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Homework Help: 2D Kinematics Problem

  1. Sep 22, 2010 #1
    1. The problem statement, all variables and given/known data

    A moderate wind accelerates a pebble over a horizontal xy plane with a constant acceleration = (4.60 m/s2) + (7.00 m/s2). At time t = 0, the velocity is (4.4 m/s). What are magnitude and angle of its velocity when it has been displaced by 11.5 m parallel to the x axis?

    2. Relevant equations

    rf = (xi + Vxit + 1/2 axt2) i + (yi + Vyit + 1/2 ayt2) j
    ... I think

    3. The attempt at a solution
    I missed a day of class due to the flu, and our text book sucks at explaining. I think the variables should be:
    ax = 4.6
    ay = 7
    vxi = 4.4
    vyi = 0
    xi = 0
    yi = 0
    xf = 11.5
    So all I need to solve for is time, yes?
    I tried to use a 1D equation to find t, since we have the Vi and xf.
    I plugged the x values into:
    xf - xi = Vxit + 1/2 axt2.
    11.5 - 0 = 4.4t - 2.3t2
    t2 + 1.91t + (0.91) = 5 + 0.91 (divided by 2.3 and then completed the square)
    (t + 0.96)2 = 5.91
    t + 0.96 = 2.43
    t = 1.47 sec

    When I plug in 1.47 as the time though, and then take the magnitude of the rf vector, I get 13.7 m, but my online homework says the answer is 15.2.

    I've tried a few other equations, but I just can't seem to get it... and that's just the first half of the problem. I'm not even sure where to start to find the angle. :/
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 22, 2010 #2
    It seems that you changed a sign. it should be 4.4t+2.3t^2 and you wrote -2.3t^2
     
  4. Sep 22, 2010 #3
    Oh, yes. That was just a typo. when I did the calculation, I did it with 11.5 - 0 = 4.4t + 2.3t2.
     
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