What can be inferred from the relation $f_S \leqslant \mu_S N$?

Is it possible to jump off a perfectly smooth surface? Why?

$A$ block of mass $2 \, kg$ is kept on the floor. The coefficient of static friction is $0.4$. If a force $F$ of $2.5 \, N$ is applied on the block as shown in the figure,the frictional force between the block and the floor will be ........ $N$.

The coefficient of static friction,$\mu_s$,between block $A$ of mass $2\, kg$ and the table as shown in the figure is $0.2$. Find the maximum mass value of block $B$ in $kg$ so that the two blocks do not move. The string and the pulley are assumed to be smooth and massless. $(g = 10\, m/s^2)$

If ${\mu _s}$,${\mu _k}$,and ${\mu _r}$ are coefficients of static friction,sliding (kinetic) friction,and rolling friction respectively,then:

$A$ block of $1\, kg$ is stopped against a wall by applying a force $F$ perpendicular to the wall. If $\mu = 0.2$,then the minimum value of $F$ will be ....... $N$.