FPGEE 2026 Practice Exam for NABP – The Complete All-in-One Guide to Exam Success!

A common characteristic of a Newtonian fluid is that its viscosity remains constant under shear stress. This means that regardless of how much force is applied to it, the fluid's internal resistance to flow does not change.

In practice, Newtonian fluids have a linear relationship between shear stress (the force per unit area) and shear rate (the rate at which the fluid is deformed). This linearity is represented by the equation τ = μ(du/dy), where τ is the shear stress, μ is the dynamic viscosity, and du/dy is the shear rate. As a result, the viscosity is a constant value for a given temperature and pressure, which is a characteristic property of these fluids.

In contrast, other types of fluids exhibit different behaviors. For example, fluids that show time-dependent viscosity would be categorized as non-Newtonian fluids, specifically thixotropic or rheopectic fluids, where the viscosity changes over time or with varying shear rates. Similarly, fluids that become thicker when agitated also fall into the non-Newtonian category and do not maintain a constant viscosity. Thus, the defining feature of Newtonian fluids is their invariant viscosity under varying stress conditions.