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Having trouble finding impulse

  1. Apr 4, 2012 #1
    1. The problem statement, all variables and given/known data
    Horizontanl force F is giving by F(x) = (2.00x + 1.00x^2) newtons, acting on a 12.0kg box which is initially at rest and on a frictionless horizontal surface. Force is applied from x=0 to x=2.50 meters. What's the total impulse the force applies to the box?


    2. Relevant equations
    v^2 = sqr(2ax)
    Impulse = mv1 - mv0
    F = ma

    3. The attempt at a solution
    I already got the work, which was 11.5 J

    2.00(2.50) + 1.00(2.50)^2 N = 11.25 N
    Now I must find velocity
    F = ma
    a = f/m = 11.25 N/ 12kg = .9375 m/s^2

    v = sqr(2 * .9375 m/s^2 * 2.50m) = 2.16 m/s
    12kg * 2.16 m/s = 25.9 momentum

    Since it started at rest, the momentum should be 25.9 . However the answer is 16.6 N * s which I can't understand how to get

    Another attempt using joules would be , since Joules is kg * m^2/s^2

    11.5 J/ 2.16 m per s = 5.32 momentum which also isn't right. Any help would be appreciated
     
  2. jcsd
  3. Apr 4, 2012 #2

    BruceW

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    Homework Helper

    This equation is not right because it is not constant acceleration. (it will only work when the acceleration is constant).
     
  4. Apr 4, 2012 #3

    BruceW

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    Homework Helper

    Your value for the work is correct. You can use this to find the answer. What is the equation for the kinetic energy?
     
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