question_answer

A rough vertical board has an acceleration \[d\] so that a \[2kg\] 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, ie,           \[F>mg\] \[\Rightarrow \]               \[\mu R>mg\] \[\Rightarrow \]               \[\mu (ma)>mg\] \[\Rightarrow \]               \[\mu >\frac{g}{a}\]