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Model of Physical Pendulum

  1. Feb 18, 2010 #1

    Would appreciate any help anyone has for me.

    I am building a physical pendulum of sort, which consists of a hollow cylinder, which I am going to fill with sand. I am going to let the sand flow out of the pendulum and investigate the change in period with changing mass.

    I also am mathematically/computationally modelling this. I am currently trying to figure out the equations of motion that I'm going to need to model this. I know the equation for the period of a physical pendulum is T=2*pi*sqrt(I/mgh), where h will be changing at a constant rate.
    I'm wondering how to calculate the moment of inertia. I've found equations on hyperphysics for common moments of inertia (http://hyperphysics.phy-astr.gsu.edu/HBASE/mi.html#) but as my pendulum is going to be partly a solid cylinder and partly a hollow one, I'm not sure how to merge these equations. Has anyone got any advice?

    Thanks in advance :)
  2. jcsd
  3. Feb 18, 2010 #2


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    Hi sajama! :smile:

    moment of inertia is additive …

    so just add the moments of inertia of the two cylinders as if they were completely separate. :smile:

    And of course, you'll also need the parallel axis theorem, since your moment of inertia is not about the centre of mass. :wink:

    (btw, there's a better list at http://en.wikipedia.org/wiki/List_of_moments_of_inertia" [Broken] … and you need to learn all of them :wink:)
    Last edited by a moderator: May 4, 2017
  4. Feb 18, 2010 #3
    You can just algebraically add moment of inertia of various bodies if they have the same axis of rotation.
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