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Princip. of linear impulse and momentum , diff. btw. impulsive force and non-impulsiv

  1. Jan 29, 2012 #1
    when applying the principle of linear impulse and momentum , how do I know if the force should be considered impulsive or non-impulsive , how should I know if I should consider it in the equation , I already know that an impulsive force is a force that is applied for a very short time ,but in some problems forces such as the normal force were considered impulsive ,for example , there is one containing a crate where the only forces applied are the weight ,normal force ,and friction and still momentum was not conserved , for example , the princip. of impulse and momentum is m(v1) + ∑ ∫ (F)dt =m(v2)
    when do I consider the integral to be 0 and momentum conserved
    Thank You
     
  2. jcsd
  3. Jan 29, 2012 #2

    tiny-tim

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    hi tj00343! :smile:
    i don't understand :redface:

    friction isn't for a very-short time …

    obviously friction over a finite distance (and time) will change the momentum :confused:

    can you give a more specific example?​
     
  4. Jan 29, 2012 #3
    Re: princip. of linear impulse and momentum , diff. btw. impulsive force and non-impu

    And Momentum is conserved only when there are no external forces acting on the system.
    ∫Fdt gives the change in momentum for both impulsive and non-impulsive forces,so what is the problem?
     
  5. Jan 29, 2012 #4
    Re: princip. of linear impulse and momentum , diff. btw. impulsive force and non-impu

    I thought that momentum is conserved when there are no external forces on the system or the forces acting are non-impulsive forces ....I'm confused because in problems my professor solved , in some problems there was external forces acting on the system ,but they were not considered ,if for example 2 balls collide , their weights and normal forces are external to the system yet we apply conservation of momentum to find their velocities ........
     
  6. Jan 29, 2012 #5

    tiny-tim

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    ah, but momentum is a vector,

    so conservation of momentum is a vector equation

    (and so is Newton's second law)

    so it works in each direction separately …

    in your professor's examples, the weights and normal forces are vertical,

    so there is no horizontal external force or impulse,

    so horizontal momentum is conserved :smile:
     
  7. Jan 30, 2012 #6
    Re: princip. of linear impulse and momentum , diff. btw. impulsive force and non-impu

    ahhhhh thank youuu tiny tim................and pabloenigma
     
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