In fluid mechanics, what does viscosity measure?

Viscosity is a fundamental property of fluids that quantifies their internal resistance to flow and deformation. It essentially describes how "thick" or "sticky" a fluid is, indicating how easily it can move. A fluid with high viscosity flows slowly and resists motion, while a fluid with low viscosity flows easily and quickly.

In practical terms, viscosity relates to how well a fluid can overcome intermolecular forces when subjected to shear stress or when layers of the fluid attempt to slide past each other. This property is crucial in various engineering applications, such as predicting how fluids behave in pipelines, the design of lubrication systems, and understanding the dynamics of fluid flow in natural and industrial processes.

The other choices presented focus on different properties of fluids that do not relate to their resistance to flow. For instance, density concerns the mass of a substance for a given volume, evaporation temperature refers to the phase change from liquid to gas, and pressure in motion describes the impact forces exerted by a moving fluid rather than its flow characteristics. These distinctions underscore why the measurement of viscosity is specifically relevant in fluid mechanics when analyzing how fluids interact with forces and each other.