Angular Acceleration of a Pinned Stick

1. Oct 9, 2011

hughes

I have no idea how to approach this problem. I'm not even sure if my visualization is right. Where do I start? Somebody please guide me; I'm not asking for a step by step solution to the answer. I just want to know how to do it.
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A thin, uniform stick of length 2 m and mass 3.4 kg is pinned through one end and is free to rotate. The stick is initially hanging vertically and at rest. You then rotate the stick so that you are holding it horizontally. You release the stick from that horizontal position. What is the magnitude of the angular acceleration of the stick when it has traveled 22.7 degrees (the stick makes an angle of 22.7 degrees with the horizontal)?
Remember that the moment of inertia for a stick of mass m and length L about its end is (1/3)m L^2.

2. Oct 9, 2011

WJSwanson

The angle between the gravitational force vector and the translational displacement vector (of a given element of the stick) depends on the angular displacement of the stick. Think of how you might be able to take that fact and the given moment of inertia to derive two different equations for the torque due to gravity.