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

Options

A. `2 sin phi`
B. `2 cos phi`
C. `2 tan phi`
D. `tan phi`

Answer

3

Explanation

According to work -energy theoram,
`W=DeltaK=0`
`:.` work done by friction+work done by gravity =0
`-(mu mg cos phi)l/2 +mg l sin phi=0`