Two blocks $A$ and $B$ of masses $4 \mathrm{kg}$ and $6 \mathrm{kg}$ are as shown in the figure. A horizontal force of $12 \mathrm{N}$ is required to make $A$ slip over $B$. Find the maximum horizontal force $F_B$ that can be applied on $B$ so that both $A$ and $B$ move together. (take $g=10 \mathrm{m} {\mathrm{s}}^{-2}$)

A $30 \mathrm{kg}$ slab B rests on a frictionless floor as shown in the figure. A $10 \mathrm{kg}$ block A rests on top of the slab- B. The coefficients of static and kinetic friction between the block A and the slab B are $0.60$ and $0.40$ respectively. When block-A is acted upon by a horizontal force of $100 \mathrm{N}$, as shown, find the resulting acceleration of the slab- B. $\left(\mathrm{g}=9.8 \mathrm{m} {\mathrm{s}}^{-2}\right)$