Simple angular acceleration problem

[joseg707]

Homework Statement

A 4 meter long rod is hinged at one end. The rod is initially held in the horizontal position, and then released as the free end is allowed to fall. What is the angular acceleration as it is released?

Homework Equations

Ang. Accel. = (Tan. Accel.)/(radius)

The Attempt at a Solution

(9.8m/s^2)/(4m)= 2.45rad/s^2

The answer is 3.6 rad/s^2.

I don't know what I'm doing wrong. Gravity is the only force acting on the rod, but there is something that I'm not taking into account. Can someone point me in the right direction please?

 

[LowlyPion]

Consider the torque about the pivot.

What is the force acting to create the angular acceleration.