Angular speed of a swinging rod

[chouZ]

Homework Statement

A nonuniform 2-0 kg rod is 2.0 m long. The rod is mounted to rotate freely about a horizontal axis perpendicular to the rod that passes through one end of the rod. The moment of inertia of the rod about this axis is 4.0 kg.m2. The center of mass of the rod is 1.2 m from the axis. If the rod is released from rest in the horizontal position, what is its angular speed as it swings through the vertical position?

Homework Equations

The Attempt at a Solution

I tried to use conservation of energy but I am not really sure whether the height in the potential energy initially is the distance from the center of mass to the rotational axis or the length of the rod...?

 

[Doc Al]

In calculating the change in gravitational PE of the rod, what matters is the change in height of its center of mass.

 

[chouZ]

still a lit bit confused...the change in height..how can u calculate it? I mean I thought about it: the height h between the position of the center of mass initially and its position at the final moment...but i can find a way to find the answer...

 

[Doc Al]

Think it through. When the rod is horizontal, where is its center of mass? What about when it swings down to be vertical? Draw yourself a picture and figure out the change in height.

 

[chouZ]

got it...thanks for the help!