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Homework Help: Momentum and impulse question

  1. Jan 17, 2006 #1
    A 230N force causes a 4.0kg to accelerate from 5.0m/s North to 3.0m/s east. for how much time does hte force act?

    t = m(vf-vi) / F

    i got this but the trouble is vf and vi are in different directions...and since t is not a vector i can't break it down to components. what should i do?
  2. jcsd
  3. Jan 17, 2006 #2
    Maybe you can do it by this way.
    let [tex] \vec{v_i} =(0,5)[/tex] [tex] \vec{v_f}=(3,0)[/tex]
    so [tex]\Delta \vec{v} =(3,-5) [/tex]
  4. Jan 18, 2006 #3


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    You can integrate

    [tex]\frac{d\vec{v}}{dt} = \frac{\vec{F}}{m}[/tex]

    Your solution will be two equations (one for each component of velocity). You know the initial and final velocity vectors. Your unknowns are the two components of [itex]\vec F[/itex] and t but you also know

    [tex]F^2 = F_x^2 + F_y^2[/tex]

    where F = 230 N. You should be able to handle the details.
    Last edited: Jan 18, 2006
  5. Jan 18, 2006 #4


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    [tex]F_x\Delta t\ =\ m\Delta v_x\ =\ a[/tex]
    [tex]F_y\Delta t\ =\ m\Delta v_y\ =\ b[/tex]
    and then
    [tex]a^2\ +\ b^2\ =\ \Delta t^2 \mid F\mid^2[/tex]
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