question_answer

A rough vertical board has an acceleration a along the horizontal so that a block of mass M pressing against it does not fall. The coefficient of friction between block and the board is

A) \[>\frac{a}{g}\]            

B)        \[<\frac{g}{a}\]

C) \[=\frac{a}{g}\]            

D)        \[>\frac{g}{a}\]

Correct Answer: D

Solution :

[d] Force on M = Mg Reaction force=Ma Force of friction\[=\mu R=\mu .Ma\] Force of friction will balance the weight .So \[\mu Ma\ge Mg\,\,;\] \[\mu \ge \frac{g}{a.}\]