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Homework Help: Quadratic Equations and kinetics

  1. May 2, 2010 #1
    1. The problem statement, all variables and given/known data
    Find the time required for a train to reach 100m if V0=20m/s and acceleration is

    2. Relevant equations

    3. The attempt at a solution
    i am not sure of how to go about getting s=(20)t+(1/2)0.5t2 into quadratic form for use in the quadratic formula
    this is my solution without the quad formula
    cube root of 5 is 1.71 which is def wrong:(
  2. jcsd
  3. May 2, 2010 #2


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    Just bring the '5' on the other side to get

    0.25t2+t-5 = 0

  4. May 2, 2010 #3
    in my book its sets up the quadratic form as t2+80t-400=0
    the answer is still wrong i get 10??

    it is supposed to be 4.5
  5. May 2, 2010 #4

    There's your kinematic equation.

    Edit: sorry didn't see your attempt.




    Quadratic form
  6. May 2, 2010 #5
    i know thats the equation but i need the quadratic formula to solve for time when only acceleration, distance and initial velocity is known
  7. May 2, 2010 #6
    Those are the only variables you need to know. I edited my above post, sorry for reposting what you knew.
  8. May 2, 2010 #7
    don't worry about it. My problem does not lie within my use of the quadratic formula. My question is just pretty much how the book got t^2+80t-400=0
    from 100=(20)t+(1/2)0.5t2
  9. May 2, 2010 #8


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    Multiply your equation throughout by 4.
  10. May 2, 2010 #9
    any specific reason why? or is it just so?
  11. May 2, 2010 #10


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    If you plan to manually factorize it, it is easier to find the factors.

    So instead of factorizing (1/4)t2+20t-100=0, it is easier to factorize out t2+80t-400.

    If you plan to use the quadratic equation formula, then it really does not matter what form you put it in.
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