In a figure shown mass of `A` and `B` is equal to `M` each. Friction between `B` and lowermost surface is negligible. Initially both the blocks are at rest. The dimensions of the block `A` are very small. A constant horizontal force `F` is applied on the blocks `B` and both the blocks start moving together without any relative motion. Suddenly, the block `B` encounters a fixed obstacle and comes to rest. The block `A` continues to slide on the block `B`. The block `A` just manages to reach the opposite end of the bolck `B`. What is the coefficient of friction between the two blocks? (Required length are shown in figure)