I must be thinking of a different Hook's law...

All you have to do is this: give the magnet an action that keeps scanning around it for physics entities. You can use events like event_scan and event_detect, and a flag to show which entities are physics entities. Then, if one gets close enough to the magnet, use forces to push the ball at the magnet... phent_addcentralforce, etc.

It would have to be a pretty strong force to overcome other forces + gravity... also, magnetic attraction grows stronger the closer the 2 objects become, just something to keep in mind for realism...

I'll admit I didn't look very carefully at Hook's law. But it seems like it's just an algebraic expression. XSI does this rule in simulation in real time so it's not impossible to make it work in real time. Then again it may have taken a mathematician to figure out how to do Hook's law in real time. I really have no idea.