A block of mass $m$ rests on a rough inclined plane. The coefficient of friction between  the surface and the block is $\mu$. At what angle of inclination $\theta$ of the plane to  the horizontal will the block just start to slide down the plane?
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The various forces acting on the block are as shown in the figure.

From figure

$\operatorname{mgsin} \theta=\mathrm{f}$            $...(i)$

$\mathrm{mg} \cos \theta=\mathrm{N}$              $...(ii)$

Divide $(i)$ by $( ii),$ we get

$\tan \theta=\frac{\mathrm{f}}{\mathrm{N}}=\frac{\mu \mathrm{N}}{\mathrm{N}}$

or $\theta=\tan ^{-1}(\mu)$

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