A block of weight W is held stationary against a rough vertical wall by applying a horizontal force P. The coefficient of static friction between the block and the wall is μs.
a. (i) Derive an expression for the minimum value of P required to keep the block at rest.
b. (ii) If W=20N and μs=0.4, calculate (a) the minimum horizontal force, and (b) the angle that the resultant reaction of the wall makes with the horizontal.