Question

A rough vertical board has an acceleration ‘a’ so that a 4 kg block pressing against it does not fall. The coefficient of friction between the block and the board should be

• A. >$\frac{g}{a}$
• B. <$\frac{g}{a}$
• C. =$\frac{g}{a}$
• D. >$\frac{a}{g}$

Answer 1

The correct option is A >$\frac{g}{a}$

For the limiting condition upward friction force between board and block will balance the weight of the block.

i.e. F > mg
$\Rightarrow \mu \left(R\right)$ > mg
$\Rightarrow \mu \left(ma\right)$ > mg
$\Rightarrow \mu$ > $\frac{g}{a}$