Question

A block of mass $m$ is at rest under the action of force $F$ against a wall as shown in figure. Which of the following statement is incorrect ?

• A. $f=mg\text{(where}f\text{is the frictional force})$
• B. $F=N\text{(where}N\text{is the normal force)}$
• C. $F\text{will not produce torque}$
• D. $N\text{will not produce torque}$

Answer 1

The correct option is D $N\text{will not produce torque}$

This is the equilibrium of coplanar forces. Hence,
$\sum F_x=0;\sum F_y=0;\sum {\tau }_c=0$

$\sum F_x=0$
$\therefore F=N$

$\sum F_y=0$
$f=mg$

$\sum {\tau }_c=0$
$\therefore {\tau }_N+{\tau }_f=0$
Since, ${\tau }_f\ne 0$
$\therefore {\tau }_N\ne 0$

Hence, option $\left(D\right)$ is correct.