# Rotational Inertia of Thin Rod

1. Feb 10, 2009

1. The problem statement, all variables and given/known data
A rigid body is made of three identical thin rods, each with length L=0.600m, fastened together in the form of a letter H. The body is free to rotate about a horizontal axis that runs along the length of one of the legs of the H. The body is allowed to fall from rest from a position in which the plane of the H is horizontal. What is the angular speed of the body when the plane of the H is vertical?

2. Relevant equations
Not quite sure, possibly I = 1/3*MR2, the perpendicular axis theorem, and the parallel axis theorem

3. The attempt at a solution
I am not really sure where to start since all that is given is L. I think that each rod must have its moment of inertia calculated separately about the axis using the perpendicular and parallel axis theorems, but we have not reviewed these subjects extensively and I am not sure of how I would go about doing this. I have read through the textbook and found no examples that are even close to this with so few variables given.
I am not necessarily looking for a worked out solution, but rather a finger to point me in the right direction (i.e. formula or concept)!

2. Feb 11, 2009

### Brian_C

You can obviously divide the H into three component rods, then calculate the I's for each rod about the given axis. As for finding the final angular speed of the body, it is most convenient to use an energy principle.

Last edited: Feb 11, 2009