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Homework Help: Did I do this right?

  1. Nov 24, 2004 #1
    Did I do this right??


    this is my work for the this question:

    A pendulum is constructed from a thin, rigid and uniform rod with a small sphere attached to the end opposite of the pivot. This arrangement is a good approximation to a simple pendulum (period=0.66s),m because the mass of the sphere (lead) is much greater than the mass of the rod (aluminum). When the sphere is removed, the pendulum no longer is a simple pendulum, but is then a physical pendulum. What is the period of the physical pendulum.

    I used this formlua: f=1/2pie x (mgL/I)^1/2, and the I= 1/3mD^2. I don't have length. But can I assume that L=D/2. L=the length from pivot to the center of gravity of pendulum. So I sub everything in, the masses will cancel, so will some of the D values and I get left with:

    f=1/2pie x (g/(2/3 D))^1/2. I don't have D, but I do have the period back up there to be 0.66s. I punch that into this formula 2pief=(g/L)^1/2 and solved for L, which was 0.10. Since D is twice that, D=0.20m?

    Well I sub D as 0.20 into the equation and I get f=4.26, or the T=1/f = 0.2346s. Is this right? Thanks.
  2. jcsd
  3. Nov 25, 2004 #2

    Doc Al

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    Staff: Mentor

    You are mixing up L and D. In your first equation, D = length of rod, L = distance to center of mass, so L=D/2.
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