Question

Three blocks A, B and C of masses $2\text{kg},3\text{kg}$ and $4\text{kg}$ are placed as shown. Coefficient of friction between A and B is 0.5 and that between B and C is 0.1. The surface is frictionless. The maximum force $F$ in N that can be applied horizontally onto A such that the three blocks move together is (Answer upto 2 decimal places and take $g=10m{s}^{-2}$
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Answer 1

$F_{s_1}=0.5\times 2\times 10=10\text{N}$
$F_{s_2}=5\text{N}$
Let acceleration of whole system be a
For C, then, $ma=F_s^{max}$
$\therefore a=\frac{5}{4}{\text{m/s}}^2$
For B, $a=\frac{5}{4}{\text{m/s}}^2$
So, $F=9\times 5/4=11.25\text{N}$