$A$ block of weight $W$ is held against a vertical wall by applying a horizontal force $F$. The minimum value of $F$ needed to hold the block is $[\mu < 1]$.

What is the maximum acceleration of a train such that a $50 \ kg$ box lying on its floor remains stationary (in $m \ s^{-2}$)? (Given: Coefficient of static friction between the box and the train's floor is $0.3$ and $g = 10 \ m \ s^{-2}$)

$A$ person initially at rest starts walking towards the east without slipping or skidding. What is the type of friction acting on the person from the ground, and in which direction does the frictional force act?

$A$ block of mass $70\,kg$ is kept on a rough horizontal surface $(\mu = 0.4)$. $A$ person is trying to pull the block by applying a horizontal force,but the block is not moving. The net contact force exerted by the surface on the block is $F$,then

$A$ force $\vec{F} = \hat{i} + 4\hat{j} \text{ N}$ acts on the $1 \text{ kg}$ block shown in the figure. The coefficient of friction between the block and the surface is $\mu = 0.3$. The force of friction acting on the block is:

If the normal force is doubled,the coefficient of friction is