Why is there force on a dielectric inserted into a capacitor

[BearShark]

Hey, we have learned in class that if you partialy insert a dielectric slab into a capacitor where voltage is held constant, there is a force pulling the dielectric farther into the capacitor. This gets me consfued. I know the more dielectric material there is in the capacitor, the capacitence would increase. Since this capacitor is connected to a voltage source, and voltage is held constant, then the charge on the plates would have to increase. However, this would mean the potential energy inside of the capacitor would increase. Wouldn't that mean I would have to apply force to push the dielectric in? How come there's a force pushing it in?

 

[theodoros.mihos]

There is two different cases:
1. Push dielectric material on a capacitor C with initial charge Q, disconnected to voltage source.
2. Push dielectric material on a capacitor C connected to a constant V voltage source.
Please, think about these cases.