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Velocity dependant drag

  1. Oct 10, 2007 #1
    1. The problem statement, all variables and given/known data
    An object dragged through an unknown fluid experiences a force opposite to that of its initial velocity (Vi) that is equal to -k(v^1/2). find the equation that models its instantaneous velocity

    Fn = Force Net
    Ff = Frictional force
    Vi = Initial Velocity
    V = instantaneous velocity

    2. Relevant equations

    3. The attempt at a solution

    v=(-kt/2m)^2 + Vi

    which can't be right because that would mean velocity increased as time moves positively...

    i'm lost. help lol
  2. jcsd
  3. Oct 10, 2007 #2


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    Staff: Mentor

    Does the object have mass m? Is another force being applied to the object?

    If the force is subject only to drag, then it will deceleration in proportion to kv1/2 according to the problem as stated.

    If the mass falls under gravity then the mass will decelerate or even accelerate to a constant velocity where the drag force = the force of gravity.

    a = dv(t)/dt = F(t)/m, where F(t) = applied force - drag force,

    and the initial condition is v(t=0) = vi/.
  4. Oct 10, 2007 #3
    there is no mass stated and the object is traveling horizontally and not subject to gravity.

    i guess I'm having trouble deriving a velocity equation more than i am having trouble understanding the situation. i'm not even sure if the way i tried it first is the right way.

    all i know for sure is that the only force acting on the point is -kv^(1/2)

    could u perhaps help me find an equation for its instantaneous velocity with respect to time?
  5. Oct 11, 2007 #4


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    Homework Helper

    you need to add a constant here:

    2(v^1/2)=-kt/m + C


    v^1/2=-kt/2m + D

    v=(-kt/2m)^2 + 2(-kt/2m)D + D^2

    So know solve for D using initial conditions.
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