Question

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?

• A. $\mu Mg$
• B. Mg
• C. $\frac{Mg}{\mu}$
• D. $2\mu Mg$

Answer 1

Answer: (C)

$\frac{Mg}{\mu}$

Answer 2

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

}{\mu N = Mg(\therefore f = \mu N) }{\mu F = Mg(\therefore N = F) }{F = \frac{Mg}{\mu}}$$