What Is the Angular Acceleration of a Released Uniform Plate?

[Himanshu_123]

Homework Statement

A uniform plate of mass m is suspended as shown.What will be the angular acceleration immediately after connection at B has been released?

http://i.stack.imgur.com/1hMuk.png

2. My attempt-

I used torque=M.I*angular acceleration

I chose the axis to be perpendicular to the plane at A where M.I=5mc^2/12 and at a distance of c/2 from line of action of mg by putting it in equation we get angular acc.=1.2g/c but the given ans. is 2.4g/c

But when I solve choosing the axis as centre of mass -
The tension/force by which string will pull= mg/2 and distance of line of action from axis =c/2 and M.I about c.m=5mc^2/48 and using torque =M.I*angular acceleration i get the right ans. I don't know what's the mistake in my 1st approach.

 

[Jonathan Yang]

Where is the diagram?

 

[Himanshu_123]

Where is the diagram?

Thanks for pointing. I corrected it.

 

[Jonathan Yang]

I think you calculated the lever arm incorrectly. If you choose point A, the lever arm that is perpendicular to the line of action is (c/2)/2 or c/4