The pressure of a fluid at a given point is defined as F⊥/A, where A is the area of a small surface inserted in the fluid at that point, and F⊥ is the component of the fluid's fo...The pressure of a fluid at a given point is defined as F⊥/A, where A is the area of a small surface inserted in the fluid at that point, and F⊥ is the component of the fluid's force on the surface which is perpendicular to the surface. (In the case of a moving fluid, fluid friction forces can act parallel to the surface, but we're only dealing with stationary fluids, so there is only an F⊥.)