In fluid dynamics, what does "viscosity" measure?

Viscosity is a critical property in fluid dynamics that quantifies a fluid's internal resistance to flow. This resistance arises from the interactions between the molecules within the fluid. When a fluid has high viscosity, it flows more slowly and with more difficulty compared to a fluid with low viscosity, which flows easily and smoothly. For example, honey has a higher viscosity than water, meaning it is thicker and flows more slowly.

Understanding viscosity is essential in predicting how fluids will behave under different conditions, especially in various engineering applications and processes. It plays a crucial role in designing systems involving fluid transport, lubrication, and other scenarios where fluid movement is key.

The other options do not accurately capture the concept of viscosity. Density refers to the mass of a fluid per unit volume, velocity pertains to the speed at which a fluid is moving, and pressure is the force exerted by a fluid per unit area. None of these terms encompass the idea of resistance to flow that viscosity specifically measures.