  This article analyses the assumptions regarding the influence of pressure
forces during the calculation of the motion of a Newtonian fluid. The purpose
of the analysis is to determine the reasonableness of the assumptions and their
impact on the results of the analytical calculation. The connections between
equations, causes of discrepancies in exact solutions of the Navier-Stokes
equations at low Reynolds numbers and the emergence of unstable solutions using
computer programs are also addressed. The necessity to complement the
well-known equations of motion in mechanical stress requires other equations
are substantive. It is shown that there are three methods of solving such a
problem and the requirements for the unknown equations are described. Keywords:
Navier-Stokes, approximate equation, closing equations, holonomic system.
