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Ball thrown up, find max height - Given one velocity in midflight

  1. Jan 15, 2014 #1
    1. The problem statement, all variables and given/known data

    A ball is thrown upwards from a point near the sidewalk next to a tall office building. At some later time an occupant of the building, exactly 5.1 meters above the point from which the ball was released, sees it pass her window with a speed of 2.9 m/s.

    Calculate the maximum height reached by the ball.

    You may neglect air resistance. Take the acceleration due to gravity to be 9.82 m/s^2


    2. Relevant equations
    Kinematic Equations

    y = Vot + (1/2)at2
    Vf = Vo + at
    Vf2 = Vo2 + 2ay
    y = t * [ (Vo + Vf) / 2 ]


    3. The attempt at a solution

    Using Kinematic equation Vf^2=Vo^2+2ay I solve for Vo
    (2.9)^2=Vo^2 - 19.64*5.1
    Vo = 10.01 m/s

    To find max height I set Vf to 0 and solve for y using the same acceleration and the newly found Vo.

    0 = 10.01^2 - 19.64y
    y=5.1 m

    Obviously this doesn't work since 5.1m is where the person at the window sees the ball traveling with velocity. Any help would be great, I tried using the other kinematic equations but it got too messy too quickly.
     
  2. jcsd
  3. Jan 15, 2014 #2
    Your value for vo does not look right. I think you forgot to add the (2.9)^2 term.
     
  4. Jan 15, 2014 #3
    That must have been it. I just redid the problem and received an answer a little greater than 12 meters. Not sure where I went wrong exactly, but no worries.

    Thanks :)
     
  5. Jan 16, 2014 #4

    PeroK

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    Science Advisor
    Homework Helper
    Gold Member

    Here's how I would look at it. Start from the window.

    The ball is travelling upwards at 2.9 m/s, so how high does it go from there?

    Just add that answer to the starting point of 5.1 m.
     
  6. Jan 16, 2014 #5

    D H

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    Staff Emeritus
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    I suggest you redo the problem again. You didn't show your work, so there's no knowing where you made a mistake.

    Hint: Temporarily ignore the fact that the observer was 5.1 meters above the ground. Suppose you threw a ball upwards at 2.9 m/s. How high will it fly given g=9.82 m/s2?
     
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