NCEES FE Mechanical Practice Exam 2025 – Complete Study Resource

The principle that must be satisfied by the flow of any fluid, whether real or ideal, is the continuity equation. This equation is a result of the conservation of mass principle in fluid dynamics, which states that for an incompressible fluid, the mass flow rate must remain constant from one cross-section of a flow field to another. This means that if fluid is flowing through a pipe that narrows, the velocity must increase in that narrower section to keep the mass flow rate the same.

The continuity equation expresses this relationship mathematically, reflecting the idea that the amount of fluid leaving a volume must equal the amount entering it. This principle is universally applicable to any fluid flow scenario and serves as a foundational concept in both theoretical and applied fluid mechanics.

While Newton's second law of motion relates to forces and can describe the motion of fluids under certain conditions, it does not apply universally to all fluid flow situations. The assumption of uniform velocity distribution is specific and applies only under certain flow conditions (like fully developed laminar flow), meaning it does not hold for all flows. Newton's law of viscosity is applicable to real fluids rather than ideal fluids, and it deals with the relationship between shear stress and shear rate, which is not a fundamental requirement for all