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Homework Help: Raindrop Problem

  1. Nov 19, 2008 #1
    1. The problem statement, all variables and given/known data
    The terminal velocity of a 0.000038 kg raindrop is about 14 m/s.

    (a) Assuming a drag force FD = -bv, determine the value of the constant b.
    correct check mark kg/s

    (b) Determine the time required for such a drop, starting from rest, to reach 63 percent of terminal velocity.

    2. The attempt at a solution

    I got part a by myself, where b= 0.0000266 kg/s

    Part b has given me some trouble. I drew a diagram and set the the equation
    F=ma=mg - (-bv) Then i tried solving for a, and using that to find t. However, I realized you can't use a=Vf-Vi/t because a is not constant. I have never done an equation like this with t and a non-constant a. I really don't want the answer because I wont learn anything, but any help or hints would be appreciated.
  2. jcsd
  3. Nov 20, 2008 #2


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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi siwik39! Welcome to PF! :wink:

    Isn't it ma=mg + (-bv)?

    Anyway, just put a = dv/dt, and solve for v. :smile:
  4. Nov 20, 2008 #3
    I got part a. I was trying to solve for t. But thanks I got it with the dv/dt.
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