# Electromagnet force equation clarification

So, from a previous previous question, I know what plane the force from an electromagnet will affect a ferromagnetic material in. I thought that would be enough, but working through some problems using the force of a solenoid (electromagnet) given by $$F = (NI)^2\mu_0A/2g^2$$ I realized I was unsure where to get $g$ from.

I know that I should take $A$ from plane b, but what point should I take $g$ from? What point along the rod should I take the area from? The farthest away possible from the electromagnet, at the back? As close as possible, at the front? The middle of the rod, an equal distance from both ends? Should I calculate the force at all places along the rod, and average them all?

As of right now, I am assuming the force is acting on the middle of the rod (the average distance away from the electromagnet.)

Any help appreciated.

Edit: Thinking about this, I believe the real question I should be asking is "Should I be using volume and g^3 in my equation instead of area and g^2? Because the electromagnet would produce a field, acting on anything in it that is ferrous. So the force would be acting on whatever volume is "immersed" if you will, in the field?"

The equation you give is for the static situation in which the iron is already fixed near the solenoid (cf this site). In this set-up, see the image below, we have a coil with surface area $A=\pi d^2/4$ with diameter $d$ placed a distance $g$ below a metal plate (it also seems that the plate must have a surface area > $A$).
The volume of the field, $V=gA$, is considered in determining the potential of the field, but this factor of $g$ here is canceled by the derivative (and then two more factors of $g$ are divided through from the magnetic field equation): $$U=\frac{B^2}{2\mu_0}gA$$ $$F=\frac{dU}{dg}=\frac{B^2}{2\mu_0}A$$ the magnetic field of a solenoid is $$B=\frac{F_m\mu_0}{g}$$ where $F_m=IN$ is the magneto-motive force. This then gives us your relation, $$F=\frac{N^2I^2\mu_0}{2}\frac{A}{g^2}$$