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Homework Help: Help with parabola

  1. Jan 12, 2009 #1
    1. The problem statement, all variables and given/known data

    In an over hand serve, the ball starts from a point 2m above the ground and travels a horizontal distance of 20m before it strikes the floor.

    The ball travels in a parbolic path and reaches its highest point after travelling a horizontal distance of 9m as show in the link below:


    http://img407.imageshack.us/img407/7919/asdaq2.jpg [Broken]
    http://g.imageshack.us/img407/asdaq2.jpg/1/ [Broken]

    Determine the equation of the parabola in the form y-k=a(x-9)^2

    2. Relevant equations


    3. The attempt at a solution

    Well i know that the ball reaches its maximum height 9 feet from
    where it was served. If we let x=0 be where the ball was served,
    then we know the x coordinate of the vertex is 9, so the equation is
    of the form


    To finish the equation precisely, I need to determine the values of
    a and k. To do that, I need two equations relating the values of a
    and k. But the given information provides with what I need to
    write those two equations --I know the x and y values (1) where
    the ball was served and (2) where it hit the ground.

    but i'm still confused ! i could use some help:)
    could somebody solve it for mee pleaaase??
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Jan 12, 2009 #2
    Re: Parabola?

    btw i missed the points on the Y axes
    there's 0,10 and 20

    the value 10 is aligned to the point V
    does that mean that (9,k) V is equal to (9,10)
    and so with the given value of k i just solve for the a value?
    if so, i get a very awkward answer:S
  4. Jan 12, 2009 #3


    User Avatar
    Science Advisor
    Homework Helper

    Re: Parabola?

    Ok, so y-k=a(x-9)^2. You know x=0, y=2 and x=20, y=0 satisfy that equation. Doesn't that give you two equations in two unknowns k and a to solve? What are they?
  5. Jan 13, 2009 #4


    Staff: Mentor

    Re: Parabola?

    That would be 9 meters.
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