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!

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.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook