Question

A horizontal force of $20$ $N$ is applied to a block of mass $4$ $kg$ resting on a rough horizontal table. What can be said about the coefficient of static friction between the block and the table if the block remains at rest? [$g={10ms}^{-2}$]

• A. ${\mu }_S\ge 0.5$
• B. ${\mu }_S\ne 0.5$
• C. ${\mu }_S=0$
• D. ${\mu }_S<0.5$

Answer 1

Answer: (A)

${\mu }_S\ge 0.5$

Mass, $m=4kg$

Static friction is a self-adjusting force.

So, normal reaction = $4g$

Since the block is at rest, so limiting friction force $\ge$ applied force.

i.e., ${\mu }_{S}mg\ge 20$

$\Rightarrow {\mu }_S\ge 0.5$