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Homework Help: Challenging Physics Question (Mass moment of inertia, pendulum)

  1. Dec 1, 2005 #1
    Hello, as you can tell by the hour I have been at this problem for quite some time now.
    I am trying to find the mass moment of inertia (rotational inertia) of a baseball bat; by means of using the equations for pendular motion. Here are the equations:

    Distance from pendulum pivot to pendulum center of gravity:
    Lcg=1.144 meter
    Mass of pendulum:
    m=0.840 kilogram
    Time Period of pendulum:
    T=2.1 seconds
    Angular speed of pendulum (calculated from period):
    w=2.99 radian/second
    gravity:
    g=9.81 meters/second^2
    Mass moment of inertia about pendulum pivot:
    Io=what the equations solve (kilogram*meter^2)

    The equation is:
    Io=(m*g*Lcg)/(w^2)
    or equivalently
    Io=((T^2)*m*g*Lcg)/(4*pi^2)

    for this I get, everytime, Io=1.05 (kilogram*meter^2)

    The trouble is coming though
    Icg=mass moment of inertia about pendulum's center of gravity
    parrallel axis theorem:
    Io=Icg+m*Lcg^2
    which becomes:
    Icg=Io-m*Lcg^2

    which spits out: Icg= -0.045 (kilogram*meter^2)

    Thats right... a negative number. That violates intuition and the laws of physics for a physical solid. How did I break physics?

    What did I do wrong and how do I fix it?
     
  2. jcsd
  3. Dec 1, 2005 #2

    mezarashi

    User Avatar
    Homework Helper

    Does it have to be by this means? If yes, I think it may be related to the 'breaking physics' part you mentioned later.
     
  4. Dec 1, 2005 #3
    I have to solve the problem using the equations for pendular motion. I checked all my formulas with textbooks and the internet, and all my equations and values seem to line up...
     
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