Two blocks $A$ and $B$ of masses $m_A = 1\,kg$ and $m_B = 3\,kg$ are kept on the table as shown in figure. The coefficient of friction between $A$ and $B$ is $0.2$ and between $B$ and the surface of the table is also $0.2.$ The maximum force $F$ that can be applied on $B$ horizontal, so that the block $A$ does not slide over the block $B$ is …….. $N$. [Take $g = 10\,m/s^2$ ]

Consider the system shown below. A horizontal force $F$ is applied to a block $X$ of mass $8 \,kg$, such that the block $Y$ of mass $2 \,kg$ adjacent to it does not slip downwards under gravity. There is no friction between the horizontal plane and the base of the block $X$. The coefficient of friction between the surfaces of blocks $X$ and $Y$ is $0.5$. The minimum value of $F$ is ………… $N$ (take, acceleration due to gravity to be $10 \,ms ^{-2}$ )

Three blocks $A$, $B$ and $C$ of equal mass $m$ are placed on a smooth surface as shown. Coefficient of friction between any block $A, B$ and $C$ is $μ$. The maximum value of mass $D$ so the block $A, B$ & $C$ move without slipping over each other is