A rigid body consists out of a homogeneous thin rod $($length L$,$ mass $2$ kg$)$ and a homogeneous thin disk $($radius L$/4,$ mass $8$ kg$)$ rigidly fixed to each other. This rigid body is pin$-$connected to a point O on a collar P$,$ that can slide over the horizontal guide. The rigid body is in rest, when the collar suddenly is given an acceleration a to the left. Determine the angular acceleration of the rigid body at that instance.

Data: L $=2$ m$,$ a $=3$ m$/$s$2.$