We know that,

Force between the plates of the capacitor is given by

Where,

Q=charge on the capacitor

A=area of plates

Derivation:

Suppose charge Q and -Q are provided on plates of capacitor of area A.

Since, F=QE

Where

Q is the test charge on the point charge

E is the electric field intensity.

Field due to charge Q on one plate is

Force on the plate with charge -Q will be

Considering magnitude, each plate applies a force of

Each.

On increasing a dielectric slab between the plates of the capacitor, the charge on the plates remains constant as the plates are isolated) .

Since, area of plates does not change, force between the plates remain constant.