The upper half of an inclined plane of inc

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The upper half of an inclined plane of inclination $\theta$ is perfectly smooth while the lower half is rough. A body starting from the rest at top comes back to rest at the bottom if the coefficient of friction for the lower half is given by

A

$µ = sin$$\theta$

B

$µ = cot$$\theta$

C

$µ = 2\, cos$$\theta$

D

$µ = 2 \,tan$$\theta$

The upper half of an inclined plane of inclination $\theta$ is perfectly smooth while the lower half is rough. A body starting from the rest at top comes back to rest at the bottom if the coefficient of friction for the lower half is given by

For upper half 

${v^2} = {u^2} + 2al/2 = 2(g\sin \theta )l/2 = gl\sin \theta $ 

For lower half

$⇒$ $0 = {u^2} + 2g(\sin \theta - \mu \cos \theta )\frac{l}{2}$

$⇒$ $ - gl\sin \theta = gl(\sin \theta - \mu \cos \theta )$

$⇒$ $\mu \cos \theta = 2\sin \theta \Rightarrow \mu = 2\tan \theta $