When a dielectric slab is gradually inserted between the plates of an isolated parellel plate capacitor,the energy of the system decreases.What can you conclude about the force on the slab exerted by the electric field.

Dear student,
The energy of the system is given by:

So in the presence of a dielectric, the energy is going to decrease the Capacitance increases.
The Electric field in the presence of dielectric would decrease and therefore the force acting on the charges in the electric field would be less as compared to in the case of absence of dielectric.
Regards