Question

A rough vertical board has an acceleration $d$ 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 $

Answer 1

Answer: (A)

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

ie, $F>m g$
$\Rightarrow \mu R>m g$
$\Rightarrow \mu(m a)>m g$
$\Rightarrow \mu>\frac{g}{a}$