What does viscosity measure?

• A. The ratio of a density of a fluid to the density of air
• B. The ratio of density of a fluid to the density of reference fluid
• C. The ratio of the density of a fluid to the density of water
• D. The ratio of viscosity to density

Answer: (B)

Viscosity measures how much a fluid resists being deformed or dragged out of shape; it’s the internal friction that slows layers of fluid sliding past one another. The most common way to describe it is dynamic viscosity, which directly relates to the force needed to achieve a certain rate of shear. If you take viscosity and divide by density, you obtain kinematic viscosity, a quantity that expresses how momentum diffuses through the fluid. Among the given options, the one that corresponds to a viscosity-related measure is the ratio of viscosity to density, since that ratio defines kinematic viscosity. The other options describe relative density (how dense a fluid is compared with air, a reference fluid, or water), which is not a measure of viscosity.