Which of the following is not considered a frictional force between solids?

Viscosity is classified as a measure of a fluid's resistance to flow, rather than a frictional force between solid objects. In contrast, static friction, kinetic friction, and rolling friction are all types of friction that occur specifically between solid surfaces.

Static friction prevents surfaces from sliding past one another when at rest, kinetic friction acts on surfaces in motion to oppose their movement, and rolling friction arises when an object rolls over a surface. Each of these forces is fundamental in understanding how solids interact with each other while overcoming the tendency to move relative to one another.

Since viscosity pertains to fluid dynamics and describes the internal friction within a fluid, it is not applicable in the context of frictional forces between solids. This distinction is crucial for grasping the concepts in physics relating to motion and forces.