What does viscosity measure in fluid mechanics?

Viscosity is a fundamental property in fluid mechanics that quantifies a fluid's internal resistance to flow and deformation. When a fluid has a high viscosity, it means that it resists motion and flows less easily, while a fluid with low viscosity flows more freely. This property is essential for understanding how different fluids behave under various conditions, especially when they are in motion.

In practical applications, viscosity affects how fluids interact with surfaces and other fluids, impacting processes such as lubrication, mixing, and transport. For instance, oil has a higher viscosity compared to water, which is why it flows more slowly and can create resistance when trying to move through a narrow space. The ability to quantify this resistance is crucial for designing systems in engineering, ensuring that the appropriate fluids are used for specific tasks and that those fluids perform as intended.

The other choices relate to important fluid properties but do not adequately capture the essence of what viscosity measures. Mass flow rate is concerned with the quantity of fluid moving through a section per unit time, heat capacity refers to a substance's ability to store thermal energy, and density under pressure deals with how mass per unit volume changes under different pressures. None of these concepts define the resistance to flow that viscosity represents.