# Homework Help: Moments of Inertia of a rod

1. Nov 14, 2006

### merlos

The moment of inertia about an axis along the length of a rod is zero, correct?

2. Nov 14, 2006

### OlderDan

Almost. If the rod has a measurable radius, it is a cylinder.

3. Nov 15, 2006

### merlos

In all the other parts ot the problem though I considered it a rod.

Here's the problem:

Find the moment of inertia about each of the following axes for a rod that is 0.280 cm in diameter and 1.70 m long, with a mass of 5.00×10−2 kg.

A. About an axis perpendicular to the rod and passing through its center.
I = (1/12)ML^2
I = .012 kgm^2

B. About an axis perpendicular to the rod passing through one end.
I = (1/3)ML^2
I = .048 kgm^2

C. About an axis along the length of the rod.

4. Nov 15, 2006

### OlderDan

The rod has a diameter. It has a moment of inertia about the long axis.

5. Nov 15, 2006

### merlos

So, I = (1/4)MR^2 + (1/3)ML^2 ?

6. Nov 15, 2006

### Staff: Mentor

The length of the stick--or cylinder--parallel to the axis doesn't matter, only the radius.