The time taken by an object to slide down

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The time taken by an object to slide down $45^{\circ}$ rough inclined plane is $n$ times as it takes to slide down a perfectly smooth $45^{\circ}$ incline plane. The coefficient of kinetic friction between the object and the incline plane is

A

$\left( {1 - \frac{1}{{{n^2}}}} \right)$

B

$1+\frac{1}{ n ^2}$

C

$\sqrt {\left( {1 - \frac{1}{{{n^2}}}} \right)} $

D

$\sqrt {\frac{1}{{1 - {n^2}}}} $

The time taken by an object to slide down $45^{\circ}$ rough inclined plane is $n$ times as it takes to slide down a perfectly smooth $45^{\circ}$ incline plane. The coefficient of kinetic friction between the object and the incline plane is

$\mu = \tan \theta \left( {1 - \frac{1}{{{n^2}}}} \right)$ $ = 1 - \frac{1}{{{n^2}}}$ [As $\theta = 45^\circ $]