A block of mass M  is held against a rough vertical wall by pressing it with a finger. If the coefficient of friction between the block and the wall is $\mu$ and the acceleration due to gravity is g, what is the minimum force required to be applied by the finger to hold the block against the wall?

 

Let F be the force applied by the finger on the block. N is normal reaction of the wall on the block. Mg is weight of block acting vertically downwards. f is the force of friction between the wall and the block which acts upwards as shown in the figure. The block will not fall, if

$$\begin{gathered} \mathrm{f}=\mathrm{Mg} \\ \mathrm{\mu N}=\mathrm{Mg} (\because \mathrm{f}=\mathrm{\mu N}) \\ \mathrm{\mu F}=\mathrm{Mg} (\because \mathrm{N}=\mathrm{F}) \\ \mathrm{F}=\frac{\mathrm{Mg}}{\mathrm{\mu }} \end{gathered}$$