And of course, your since the pressure is going to be a time function so, it has to vary with respect to time. So, all these variables are involved and the pressure is going to be a function of all these three areas and the other side it will be at this location. So, you will have pressure at plus a, because this is 0, this point is 0. So, the difference in these two pressures should give you the and the difference between these two pressures acting over this surface area and integrating sum this height to this height, should give you the force, it is very simple. So, I could find this as one of the best example available in literature to explain the evolution or the meaning of Froude-Krylov force.