$($b$)$ A $4\mathrm{.}0kg(m_2)$ block is stacked on top of an $8\mathrm{.}0$ kg block $(m_1)\mathrm{.}60N$ is applied horizontally to the $8\mathrm{.}0$ kg block. The coefficient of kinetic friction between the table and the lower block is $0\mathrm{.}10.$ Calculate what is the minimum coefficient of static friction between the blocks if they are not to slip.

$(20$ marks$)$

$(25$ marks$)$

FIGURE $02$