Uniform distribution of charge on a plane and on a bar: conditions to

[bznm]

On the plane z=0 there is a superficial charge distribution such that $\sigma$ is constant.

Near to the plane, there is a bar, charged uniform with total charge q. At the extremities the bar has two constraints, so it can't turn.

If I want to find the constraints force and the force momentum needed to block the bar, can I consider the charge q as puntiform and put it in the midpoint of the bar?

 

[BruceW]

so, you want to find the total electrostatic force on the bar. And you want to know if it is OK to simply calculate the force which would act on a single charge q in the centre of the bar. Well generally, this depends on the electric field which is acting on the bar. What can you say about the electric field created by the plane?