So, therefore, I would expect that we discuss this slowly. Basically, we are looking at here a z and b z, from the heave point of view it can be any name, but we will talk generally what is added mass and damping. Now, you see let me start this from a case of a accelerating body. This is the body here, somebody the full page is fluid, there is no free surface as on now. Now, I begin to oscillate. Basically, oscillation involves acceleration, you know if you oscillate you are always having acceleration, it is not a constant motion like a ship.