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Question about freely falling objects

  1. Mar 5, 2013 #1
    1. The problem statement, all variables and given/known data
    Q: A stone is thrown vertically upwards with initial speed of 5 m/s from edge of a cliff 30 m high. Find the time needed for the stone to reach the ground (in seconds).


    2. Relevant equations
    1. (h= Vo t - 0.5 g t^2)
    2. Quadratic equation: 3ea647783b5121989cd87ca3bb558916.png


    3. The attempt at a solution
    First I will use this equation: (h= Vo t - 0.5 g t^2)
    -30 = (5) t - 5 t^2
    Now I divide by 5 and it will be like this
    t^2 - t - 6
    Then, I use the quadratic equation
    [itex]\frac{-(-1)+\sqrt{(-1)^2-4(1)(-6)}}{2(1)}[/itex]

    The answer is 3, is that right?

    Another question, I am not sure..
    maybe I need to calculate the upward vertical distance it will go first.. then I sum it with the actual height which is 30 m?
     
  2. jcsd
  3. Mar 5, 2013 #2
    Looks right to me.
     
  4. Mar 5, 2013 #3
    simple calculation..
    considering down as positive...
    h=ut+1/2at.t
    h=30 m
    a= 10 m/s.s
    u=-5m/s

    30=-5t+5t.t
    t=3 s
     
  5. Mar 5, 2013 #4
    simple calculation..
    considering down as positive...
    h=ut+1/2at.t
    h=30 m
    a= 10 m/s.s
    u=-5m/s

    30=-5t+5t.t
    t=3 s
     
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