Viscosity is defined as which property of a fluid?

• A. A fluid's density
• B. A fluid's color
• C. A fluid's temperature
• D. A fluid's resistance to flow

Answer: (D)

Viscosity is the measure of a fluid’s internal friction that resists motion between its layers as it flows. That’s why it’s defined as resistance to flow—fluids with high viscosity, like honey, flow slowly, while low-viscosity fluids, like water, flow readily. Density, color, and temperature describe other properties: density is mass per volume, color is a pigment attribute, and temperature is the fluid’s thermal energy (which can affect viscosity but is not the definition of it). In practice, dynamic viscosity is measured in units like Pa·s, and sometimes kinematic viscosity is used, which is viscosity divided by density.