First, let us calculate force on a fluid element because of pressure variation. Consider an elemental fluid volume bounded by (ABCD)-(A'B'C'D') surfaces.
(Fig. 3f)
Pressure at is
Volume of the fluid under consideration is
Force acting on the face AA'C'C: (+ ve direction)
Force acting on the
face (-ve direction)
(Neglecting the higher order terms or assuming linear variation
over
)
Differential force,
(acting along –ve x-direction)
Similarly,
Therefore, we have expression for differential pressure–force
or,
If there is no pressure variation or pressure is uniform around an element, the net force is zero,
or
is
(+ ve direction) 
(-ve direction) 
) 
(acting along –ve x-direction) 
