question_answer

The upper half of an inclined plane of inclination 9 is perfectly smooth while lower half is rough. A block starting from rest at the top of the plane will again come to rest at the bottom, if the coefficient of friction between the block and lower half of the plane is given by                                 [NEET 2013]

A) \[\mu =\frac{1}{\tan \theta }\]

B) \[\mu =\frac{2}{\tan \theta }\]

C) \[\mu =2\tan \theta \]

D) \[\mu =\tan \theta \]

Correct Answer: C

Solution :

The block may be stationary, when

\[mg\sin \theta \cdot L=\alpha mg\cos \theta \frac{L}{2}\]

or         \[\alpha =\frac{mg\sin \theta \cdot L}{mg\cos \theta \frac{L}{2}}\]

\[=2\frac{\sin \theta }{\cos \theta }=2\tan \theta \]

\[\alpha =2\tan \theta \]