When , only the first two terms become significant. The above equation becomes

                                                               L - 7.3

Similarly, pressures at the center of all the faces can be derived in terms of P (x, y, z) and its gradient.

Note that surface areas of the faces are very small. The center pressure of the face represents the average pressure on that face.
The surface force acting on the element in the y-direction is

                                          L -7.4