The upper portion of an inclined plane of inclination $\alpha $ is smooth and the lower portion is rough. A particle slides down from rest from the top and just comes to rest at the foot. If the ratio of the smooth length to rough length is $m : n$ , the coefficient of friction is
  • A$\left[ {\frac{{m\, +\, n}}{n}} \right]\tan \,\alpha $
  • B$\left[ {\frac{{m\, +\, n}}{n}} \right]\cot \,\alpha $
  • C$\left[ {\frac{{m\, -\, n}}{n}} \right]\cot \,\alpha $
  • D$\frac {1}{2}$
Diffcult
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