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Homework Help: Projectile motion- Deriving the right formular

  1. Jan 24, 2010 #1
    1. The problem statement, all variables and given/known data

    Projectile A is launched horizontally at a
    speed of 20. meters per second from the top of
    a cliff and strikes a level surface below, 3.0 seconds
    later. Projectile B is launched horizontally
    from the same location at a speed of 30. meters
    per second.
    Approximately how high is the cliff?
    (1) 29 m (3) 60. m
    (2) 44 m (4) 104 m
    2. Relevant equations

    3. The attempt at a solution
    On two occasions my answer turn out to be '3' and '4'. but the answer key says it's '2'. I have tried for hours but i cant find the right approach. Please offer me a good explanation of how I should work with this? or u can even give me a link to read more or something.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jan 24, 2010 #2


    User Avatar
    Homework Helper

    Use the equation
    h = 1/2*g*t^2
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