**1. The problem statement, all variables and given/known data**

I need to derive the moment of inertia of a thin rod with its axis of rotation at the end of the rod. http://ift.tt/1eG9r2u

The third one.

**2. Relevant equations**

I = mr^2

**3. The attempt at a solution**

I completely understand how the moment of inertia is mL^2/3, but what I don’t understand is why linear density is used. M/L. Shouldn’t it be volume density, or area density at the very least since a thin rod is 3D?

http://ift.tt/1iVXAcb