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Math Help - Centroid of composite area

  1. #1
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    Centroid of composite area

    Hello again,
    I'm not entirely sure this belongs here but it seems like a good fit. I have a question that I was pretty certain I knew how to solve but the answer that my book provides does not match up with my found answer, and I cannot understand why. I'm hoping some of the sharp minds here may be able to help me out. First a little information about finding centroids...
    I've drawn a figure of the area to help understand the problem

    Centroid of composite area-problem-7.120.jpg

    to find the x component of the centroid of an area the following formula is utilized.. ((x1A1+x2A2+x3A3+xnAn)/(A1+A2+A3+An))
    As you can see, this total area can be divided into 4 shapes(2 rectangles, a semi-circle and a circle), and we can find the area and x centroid of those shapes
    for this problem, the x components (xn) of the area shapes are given in an appendix...
    Rectangles are 1/2*Base
    Semi-circles and circles are the center point of said circles...
    (An) simply refers to the area of those shapes. Area of semi circle is 1/2pi(r^2)

    For this problem, and others similar to it, I add the total areas of Parts 1, 2, and 3 then subtract 4. This method has been proven to work on previous exercises.
    The following table consists of the numbers I come up with.

    Xi Ai XiAi
    Part 1 80 12800 1024000
    Part 2 120 3200 384000
    Part 3 120 2513.274 301592.895
    Part 4 120 1256.637 150796.447



    After substituting these values into the equation ((x1A1+x2A2+x3A3)-x4A4)/((A1+A2+A3)-A4)) I get an answer of 90.33mm. However the answer in the book suggests the answer is in fact 87.3mm. I have spent the better part of 2 hours, reworking this problem, going through it step by step, doing other problems just like it in the same fashion and getting the right answers for those problems, and still cannot see how this problems answer is 87.3.
    Anyone who is willing to go through this problem and either confirm my suspicions that the books answer is wrong, or find the mistake that has successfully eluded me, I would highly appreciate it because I am losing my mind trying to figure out what is going on here!!! If something is unclear please let me know and I will try to clarify it better. Thank you!!
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  2. #2
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    Re: Centroid of composite area

    Did you forget to divide by 2 when you calculated area 4 ?
    It makes matters worse !
    I got a final value of 91.37
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  3. #3
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    Re: Centroid of composite area

    well area 4 is a complete circle so there should be no division by 2.

    I found this problem on chegg.com or w/e the website is, and someone else did this problem and ended up with the exact same answers as me so it made me feel a little better. I should mention that this problem was on a quiz (of course!) so I guess I will figure out what the deal is on tuesday when I receive it back.

    thanks for takin a crack at it though!
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  4. #4
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    Re: Centroid of composite area

    Quite right regarding the 2, my mistake.
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  5. #5
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    Re: Centroid of composite area

    update: book was wrong

    feels good
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