What does “viscosity” measure in fluid mechanics?

Viscosity is a fundamental property in fluid mechanics that quantifies a fluid's resistance to deformation or flow. It indicates how easily a fluid can move or how thick or thin it is. A higher viscosity means that the fluid has a greater resistance to flow, causing it to behave more like a solid under certain conditions, while a lower viscosity indicates that the fluid flows easily.

For example, honey has a higher viscosity compared to water, making it thicker and slower to pour. Understanding viscosity is crucial for various applications, such as predicting how fluids will behave in pipes, engines, and various mechanical systems, as it affects the energy required to pump fluids and the overall efficiency of fluid transport.

The other options relate to different properties of fluids that are not directly related to viscosity. Density refers to the mass of the fluid per unit volume, thermal conductivity measures how well a fluid can conduct heat, and surface tension relates to the cohesive forces at the surface of a liquid. Each of these properties plays an important role in fluid dynamics but does not capture the essence of what viscosity represents.