The coefficient of static friction between a wooden block of mass $0.5 \mathrm{~kg}$ and a vertical rough wall is $0.2$. The magnitude of horizontal force that should be applied on the block to keep it adhere to the wall will be $\mathrm{N}$. $\left[\mathrm{g}=10 \mathrm{~ms}^{-2}\right.$ ]
F.B.D. of the block is shown in the diagram
Since block is at rest therefore
$\mathrm{fr}-\mathrm{mg}=0$..(1)
$\mathrm{F}-\mathrm{N}=0$..(2)
$\mathrm{fr} \leq \mu \mathrm{N}$
In limiting case
fr $=\mu \mathrm{N}=\mu \mathrm{F} \quad \ldots .(3)$
Using eq. (1) and (3)
$\therefore \mu \mathrm{F}=\mathrm{mg}$
$\Rightarrow \mathrm{F}=\frac{0.5 \times 10}{0.2}=25 \mathrm{~N}$
Ans. $25.00$
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