According to the literature, the conservation of momentum equation needs to be coupled with the mass conservation equation. However, they cannot create a coupled system of equations of motion because they ignored third Newton law. The conservation of momentum equation is Newton’s second law of motion, whereas conservation of mass belongs to kinematics that have no deal with forces at all. However, no one motion in nature can be described only by Newton’s second law without Newton’s third law (every action creates an equal and opposite reaction). Einstein (1905) referred to this as dynamic equilibrium. It is only half the task to construct the conservation of momentum equation of “action”. The second equally important part is to find the equation of “reaction”, which would satisfied flow problem. We will show that the system of the conservation of momentum and diffusion of momentum equations satisfies the dynamic equilibrium condition.
Key words: Static and dynamic equilibrium, momentum, Navier Stokes (NS) equations.
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