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Math Help - First Moment about the Line x=4

  1. #1
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    First Moment about the Line x=4

    I did the following question two ways; with vertical strips and with horizontal strips. I got two slightly different answers [ rho*(13/4 + 6ln(2)) with vertical strips and rho*(13/4 - 6ln(2)) with horizontal strips]. I can't figure out which (if either) is the correct answer or where I went wrong.

    Here is the question:

    Consider the thin plate of constant density rho [mass/unit area] bounded by the curves:
    y=ln(x), y=0, and x=2
    Find the first moment (moment of mass) of this plate about the line x=4.
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  2. #2
    MHF Contributor
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    Hi

    Are you talking about the moment of inertia

    \int_{1}^{2} \int_{0}^{\ln x} \rho (4-x)^2 dy dx ?
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  3. #3
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    No. I'm talking about moment of mass (first moment). Also, we are just using single integrals at the moment.
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