A small block B fits inside a lot cut in arm OA which rotates in a vertical plane at a constant rate. The block remains in contact with the end of the slot closest to A and its speed is 1.4 m/s for 0\le \theta \le 15{ 0 }^{ \circ }. Knowing that the block begins to slide when \theta =15{ 0 }^{ \circ }, determine the coefficient of static friction between the block and the slot.