6. A uniform rod \(A B\) of mass \(m\) and length \(4 a\) is free to rotate in a vertical plane about a horizontal axis through the point \(O\) of the rod, where \(O A = a\). The rod is slightly disturbed from rest when \(B\) is vertically above \(A\).

1. Find the magnitude of the angular acceleration of the rod when it is horizontal.
2. Find the angular speed of the rod when it is horizontal.
3. Calculate the magnitude of the force acting on the rod at \(O\) when the rod is horizontal.
   (5)