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Homework Help: Physical pendulum formula

  1. Nov 13, 2006 #1
    A rod suspended on its end and acting as a physical pendulum swings with a period of 1.74 s. What is its length? (g = 9.80 m/s2)

    Ok so in order to solve this I need to use the physical pendulum formula:

    T = 2Pi * sqrt (I / mgh) ==> I am looking for 'h'

    To get 'h' I need to first calculate 'I' which requires mass for a rod.

    mic.gif

    So, taking it from here I'm not sure where to begin. I've tried rearranging the formulas I've been given with no luck - any ideas?
     
  2. jcsd
  3. Nov 13, 2006 #2

    OlderDan

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    h is the distance from the pivot point to the center of mass of the rod. You can express that in terms of the length of the rod and you can express I in terms of the length of the rod.
     
  4. Nov 13, 2006 #3
    Ok let me try this:
    1.74=2Pi * sqrt ((1/12*mL^2) / (m*9.81*2L)
    => .27=sqrt((1/12L / 19.62))
    1/12L = 1.504
    L=~18m ??

    This is wrong. I canceled out m and L in my early steps, did this throw my calculations off?
     
  5. Nov 13, 2006 #4

    Doc Al

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    The stick is suspended from its end, not its middle.
     
  6. Nov 14, 2006 #5
    1.74=2Pi * sqrt((1/3L^2) / (m(9.81)(2L)

    *question: I haven't had algebra for a while, but you can just cancel the m's and cancel the L's, leaving only one 'L' in the numerator, correct?

    If that assumption is correct I get L=4.51m - my book disagrees with this answer though; any idea why?
     
  7. Nov 14, 2006 #6

    Doc Al

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    Looks like you are setting h = 2L, which isn't right. As OlderDan explained, h is the distance from the pivot point to the center of mass of the rod.
     
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