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Pendulum frequency

  1. Jul 17, 2010 #1

    I want to build a machine, but I need some precise calculations to complete it. I basically need to have various pendulums of the same length but different frequency

    I was thinking on using some weights that I could set higher or lower on the pendulum to adjust the frequency.

    I want to calculate the frequency given the following information:

    -Length of the pendulum (will be constant)
    -Weight of the material used throughout the pendulum (will be constant)
    -Additional adjustable weight
    -Height of the additional weight

    I hope it is not to difficult to come up with a formula that considers all of this.

    Thanks in advance!
  2. jcsd
  3. Jul 17, 2010 #2


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    Staff: Mentor

  4. Jul 17, 2010 #3
    Thanks, I was reading this article: http://en.wikipedia.org/wiki/Center_of_percussion
    which describes somewhat my problem, unfortunately I haven't been able to deduce a formula that would enable me to precisely calculate the real (I'm actually going to build it) frequency of a pendulum, given the above described criteria.

    I reckon I need something like this:


    Looking at the schematic I want to keep the length static, and move the red ball up and down along the black line (it will be rigid, not a string), thus modifying the period/frequency.
  5. Jul 17, 2010 #4

    Doc Al

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    You'll want to treat it as a physical pendulum. See: http://hyperphysics.phy-astr.gsu.edu/Hbase/pendp.html" [Broken].

    You'll need to calculate the rotational inertia about the pivot and the distance of the center of mass from the pivot.
    Last edited by a moderator: May 4, 2017
  6. Jul 17, 2010 #5
    What I understand is that this is the formula that's going to help me:


    -"m" is the total mass of my pendulum
    -"g" is gravity
    -"L" is the length from the pivot to the end of the pendulum
    -"Isupport" is the moment of Inertia which I have to calculate with:


    -"r" is the distance from the pivot to the center of mass?

    I'm not sure if I'm interpreting things correctly on this one, I need to perform the calculation for about 20 different pendulums, so deducing a formula where I can input my data and get the result is my priority.

    Now, it troubles me that this will work only for "small displacements", what exactly is a small displacement on this case?

    Thanks for your help so far!
  7. Jul 17, 2010 #6

    Doc Al

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    Depending on how you are distributing the mass along the pendulum, you can simplify the calculations for the center of mass and rotational inertia.

    The link I gave describes where the 'small angle' approximation comes in. You can always extend it to greater accuracy, if you like.
  8. Jul 17, 2010 #7
    The pendulum will be a straight wooden stick and the weight will be a lead or steel piece that I can move along the length of the stick, thus changing the center of mass.

    Ideally all the pendulums will have the same weight, but just by offsetting the steel weight I will be able to change the frequency significantly.

    I guess I need to figure out how to calculate the center of mass, based on the position and mass of the steel weight as well as the rotational inertia, I could then input those on the above mentioned formula and it should work?
  9. Jul 17, 2010 #8

    Doc Al

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    If you can neglect the weight of the stick compared to the steel weight, you can treat it as a simple pendulum. Otherwise, you'll need to treat it as a physical pendulum as we've been discussing.

    I suspect you'll be fine by approximating the steel weight as a point mass. Then you can easily calculate the combined center of mass and combined rotational inertia. The rotational inertia of a point mass is mr²; that of a stick rotating about one end is 1/3mL². (All of this can be found on the hyperphysics site.)
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