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Homework Help: Freefall physics homework

  1. Sep 13, 2004 #1
    I'm having some problems with this question, A ball is thrown directly downward, with an initial speed of 8.25 m/s, from a height of 29.4 m. After what time interval does the ball strike the ground?

    I tried 29.4 + 8.25 m/s(x) + 1/2(-9.8m/s)(x), but that didn't turn out to be right. Can I get some help with this one?
  2. jcsd
  3. Sep 13, 2004 #2


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    Homework Helper

    Well in a xy coordinate system, initial position will be its height, and when it hits the ground it will have a position of 0, so it's final position must be 0
  4. Sep 13, 2004 #3

    Doc Al

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    Staff: Mentor

    wrong sign... and more

    I assume you are trying to apply the following kinematic equation:
    y = y_0 + v_0 t + (1/2)a t^2
    Be sure to use a consistent sign convention: not only is the acceleration negative (a = - 9.8 m/s^2), don't forget that the initial velocity is also negative since it is thrown downward.
    And, as Cyclovenom points out, the final postion is where y = 0. Solve for t.
    Last edited: Sep 13, 2004
  5. Sep 13, 2004 #4


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    Staff Emeritus
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    Gold Member

    Don't forget that the x (I would prefer t!) in the bold quantity needs to be squared.
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