The problem statement, all variables and given/known data Hello, I am suppose to provide an equation to calculate the force that a disk magnet pulls a steel ball with respect to the distance between those two objects. My idea is to calculate the difference between the energy density with and without the ball and then calculate the gradient of this change. It will look something like F=∇(W0-W), where W0 is the energy density with the ball and W is the energy density without it. The difference between them would be the permeability of the ball which varies on its size and material. I don't have to calculate it exactly, just present some thought process. My question is: Is the method acceptable or should i rather focus on Maxwell tensor? Thank you in advance.