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Homework Help: Air resistance - find Vx(t) and Vy(t)

  1. Sep 29, 2011 #1
    Suppose at time t= 0 the football is kicked with initial velocity components Vxo and Vyo. Solve the equations above to find Vx(t) and Vy(t). You can make use of the formulas we worked out in class.

    "Equations above":
    ax = -kvx
    ay = -g-kvy

    formula worked in class:

    v(t) = (g/k)(1-e^-k(t-to)) + Voe^-k(t-to)


    Vx(t) = Vocosθe^-kt

    Vy(t) = (9.8m/s^2/k)(1-e^-kt) + Vysinθe^-kt

    Is this correct?
  2. jcsd
  3. Sep 29, 2011 #2
    Next problem is to integrate that to get a formula for x(t) and y(t)

    Assuming the above is correct, here's what I got:

    x(t) = -V0sin[itex]\theta[/itex]e-k(t-to)/k +V0sin[itex]\theta[/itex]/k

    y(t) = -ge-k(t-t0)/k2 + V0cos[itex]\theta[/itex]e-k(t-t0) + g/k2 - V0cos[itex]\theta[/itex]/k
  4. Sep 29, 2011 #3
    Please help [URL]http://img.photobucket.com/albums/v253/Hatsuharu1399/gdkifmqq.gif[/URL]
    Last edited by a moderator: Apr 26, 2017
  5. Sep 29, 2011 #4
    Come on guys

    Let me rephrase my question

    How to I include air resistance (k) into a projectile motion problem?

    In this problem, k = 0.1/s

    so do I stick that into the acceleration equation to get a new acceleration?

    ax = -kVx
    ax = -0.1/s * Vx

    ay = g - kvy
    ay = -9.8m/s^1 - 0.1 * Vy

    This part seems okay, but what do I use for Vx and Vy? Is it the initial velocities???

    In this problem I was given Vx0 = 12 m/s and ax0 = 10 m/s

    So if I use that, I get these constant velocities

    ax = -1.2 m/s^2
    ay = -10.8 m/s^2
  6. Sep 29, 2011 #5


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    Last edited by a moderator: May 5, 2017
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