Constrained motion question (using moments of inertia/energy)

Mutant_Fruit

I've attached a screenshot of the question. This is what i've tried so far:

I've calculated the initial energy of the system (which is all potential energy):
Energy of disc:
0.5*I*w^2 + 0.5 *m*v^2 + m*g*h
= 0 + 0 + 0

Energy of arm:
0.5*m*v^2 + m*g*h
= 0 + 4*9.81*1.5Cos(45)
= 41.62 joules

Energy of collar:
0.5*m*v^2 + m*g*h
= 0 + 1*9.81*3Cos(45)
= 20.81 joules

Then i moved time on until the supporting arm BC was horizontal and calculated the energy at that stage (which would be all kinetic). When i do that, i end up calculating that the speed of the collar is 1.04 m/s. However, the answer is supposed to be 7.315 m/s.

I'm doing something wrong, but i can't figure out what it is. Any hints on how this is supposed to be solved would be great.

Thanks.

arildno

In all probability, your flaw lies in determining the kinetic energy term of the arm. Specify what you did.