The figure shows an L shaped body of mass M is placed on smooth horizontol surface. The block A is connected to the body by means of an inextensible string, which is passing over a smooth pulley of neglegible mass. Another block B of mass m is placed against a vertical wall of the body. Find the minimum value of the acceleration of block A so that block B remains stationary relative to the wall. Coefficient of friction between the block B and the vertical wall isµ=2. (g=10m/s2) 
The L shaped block is on a frictionless table. Let mass of the block A be MA.
Let A be the acceleration of the block A, then this will also be the acceleration of block of mass M.
For the block B, the normal reaction on the L shaped block is, N = ma
The frictional force upward is, f = µN = µma
The gravitational force downward is = mg
Now, since, the block is not moving down,
mg = µma
=> a = g/µ = 10/2 = 5 m/s2