See, if I, now this v was a vector, so I got a velocity this side some that is x o g dot but, I can, let me define this velocity as to be small u right I am defining, I am calling that a velocity this side is small u, velocity this side is small v and rotation of course, remains same which will be psi dot, which which I can call r, normally it is called r when I r equal to psi dot d psi by the rate of change of this heading angle, we will come to that afterwards, I will I’ll come to that little later on; but understand this that I let me call this to be x this to be y. And now I am going to call the force on this side to be X and force on this side to be Y, so what we are doing think of that, there are two systems one is that along this line I have got, a velocity which is x o g dot, you can say if I want, I can call it u dot velocity and I can call this to be v dot as velocity.