Question about capacitor and dielectric

[alexio]

The problem is that I am complicated what the direction of the force between capacitor and dielectric is while inserting dielectric in capacitor, when the voltage of capacitor is constant.
Also I want to know the direction of the force, when the charge of the capacitor is constant.

 

[Meir Achuz]

The dielectric is pulled into the capacitor in each case.
The energy change is different, but not the force.