Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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
    0.5m/s2


    2. Relevant equations
    s=v0t+(1/2)at2


    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
    s=v0t+(1/2)at2
    100=(20)t+(1/2)0.5t2
    5=t+(1/2)0.5t2
    5=t+.25t2
    5=t3
    cube root of 5 is 1.71 which is def wrong:(
     
  2. jcsd
  3. May 2, 2010 #2

    rock.freak667

    User Avatar
    Homework Helper

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

    0.25t2+t-5 = 0

    at2+bt+c=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
    [tex]\Delta[/tex]x=vit+[tex]\frac{1}{2}[/tex]at2

    There's your kinematic equation.

    Edit: sorry didn't see your attempt.

    [tex]\Delta[/tex]x=vit+[tex]\frac{1}{2}[/tex]at2

    0=vit+[tex]\frac{1}{2}[/tex]at2-[tex]\Delta[/tex]x

    [tex]\frac{1}{2}[/tex]at2+vit-[tex]\Delta[/tex]x=0

    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

    rock.freak667

    User Avatar
    Homework Helper

    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

    rock.freak667

    User Avatar
    Homework Helper

    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.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook