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!

Rain Drop VTerminal

  1. Jan 29, 2009 #1
    1. The problem statement, all variables and given/known data

    The terminal velocity of a 3×10−5kg raindrop is about 5m/s . Assuming a drag force , and Drag Force = -bv

    1. Assuming a drag force determine the value of the constant .

    2. Assuming a drag force determine the time required for such a drop, starting from rest, to reach 63% of terminal velocity.

    3. The attempt at a solution

    I solved Q1 with an answer of b=6*10^-5 kg/s

    for #2. I used Vfinal = Vinit + at

    Equation 1 = V final = 0.63 of Vterm and therefore is 3.15 m/s
    V init = 0
    a = ?
    Time = ?

    for a I used the constant b and did FBD and N2

    got Fdrag - mg = ma


    -(6E-5kg/s)(3.15m/s) - (3E-5kg)(9.8m/s2) = (3E-5kg)a

    but that gives a = -16.1 m/s2 which doesn't make sense to me. And i dropped that number in equation 1 above.

  2. jcsd
  3. Jan 29, 2009 #2
    Think about the direction of the forces. The raindrop is falling down, thus accelerating downwards. The drag force is slowing the drop down and is working against the direction of motion. Isn't it gravity, on the other hand, that pulls the drop downwards and therefore causes the acceleration?
  4. Jan 29, 2009 #3


    User Avatar

    The equation of motion for your situation is [tex] ma = -mg + F_{drag} [/tex]

    This means that you cannot use [tex] V_{final} = V_{init} + at [/tex] !!! That is the solution for motion without drag. You need to integrate the equation of motion with a drag force and use that, or the appropriate relationship derived from it, to find your answer.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook