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Homework Help: Projectile motion football kick problem

  1. Jan 20, 2010 #1
    1. The problem statement, all variables and given/known data
    A football kicker can give the ball an initial speed of 25 m/s. what are the least and greatest angles of elevation at which he can kick the ball to score a field goal from a point 50m in front of the goalposts whose horizontal bar is 3.44m above the ground?

    2. Relevant equations
    I really don't know what to do

    3. The attempt at a solution
    I figured I could write

    Then I tried to find t in terms of X and got

    Then I plugged that in to the other equation and I got

    I thought then I could find the zeros and that would be my answer but it doesn't seem to be working... I can't figure out why. Maybe I totally missed the answer but either way I still need help lol. Thanks.
  2. jcsd
  3. Jan 20, 2010 #2


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    Homework Helper

    you can write the equation of the projectile as
    y = x*tanθ - (g/2*u^2)*x^2*sec^2(θ)
    Put sec^2(θ) = 1 + tan^2(θ) and solve the quadratic for tanθ.
  4. Jan 21, 2010 #3
    I am confused, what are you referring to with u?
  5. Jan 21, 2010 #4


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    Homework Helper

    Sorry. u is Vo.
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