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Which of the above affect(s) the terminal velocity with which rain falls?
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*When an object moves through a fluid, there is a drag force which retards its motion. Its magnitude is given by*

*F*_{drag} ≈ *C**ρ**Av*^{2} (1)

*where C (= 0.2) is a constant, ρis the density of the fluid, A is the cross-sectional area of the object normal to the flow direction, and v is its velocity relative to the fluid.*

* *

*Equation (1) is valid only if the fluid flow develops whirls and eddies, that is, approaching the onset of turbulence. (If the fluid is essentially undisturbed, then the drag force is actually greater than the value given in equation [1].) The extent to which a fluid is disturbed is determined by a dimensionless constant called the Reynolds number, defined by*

*Re* = *ρ**vl*/*η* (2)

*where l is the linear size of the object and η is the viscosity of the fluid, a measure of its stickiness. A table of densities and viscosities is shown below.*

__substance __ __ρ____(kg/m ^{3}) __

__η____(kg/m s)__

*air 1.29 1.8 x 10 ^{–5}*

*water 1.0 x 10 ^{3} 1.0 x 10^{–3}*

*mercury 1.36 x 10 ^{4} 1.6 x 10^{–3}*

* *

*If Re is greater than about 100, then equation (1) for F_{drag} is fairly accurate. The Reynolds number also determines when turbulence begins. If Re is greater than about 2 x 10^{5}, then the fluid develops whirls and eddies that break off from the flow in an essentially unpredictable manner, i.e., turbulence.*

*(Note: g = 9.8 m/s^{2}.)*

On Venus, rain presumably consists of sulfuric acid droplets in a carbon dioxide atmosphere. Consider a water raindrop on Earth, and a drop of equal size and mass of sulfuric acid on Venus. The acceleration due to gravity on Earth's surface is approximately the same as the acceleration due to gravity on the surface of Venus. Consider the following possibilities:

I. The chemical composition of the drop.

II. The temperature of the atmosphere.

III. The pressure of the atmosphere.