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4-2.Friction
medium
The coefficient of static friction between a wooden block of mass $0.5\, kg$ 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 $N$ $\left[ g =10\, ms ^{-2}\right]$
A
$25$
B
$30$
C
$30$
D
$20$
(JEE MAIN-2021)
Solution

$F.B.D.$ of the block is shown in the diagram
Since block is at rest therefore
$fr – mg =0$ $……..(1)$
$F- N =0$ $……….(2)$
$fr \leq \mu N$
In limiting case
$fr =\mu N =\mu F$ $……..(3)$
Using eq. $(1)$ and $(3)$
$\therefore \mu F = mg$
$\Rightarrow F=\frac{0.5 \times 10}{0.2}=25 N$
Standard 11
Physics