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 , calculate the minimum force required to be applied by the finger to hold the block against the wall.

Options

A. `muMg`
B. Mg
C. `(Mg)/(mu)`
D. `2muMg`

Answer

C

Explanation

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
f=Mg `muN=Mg (therefore f=muN)`
`muF=Mg(therefore N=F)=F=(Mg)/(mu)`