Fluid Mechanics
The terminal velocity of a solid spherical particle falling through a stationary fluid mass in the Stoke's law range is proportional to the

Difference in the densities of the particle & fluid
Inverse of fluid viscosity
Square of particle size
All of these

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Fluid Mechanics
In case of hydraulically smooth pipe, the resistance to flow depends only on the Reynolds number, whereas for a hydraulically rough pipe, the resistance to flow is governed by the relative roughness. Two pipes are said to have the same hydraulic roughness, when they have equal values of

Relative roughness
Absolute roughness
Friction co-efficient for flows at equal Reynold number
All of these

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