Question

A block of mass $4kg$ is kept on a rough horizontal surface. The coefficient of static friction is $0.8$. If a force of $19N$ is applied on the block parallel to the floor, then the force of friction between the block and floor is:

• A. $32N$
• B. $18N$
• C. $19N$
• D. $9.8N$

Answer 1

Answer: (C)

$19N$

Static friction is a self-adjusting force.
The limiting value of friction up to which the body stays at rest till it starts moving is given by, $F_s=\mu mg$
where $\mu$ is the coefficient of static friction.
Below that limit, the frictional force is equal to the force applied.
Here $F_s=0.8\times 4\times 10=32N$ ; g=$10m/s^2$
Whereas force applied is 19N less than the limiting value of friction.
Hence Frictional force is $19N$.