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Homework Help: 2d motion help

  1. Oct 25, 2012 #1
    An object is launched from the origin with a velocity of 20 m/s at an angle of 30 degrees above the x axis. The object lands on a roof that is 4 meters high. What is the time when the object hits the roof?

    a) 1.11 s
    b) 1.27 s
    c) 1.33 s
    d) 1.49 s
    e) 1.67 s

    I did 20 (sin(30)) = 10 m/s. Then used the equation

    vf = vi + at

    0 = 10 - 9.8t

    t = 1.02 s

    What am I doing wrong.
  2. jcsd
  3. Oct 25, 2012 #2


    User Avatar
    Homework Helper
    Gold Member

    vf is not 0, for starters. Consider using the fact that yi=0 and yf=4.
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