Two blocks A and B are separated by some distance and tied by a string as shown in the figure. The force of friction in both the blocks at $\mathrm{t}=2\mathrm{s}$ is 

$$\begin{gathered} {\left(f_1\right)}_{\max }={\mu }_1{m}_{1}g=6 \mathrm{N} \\ {\left(f_2\right)}_{\max }={\mu }_2{m}_{2}g=10 \mathrm{N} \\ \text{ At }t=2 \mathrm{s},F^{\text{'}}=4 \mathrm{N} \end{gathered}$$

Net pulling force,

Total maximum resisting force,

$F_2={\left(f_1\right)}_{\max }+{\left(f_2\right)}_{\max }=16 \mathrm{N}$

System will not move and free body diagrams of the two block are as shown in figure.