Here the same concept we have to use it, because here what happen, my ship has now become like that, so, now I have to find out that. Because, it is going the difference being here, the pressure that is acting on the hull is now dynamic pressure not hydrostatic pressure. Because of, the flow around it and now this ever we integrate them I get the net force on the hull, and net moment on the hull.
And that must balance in some sense or that plus the rudder forces, the control forces must come to a 0 or in in other words, in this case the static stability case, what I am doing,? Is the, see here we    (Refer Slide Time: 28:41) the ship was going like that and certainly at one point I disturbed it. I kind of made it like that, you know at one point, it was there was no rudder, I somehow pushed it. Eventually, what happen, forces got created on that, because that flow is not anymore straight line.
Now, what I want to know does the force created and the moment created is such that, it will tend to make this theta this angle 0. This angle 0 meaning, it will try to align itself in the direction of motion. This is what we the question that, we want to ask. Of course, it cannot go back to the same position, because there is no restoring force. But, it can come back to the same straight line.
Because, you see what it means that (Refer Slide Time: 29:28), again I mean this is only for discussion purpose purpose here. Look at this vectorially this is my direction u and this is my v. Actual, as if it is going this side, see it was going this side and I just give a push. So, it became like that, now what I want to know that this angle must diminish. Because, when see basically this v means, I have a small velocity here, because no, not that small not this side u here. I have a small velocity here. Because vectorially this is my u, this is my small v, this is my vector v.
So, I have a small v created sway velocity. And I want to know that sway velocity must come to 0, because only then small u and vector v will align itself to each other. So, these are the kind of the questions, I am asking and these are the answers of that I am trying to find out. And, if I want to do that, as I repeatedly said and I will just now come to this, you know that expressions expressions basic coordinate system.
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We need to understand and write equation of motion. Equation of motion, obviously, would say mass into acceleration equal to force the standard form. We have to come to up to that equation. Then, find out the property of the kind of hull forces that gets created. Then I will also find out what is it that it takes. For me, to make sure the hull is, you know like stable or and the related studies studies.