In the given figure the coefficient of friction between 4kg and 5 kg blocks is 0.2 and between 5 kg block and ground is 0.1 respectively. Choose the correct statements : -

 

So block ‘Q’ is moving due to force while block ‘P’ due to friction. Friction direction on both +Q blocks as shown.

First block ‘Q’ will move and P will move with ‘Q’ so by FBD taking ‘P’ and ‘Q’ as system $F-9=0\Rightarrow F=9N$ When applied force is 4 N then FBD

4kg block is moving due to friction and maximum friction force is 8 N. So acceleration = $\frac{8}{4}=2m/s^2=a_{\max }$

Slipping will start at when Q has +ve acceleration equal to maximum acceleration of P i.e. $2m/s^2$

$F-17=5\times 2\Rightarrow F=27N$