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Homework Help: Pendulum-why is this wrong?

  1. Apr 20, 2008 #1
    Pendulum-why is this wrong??

    1. The problem statement, all variables and given/known data
    A 14 cm long 230 g rod is pivoted at one end. A 17 g ball of clay is stuck on the other end. What is the period if the rod and ball swing as a pendulum?

    2. Relevant equations

    T= 2pi*sqrt(L/g)

    3. The attempt at a solution

    I used the above equation

    T=.75 s which is wrong

    I thought that the period of a pendulum didn't depend on mass, so why is this different?
  2. jcsd
  3. Apr 20, 2008 #2


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    The formula you posted in relevant equations applies to a simple pendulum, where a mass is contained all at the end of a massless string. This is a physical pendulum and has a different formula for the period.
  4. Apr 20, 2008 #3
    So would i find the center of mass by doing this:

    let the position of the ball be x=0

    cm= [(.02kg)(0m) + (.200kg)(.07m)]/.22kg

    Then I would plug that into omega= sqrt[Mgl/I]
    where l is the distance between the center of mass and the pivot

    Then I would find the moment of inertia by first finding the moment of inertia for the rod and then for the ball and adding those together. Then I do 2pi/omega to get the period
  5. Apr 20, 2008 #4
    hmmm...I also tried to find the moment of inertia by using the total mass and the distance between the center of mass that I calculated and I didn't come up with the same moment of inertia as I did when I calculated them individually. Does this mean I calculated the center of mass wrong?
  6. Apr 20, 2008 #5


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    In your formula l is the distance between the center of mass and the pivot, so you probably want to make let the position of the pivot be x=0. (The way you did it is fine, but it gives the distance of the center of mass from the ball, so you would need to do a subtraction to find the l to plug in.)

    c.o.m. = ( 0.017 * 0.14 + 0.230 * 0.07 ) / (0.0247)

    I'm not sure I understood what you were doing in your last post (#4). Were you asking if you could find the moment of inertia by finding how far the center of mass is from the pivot and doing something like Mr^2? If so, that's not correct. It would predict, for example, that the I for a uniform solid sphere is 0 for an axis about it's center. (Because its center of mass is at its center.) But we know that's not true.
  7. Apr 20, 2008 #6
    oops had the wrong #s in there

    Ok- so is this the correct way to do the problem:
    for a pivoting rod: I=(1/3)ML^2
    so I= (1/3)(.230kg)(.14m)^2
    I= .0015 kgm^2 for rod

    For the ball

    add them together: = .0018 kgm^2

    Then for the center of mass:
    ( 0.017kg * 0.14m + 0.230kg * 0.07m ) / (0.247kg) = .0748 m

    omega= sqrt[Mgl/I]
    so omega= sqrt [.247kg*9.80*.0748/.0018]
    = 100.59 rad/s

    2pi/100.59 = .062 sec?
  8. Apr 20, 2008 #7


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    Hi bcjochim07,

    I noticed that's rather small for the period! I think you forgot to take the square root when you calculated omega.
  9. Apr 20, 2008 #8
    oh yep.... so T=.6265s
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