Now I am having some parameter here so y actually goes like that initially 0y because there was no y there right so what we I now want this is my y this is my y at u v w etcetera so what is happening therefore, I’ll write it big way y u v psi dot u dot v dot psi double dot this has become equal to u minus u1 this we agree now there more to it now we will see some picture what is du by dy essentially variation of Y against u how much Y changing how much is a sway force Y force see here this is my Y force I want to find out if I change U make changes here U minus dudy by dudy by du is change of Y with respect to U what is the change. if change this side is there a change in the Y force no so it is because of symmetry so this goes to 0 similarly, if I were to but, this does not go to 0 of course, because there obviously is if I give a V here there will be Y here so this is not 0 if I give a rotation here this psi dot is rotation also is not 0 because if I rotate there will be this thing but, about this again what is this I accelerate here what is my force0. So you see I therefore, end up getting an expression which is only these 4 terms so we what I end up getting is essentially these 4 terms that means I will now write