(a)

Given: acceleration of block $A$ varies with time as shown in figure To find the value of coefficient of kinetic friction between the block $A$ and $B$ Solution:

Force is applied on block $B$. The moment when the block $A$ starts sliding is the moment when the applied force $F$ is greater than the frictional force between the two blocks.

Apply Newton's Second law of motion we get

$\operatorname{ma}_i=\mu g m$

$\Rightarrow a _{\Lambda}=\mu g$

$\Rightarrow 4.9=\mu(9.8)$

$\Rightarrow \mu=0.5$

is the value of coefficient of kinetic friction between the block $A$ and $B$