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A uniform steel bar swings from a pivot

  1. Apr 23, 2013 #1
    1. The problem statement, all variables and given/known data
    A uniform steel bar swings from a pivot at one end with a period of 1.3 sec.
    Solve for Length of bar

    2. Relevant equations
    T=2pi*sqrt(L/g)


    3. The attempt at a solution
    since the period is 1.3 seconds, I just plug in 1.3 for T and 9.8m/s^2 for g and solve for L
    i get L= .42m, which is wrong.
    What am I doing wrong?
     
    Last edited: Apr 23, 2013
  2. jcsd
  3. Apr 23, 2013 #2
    So are you solving for the length of the bar? It is unclear in the information above what you are solving for.
     
  4. Apr 23, 2013 #3
    yes I am supposed to solve for the length of the bar, sorry about that
     
  5. Apr 23, 2013 #4

    ehild

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    Your formula is valid for a simple (mathematical) pendulum. It is a "physical pendulum" now.

    ehild
     
  6. Apr 23, 2013 #5
    I used T=2pi * sqrt(2L/3G) and got the correct answer.
    is this the formula for period of a pendulum with a bar or something?
     
  7. Apr 23, 2013 #6

    ehild

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  8. Dec 10, 2013 #7

    I read your source and can't figure out how they came to that equation. When I do the math out of [tex] \frac{I}{Mgh} [/tex] and substitute [tex]\frac {ML^2}{3}[/tex]for I, I get [tex]2\pi\sqrt{\frac {L}{3g}}[/tex]instead of [tex]2\pi\sqrt{\frac{2L}{3g}}[/tex] Where did the 2 come from in the numerator in the correct equation?

    EDIT: Was missing the equation higher up in your source article where it stated that [tex]h=\frac{L}{2}[/tex] where I was assuming h was the same as L.
     
    Last edited: Dec 10, 2013
  9. Dec 10, 2013 #8

    ehild

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    The time period of a physical pendulum is

    [tex]T=2\pi \sqrt{\frac{I}{mgh}}[/tex]

    where I is the moment of inertia of the swinging body with respect to the pivot,
    m is the mass,
    h is the distance of the centre of mass from the pivot.

    In case of a homogeneous thin bar of length L, I=mL2/3, and the CM is at the middle, so h=L/2.


    ehild
     
    Last edited: Dec 10, 2013
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