Question 191 of 210

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

A$2 \sin \phi$B$2 \cos \phi$C$2 \tan \phi$D$\tan \phi$

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Answer: (C) $2 \tan \phi$

$m g s \sin \phi=\mu m g \cos \phi \cdot \frac{s}{2}$