1. Defining Viscosity: Viscosity is the physical property that quantifies a fluid's internal resistance to flow or shear. It can be thought of as "fluid friction." A fluid with high viscosity, such as honey or motor oil, flows slowly and resists deformation. A fluid with low viscosity, like water or air, flows easily.
2. Mechanisms at the Molecular Level: In liquids, viscosity is primarily caused by cohesive forces between molecules. In gases, it is caused by the collision and exchange of momentum between molecules as they move between layers of different velocities.
3. Distinguishing Terms:
• Humidity: Refers to the moisture content in the air.
• Velocity: Refers to the speed and direction of the fluid movement.
• Flow: A general term for the movement of the fluid mass.
Therefore, viscosity is the specific property that acts as the "resistance" to the flow.