Mechanics help

Ive gotten part i but now part ii.

So as no sliding takes place between the boxes, the frictional force there has to be equal or more than the force acting on it.

Nr= 4000 N

Fr= 0.4 x 4000
= 1600 N

From here i have no idea what to do. Any help would be appreciated. Thanks

As you've ascertained, the maximum frictional force on the top box is 1600N.

Since this is the only force accelerating the top box, then apply F=ma to get the maximum acceleration.

i dont get it. Wdym that is the only force accelerating the top box, thats the frictional force.

Yes. The external force is being applied to the bottom box. The only force being applied to the top box is via the friction between the two.

As the bottom box accelerates it drags the top box with it, via the friction between the two.

Right, so the max acceleration of the top box without it sliding is the overall max acceleration because it does not slide ? Then ill just use the acceleration on the entire thing to find max possible value of P. Yes yes?