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Homework Help: Vertical Motion Question

  1. Sep 6, 2008 #1
    1. The problem statement, all variables and given/known data

    A ball is thrown vertically from the edge of a cliff. It rises 10 m above the cliff, then falls 50 m below the cliff.
    Find the inital velocity and determine the time the ball was in the air.
    2. Relevant equations

    (v final )^2 - (v initial)^2 = 2a (delta y)

    3. The attempt at a solution

    I'm not sure if I should use 10 or 60 m for the displacement in this equation.

    I used 60 m and got v inital = 34. 292 m/s.
  2. jcsd
  3. Sep 6, 2008 #2


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    Displacement is a vector with a magnitude equal to the distance between the initial point & final point. In this case your initial point is 0m and your final point is -50m.
  4. Sep 6, 2008 #3


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    Hi veronicak5678! :smile:

    I don't get it … what did you use for vfinal? :confused:

    (and why 60 rather than 40?)

    Hint: you need at least one known v … so what y must you use? :smile:
  5. Sep 6, 2008 #4
    I used 0 for v final. And I used 60 because it travels 10 above, then 50 below. So I should just use 50 for delta y?
  6. Sep 6, 2008 #5


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    Good … but if you use 0 for vfinal, you must use the y where v = 0, which is … ? :smile:
  7. Sep 6, 2008 #6
    So I do use 50 for displacement? Using that, I come up with initial velocity = 31.3 m/s, and time in the air = 3.194 s.
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