Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Deriving Moment of Inertia

  1. Dec 7, 2006 #1
    1. The problem statement, all variables and given/known data
    A uniform rod of mass M and length L is free to rotate about a horizontal axis perpendicular to the rod and through one end. A) Find the period of oscillation for small angular displacements. B) Find the period if the axis is a distance x from the center of mass

    2. Relevant equations
    I = summation(Mx^2)
    T restoring = k*theta = mgtheta*x
    period = 2pi*root(I/k)

    3. The attempt at a solution
    The first part is no problem. I = 1/3ML^2 and the restoring constant is LMg/2. T = 2pi*root(2L/3G)

    For the second part, I know that mgtheta acts at the distance x, but how do I derive moment of inertia for an axis x distance from the com? I know it changes with the axis and it has to fall inbetween 1/3ML^2 and 1/12ML^2. But Im not familliar with the integration that goes into deriving I.

    Should I leave it as 2pi*root(newI/xMg) ?
  2. jcsd
  3. Dec 7, 2006 #2


    User Avatar
    Homework Helper

    This should help: http://en.wikipedia.org/wiki/Parallel_axis_theorem" [Broken].
    Last edited by a moderator: May 2, 2017
  4. Dec 7, 2006 #3
    Thanks a lot. So the period is 2pi*root[(1/12L^2 + x^2)/(gx)]
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook