What is viscosity?

• A. A measure of a fluid's resistance to flow
• B. A measure of fluid density
• C. A measure of thermal conductivity
• D. A measure of color

Answer: (B)

Viscosity describes how strongly a fluid resists flowing when a force is applied. It’s like the difference between honey and water: honey clings and pours slowly, water flows easily. This resistance to flow is what viscosity measures, and it can change with temperature, pressure, or the fluid’s composition. In ALREMP contexts, fluids with the right viscosity ensure smooth actuation and reliable lubrication in hydraulic and mechanical systems. Density, by contrast, is about how much mass a volume contains, thermal conductivity is about heat transfer, and color is about appearance. So the idea that best defines viscosity is the fluid’s resistance to flow.