct to me. I get

    [tex]\bar{x} = \bar{y}= \frac{r(10-3\pi)}{12-3\pi}[/tex]
     
  4. Jul 14, 2010 #3
    That is exactly the answer. Can you please go into some detail about how you obtained it? Can you also explain why I am not getting the correct answer when I try integrating for the area under the curve?
     
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