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Homework Help: Kinematics Question -- A soccer ball bounces off a wall

  1. Feb 23, 2015 #1
    1. The problem statement, all variables and given/known data
    "A person kicks a soccer ball at an angle 30° above the horizontal on the ground. The ball then hits a wall at an angle 15° below the horizontal 4.9 seconds later. Determine the height at which the ball hit the wall."

    (Assume acceleration due to gravity is 9.80m/s2 [Down].)

    2. Relevant equations
    Not entirely sure where to start, but perhaps cos A1 = vx / vi and sin A1 = viy / vi as well as cos A2 = vx / vf and sin A2 = vfy / vf.

    3. The attempt at a solution
    Use the above equations to find vx (?) by subbing them into one of the five kinematic equations. Then find viy and vfy using ratios. Lastly, use viy and vfy to determine the displacement at the max height and the displacement downwards from the max height to the wall. Find the difference of the two values to determine the answer to the question.

    I feel like I might not be seeing something here. Anyone care to provide some sort of solution?

  2. jcsd
  3. Feb 23, 2015 #2

    Suraj M

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    I hope you used the concept of constant horizontal velocity which will lead you to the ratio ##\frac{v_f}{v_i}##
    Then from there by just substituting the answer into the equation for the y coordinate of the velocity, you can find displacement, use third equation.
    Post your calculations(working), it'll be easier to say, if your on the right track or not!
  4. Feb 24, 2015 #3


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    Be prepared to create additional variables so that you can write out the standard equations. In the present case, I would suggest setting s as the horizontal distance.
    When you post your working, please keep everything symbolic, e.g. write g, not 9.8, and t, not 4.9.
  5. Feb 24, 2015 #4


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    If you need a hint, think of trigonometry and in particular the tangent of an angle.
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