Hello, as you can tell by the hour I have been at this problem for quite some time now.(adsbygoogle = window.adsbygoogle || []).push({});

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?

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

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