# Solenoid force to stroke equation

What does the variable "k" mean in the solenoid equation in the picture attached? Many thanks for the help.

• Link to the original document? – Andy aka Mar 9 '18 at 0:40
• consult-g2.com/course/chapter11/chapter.html – Martin K Mar 9 '18 at 0:56
• @MartinK The equations are immediately above the curve. Can't you guess, just by inspection, the implied meaning of $k$? Just look up slightly and I think you can see the factors in play there. (Nice page, by the way.) – jonk Mar 9 '18 at 2:00
• See the link, equations 11.41 and 11.50. – WhatRoughBeast Mar 9 '18 at 3:37
• so k=Uo/2*Ag*n^2*i^2 ??? Sorry but I am not good at maths :) – Martin K Mar 9 '18 at 5:07

Force = $(N\cdot I)^2\cdot 4\pi 10^{-7}\cdot \dfrac{A}{2g^2}$
If current, area and turns are constant, Force is inversely proportional to the gap squared ($\ell^2$). Formulas taken from this question.