Thank you for your answer!
I found sth. in the internet: http://www.spiralwishingwells.com/guide/physics.html
The form of a funnel is -1/x^2.
But it doesnt help me.
I dont know the lagrangian but i know the other moments... but i dont have any approach / idea how to start with my calculation. Can anyone help me there pls. is it that complicated?
has anyone a equation which i could use to start?