You know what I am saying that, the municipal car that engines that fellow is going this side, he his   motor is here, the last bogie is here, but it is still following this line, should it have followed. It is not necessarily mandatory, that it should follow, I mean in it is not necessarily true. Unless and until you have ensure it has certain properties. Remember it is not an it is not a train line. You know all of them might be having just 2 wheels, just a in a bogie with with. So, called bogie with a with a 2 wheels, 1 wheel and another wheel
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So, you know the even here, this concept of the stability comes in directional stability or possession of stability. Because, until and unless this is different dynamics of course, but the concept is similar. That there is something, some mechanism forces which should be in such a way that, if I pull this side they just follow in the same line. So, this property what I was trying to say is what is called that straight line stability (Refer Slide Time: 24:43).And a ship can only of the straight line stability or lack of it.
So, naturally what when we are starting initially, what we are going to be talking about is to develop an equation of motion. To try to find out, when the ship is disturbed what kind of forces get created on it. This is similar to what we have talked, in roll for example, because we said that as soon as I heel the ship by theta degree. What are the, what are the moments that got created, because of hydrostatic pressure.
You see the difference is that in a ship (Refer Slide Time: 25:23) in a in a static roll case. I have rolled it by a small angle. Moment I did small angle the only force acting on the hull is what hydrostatic pressure. So, what I essentially did, that this hydrostatic pressures on that new weighted line. I added them up to find, what is the net moment acting on that hull. Only thing we called it hydrostatic is because, the moment I calculated arise, arose, because of hydrostatic pressure only.