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Integrating velocity equation to find position?

  1. Oct 5, 2011 #1
    1. The problem statement, all variables and given/known data

    For t < 0, an object of mass m experiences no force and moves in the positive x direction with a constant speed vi. Beginning at t = 0, when the object passes position x = 0, it experiences a net resistive force proportional to the square of its speed: Fnet = −mkv2, where k is a constant. The speed of the object after t = 0 is given by
    v = vi/(1 + kvit).

    (a) Find the position x of the object as a function of time. (Use the following as necessary: k, m, t, and vi.)

    (b) Find the object's velocity as a function of position. (Use the following as necessary: k, m, t, vi, and x.)

    2. Relevant equations

    a = Δv/Δt
    v = Δx/Δt

    3. The attempt at a solution

    I am suspecting that I need to integrate the given function to find the position function? I know that v = Δx/Δt, so we should have:

    dx/dt = vi/(1 + kvit)
    then we need to take the definite integral from x0 to xf or just 0 to xf. My calculus is a bit rough, but isn't this some sort of a separable differential equation? So we need to make all the x's and t's all on one side? Right now that seems, well, impossible since we have no x's? So do I need to find something involving v and x, then relate/substitute so I only have x's and t's? Help please...
  2. jcsd
  3. Oct 5, 2011 #2


    User Avatar

    Staff: Mentor

    Yes, integrate dx/dt = vi/(1 + kvit)

    k and vi are constants, so this is of the basic form 1/(1 + a.t)
    and you just integrate with respect to t.
  4. Oct 5, 2011 #3
    Ok, thanks!
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