Un-dampened vibrating systems, Dynamics

[MightyG]

Homework Statement

having my brain fried with some dynamics problems for uni and my lecturer is on holiday so I don't have anyone to ask.

Currently working on un-dampened vibration and simple harmonic motion and I can work out basic questions such as a weight hanging on a spring but when it starts to involve rotation as well I just can't seem to get my head round it.this is the type of question I mean:

http://img88.imageshack.us/img88/1142/wtfxr2.jpg

any help with this would be greatly appreciated!

The Attempt at a Solution

is there any way of simplifying the problem in a similar way to parallel/series spring problems? or is this too simple a problem to simplify?

ive been trying to rethink the problem as a massless wheel of radius L with a tangential downwards force (forcing the wheel clockwise) of WL Sin (theta) and a tangential upwards force (anti-clockwise) of Ka Sin(theta) but I am having a hard time trying to work out how to get from the simple free body diagram to a function describing the motion of the system.

am I making fundamental/retarded mistakes or is this the completely wrong way to go about it?

need to try and get my head around these problems asap as I've been told that there is going to be a question almost the exact same as this in my up coming exam! lol

 

[CEL]

The displacement of the mass is L.sin(theta) and the displacement of the spring a.sin(theta). The momentums to be balanced are m.g.L.cos(theta) and K.a.sin(theta).a.cos(theta).