(4) reger the fig below
weight of block = mg and force acting on block upwards = FSinΘ
if R is the reaction of the block on the ground then R = mg- FSinΘ
coeff. of riction = tan?
frictional force = Rtan? = (mg- FSinΘ) tan?
this force should be equal to the horizontal component of the force applied, hence
FCosΘ = (mg- FSinΘ) tan?
simplifying F = mgtan?/ (CosΘ + SInΘtan?)
hence F = mgSin?/(CosΘCos? + SinΘSin?)
F = mgSin? / Cos(Θ-?)