Finding raindrop radius at terminal velocity using Stokes law

[IDK10]

Homework Statement

Using the viscosity of air as 1.8x10^-5Pa s, and the density of water as 1x10³kg m^-3, find the radius of a raindrop traveling at a terminal velocity of 7.05ms^-1. Assume Stokes law can be used.

The problem is finding the density of air, I could use F = 6rvπη but I don't know F, because it says in a previous question to assume that in D + U = W, to assume U is negligible, and I don't know the weight, because I don't know the mass or volume of the raindrop.

Homework Equations

r = sqrt((9ηv)/(2g(ρ-σ)))

The Attempt at a Solution

Without a density of air, I get 9.31x10^-5m, but that would be wrong since the density of air is not being included.

 

[BvU]

Density of air is a small correction, so you can use the ideal gas law to find its approximate value.

 

[ehild]

The Attempt at a Solution

Without a density of air, I get 9.31x10^-5m, but that would be wrong since the density of air is not being included.

Google "density of air" .(It is about 1.225 kg/m³)
And check your numerical result.

 

[Chestermiller]

In your judgment, is buoyancy going to contribute significantly to the net force on the raindrop?

 

[IDK10]

If by buoyancy you mean upthrust, then no, a previous part of the question says it is neglibible. So its just F = W.

 

[ehild]

The Attempt at a Solution

Without a density of air, I get 9.31x10^-5m, but that would be wrong since the density of air is not being included.

It is not correct, but not because of neglecting the density of air.