What term is used to describe the richness or thinness of a fluid?

• A. Viscosity
• B. Density
• C. Specific gravity
• D. Pressure

Answer: (A)

The amount a fluid resists flowing, its thickness or thinness, is described by viscosity. Viscosity is the internal friction between layers of a fluid as they slide past one another. A high-viscosity fluid, like honey, is thick and flows slowly; a low-viscosity fluid, like water, is thin and flows easily. Temperature, composition, and additives can change viscosity, which is why fluids used in aviation systems are specified for operating conditions. Density measures how much mass is in a given volume, not how a fluid flows. Specific gravity compares density to that of water, not flow behavior. Pressure is the force exerted per area, not the fluid’s resistance to flow. So the term that describes the richness or thinness of a fluid is viscosity.