Question

A block of mass $4\text{kg}$ rests on a rough horizontal surface. If a horizontal force of $12\text{N}$ is applied on the block, then the frictional force by the surface on the block is
$({\mu }_k=0.4,{\mu }_s=0.5$ and $g=10{\text{m/s}}^2)$

• A. $16\text{N}$
• B. $12\text{N}$
• C. $20\text{N}$
• D. $8\text{N}$

Answer 1

The correct option is B $12\text{N}$

Limiting static friction is given by
$f_{lim}={\mu }_{s}N$
$={\mu }_{s}Mg$
$=0.5\times 4\times 10$
$=20\text{N}$
$f_{lim}>F_{\text{applied}}$
If applied force is less than limiting friction, then friction force will be equal to applied force.
$f=F_{\text{applied}}$
$f=12\text{N}$