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Homework Help: Kinematics problem

  1. Jun 4, 2004 #1
    A ball is throw horizontally from the top of a building 35 m high. The ball strikes the ground at a point 80 m from the base of the building. Find the ball's initial velocity.

    What I have done so far is to find the time it takes for the ball to reach the ground, using the x=V0t + 1/2 at^2 forumula, and I found the time to be 2.67 s.

    Then I used that and plugged it into the Vf^2 = V0^2 + 2ax formula, and solved for a, finding it to be [-V0^2]/160

    and then I plugged that into the x = V0t + (1/2) a t ^2 formula, and solved the quadratic equation, and found the initial velocity to be 60 m/s, but in my book it says that the answer is 30 m/s I just can't gigure out what I did wrong.
  2. jcsd
  3. Jun 4, 2004 #2


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    Here, you have mixed up your concepts a bit.

    The ball experiences FREE FALL!!
    Therefore, you know at the start its acceleration; in vectorial form it is [tex]\vec{a}=-g\vec{j}[/tex]
    There is no acceleration in the horizontal direction; try and use the free fall condition to solve the problem properly.
  4. Jun 4, 2004 #3
    Try starting in the y-direction. As arildno said, you know the distance the ball travels, the velocity it starts with (in that direction) and the acceleration is g. See what you can find out about that and than start looking at the x-direction motion.
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