question_answer

A rough vertical board has an acceleration 'a' so that a 2 kg block pressing against it does not fall. The coefficient of friction between the block and the board should be

A) > g/a               

B) < g/a                     

C)       = g/a               

D) > a/g                                

Correct Answer: A

Solution :

For the limiting condition upward friction force between board and block will balance the weight of the block. i.e. \[\operatorname{F} > mg\] \[\Rightarrow \,\,\,\mu (R)>mg\] \[\Rightarrow \,\,\,\mu (ma)>mg\] \[\Rightarrow \,\,\,\mu \,\,>\,\,\frac{g}{a}\]