# Homework Help: Inertia Problem

1. Nov 9, 2006

### cd80187

In Fig. 10-35, two particles, each with mass m = 2.30 kg, are fastened to each other, and to a rotation axis at O, by two thin rods, each with length d = 0.670 m and mass M = 0.360 kg. The combination rotates around the rotation axis with angular speed ω = 0.211 rad/s. Measured about O, what are the combination's (a) rotational inertia and (b) kinetic energy?
(I'm not sure how to post pictures on here, because the picture is on wileyplus, and to view it, you need a password)

But for this problem, I am not even sure where to start. I know that I need to find the total inertia, but how many parts should i split it into? Should I take each rod seperately and each particle seperately, and use the summation formula for inertia, or should i use the parallel axis theorem? I am just so lost on where to start.

2. Nov 9, 2006

### OlderDan

If the configuration is axis-rod-particle-rod-particle, then you will want to use the parallel axis theorem for the rods and add the contributions from the masses. The parallel axis theorm has already been used for the rod rotating at one end in most places where you look up moments of inertia. Since the second rod is not touching the axis, you will have to compute it yourself.