1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Radius of a raindrop

  1. May 5, 2017 #1
    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. jcsd
  3. May 5, 2017 #2


    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    Density of air is a small correction, so you can use the ideal gas law to find its approximate value.
  4. May 5, 2017 #3


    User Avatar
    Homework Helper

    Google "density of air" .(It is about 1.225 kg/m3)
    And check your numerical result.
  5. May 5, 2017 #4
    In your judgment, is buoyancy going to contribute significantly to the net force on the raindrop?
  6. May 6, 2017 #5
    If by buoyancy you mean upthrust, then no, a previous part of the question says it is neglibible. So its just F = W.
  7. May 6, 2017 #6


    User Avatar
    Homework Helper

    It is not correct, but not because of neglecting the density of air.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted