Magnetic forces required to repel, one permanente and one electromagnet

[paulmax123]

Hi guys, this is my first time here.
I'm currently working on a new design, our team works for Network Rail and we really need your help.

Ok here's the problem:
I have a magnet, permanent magnet which is able to move only up and down, not laterally. I have another magnet, this time its an electromagnet. Both the magnets have been aligned so that when the electromagnet is switched on, they repel. Now the question is, if my magnets are repelling over 7mm and if a force of 0.4N is acting on the permanent magnet over the 7mm distance, how powerful do my magnets have to be? Sounds a bit vague, but if my permanent magnet is a neodynium one at 12000br, how large does my electromagnet have to be

Oh another thing, if I know that the force acting on the magnets at the distance of 7mm is 0.4N, what is the force acting on the magnets at 0 distance? The formula to express force and magnetic force with respect to distance has been a puzzling one for our entire team, please we need your help!

 

[anorlunda]

ping @Charles Link

Charles, We get so many questions similar to this one it makes we think that we should have an Insights on calculating magnetic forces.