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Homework Help: Free Fall

  1. Sep 28, 2008 #1
    1. The problem statement, all variables and given/known data
    A ball is thrown vertically upward with an initial velocity at the edge of the roof of a building which is 30m above the ground. The speed of the ball is 14.5 m/s when the ball is 12m above the ground. How long does the ball take to reach maximum height?


    2. Relevant equations
    Vf^2=Vo^2+2ad



    3. The attempt at a solution
    First, i use that equation to find distance between the highest point and 12 above ground. Then I plan to use it one more time to find the initial velocity. But the thing is that

    I find d=-10.727m as the distance between highest point and 12 above ground. This doesn't make sense, shouldn't it be higher than 18m?
     
  2. jcsd
  3. Sep 29, 2008 #2

    tiny-tim

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    Hi Thiendrah! :smile:

    Show us your calculations, and then we'll know how to help. :smile:
     
  4. Sep 29, 2008 #3
    v2=u2+2aS

    Use that equation for all the calculation 12m above the ground and then add 12m to your final result.

    So obviously v=0 (as it reaches the top) u=14.5ms-1 a= -9.8ms-1 (If up is positive), and then solve for S and add 12m.
     
  5. Sep 29, 2008 #4
    minifhncc, that's the same thing. You still get the same result.


    0^2=14.5^2+2.(-9.8).S

    S=10.727

    10.727 + 12 = 22.727...

    this doesn't make sense tho. The object is thrown vertically upward off a roof that's 30meters above the ground.

    Am i doing anything wrong?
     
  6. Sep 30, 2008 #5

    tiny-tim

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    Hi Thiendrah! :smile:

    No … you seem to be right.

    I can't see any way out of this in the question …

    the question specifically says that the initial velocity is vertical …

    your formula is correct (you could also have used KE + PE = constant, with the same result) …

    the question is wrong. :frown:
     
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