Solid friction, like viscosity is

Analyze the question: The question asks what property solid friction and viscosity have in common. Viscosity is the "friction" within fluids, and solid friction is the resistance between solid surfaces. We need the statement that is true of both.

Concept:

• Solid friction: It opposes the relative motion (or tendency of motion) between two surfaces. It depends on the normal reaction (F = μR) and is independent of the area of contact.
• Viscosity: It is the internal friction between layers of a fluid. It opposes the relative motion between the layers. It depends on the velocity gradient, not on normal reaction.

Check each option:

• A. Independent of the surface area in contact: True for solid friction (laws of friction). For viscosity, the viscous force F = ηA(dv/dx) does depend on the area of the layers. So this is not true of both.
• B. Independent of the relative motion between layers: False for viscosity, because viscous force exists precisely because of relative motion between layers and increases with velocity gradient.
• C. Dependent on normal reaction: True for solid friction (F = μR), but not for viscosity.
• D. In opposition to motion: True for both. Solid friction opposes the motion of one surface over another, and viscous force opposes the relative motion of fluid layers (or of an object moving through a fluid).

Answer: Option D is the only statement that applies to both solid friction and viscosity.

Tip: When a question says "like" or "both", test each option against both things. Properties such as "depends on normal reaction" or "independent of area" belong to solid friction only, whereas "opposes motion" is the universal property of all frictional forces.