# Homework Help: Radius of a raindrop

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1. May 5, 2017

### IDK10

1. The problem statement, all variables and given/known data
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 travelling 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.

2. Relevant equations
r = sqrt((9ηv)/(2g(ρ-σ)))

3. 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.

2. May 5, 2017

### BvU

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

3. May 5, 2017

4. May 5, 2017

### Staff: Mentor

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

5. May 6, 2017

### IDK10

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

6. May 6, 2017

### ehild

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