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Finding the initial velocity using quadratic

  1. Dec 10, 2011 #1
    1. The problem statement, all variables and given/known data

    I need to find Vi, knowing the following:

    Δt=6/Vi cos40

    Viy=Vi sin40


    ay=-g = -9.8m/s^2

    2. Relevant equations

    Quadratic: x= [-b +/- √(b^2-4ac)]/2a

    Kinematic equation to be used: Δy=ViyΔt + 1/2ayΔt^2

    3. The attempt at a solution

    Δy=ViyΔt + 1/2ayΔt^2

    4=(Vi sin40)(6/Vi cos40) -4.9(6/Vi cos40)^2

    This is where I'm stuck, I don't know how to continue the problem and solve for Vi.

    EDIT: I solved for Vi and got and answer of 9.9m/s, but according to my teacher, the correct answer should be 17.05m/s, so I'm definitely making a mistake somewhere in the algebra.

    Last edited: Dec 10, 2011
  2. jcsd
  3. Dec 11, 2011 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    IF you show your working it will be easy to see where you're going wrong.
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