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Homework Help: Momentum and impulse?

  1. Jan 10, 2010 #1
    I'm a little confused about momentum and impulse.
    Its not a single problem but just how they relate.

    For example if a 2kg block moving at 3 m/s hits a 6 kg block,
    then it will move 1 m/s because momentum is conserved with
    6 momentos on either side of the equation. If the impact time is 1 millisecond
    whats the impulse? I dont know how you get impulse from momentos.
    Impulse is force x delta time and Momentum is mass x velocity, but in this problem
    how do i get impulse using the fact that the system's momentum is 6 momentos.
     
  2. jcsd
  3. Jan 10, 2010 #2
    Airforcetwo,
    What you have to understand is that Change in Momentum = Impulse, as described in the equation F delta T=m delta v. So in this case, it is not 6 momentos on either side, but 6 momentos combined because the initial Momentum must equal the final Momentum, which is the Momentum of both blocks added together.
    -C
     
    Last edited: Jan 10, 2010
  4. Jan 10, 2010 #3

    Matterwave

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    Gold Member

    We have Newton's second law:

    [tex]F=ma[/tex]

    This can be expressed as:
    [tex]F=m\frac{dv}{dt}=\frac{d(mv)}{dt}=\frac{dp}{dt}[/tex]

    Assuming the mass doesn't change. Now, if we integrate both sides with respect to time, we get the impulse I: (assume correct limits of integration)

    [tex]I=\int{Fdt}=\int{\frac{dp}{dt}dt}=\int{dp}=\Delta p = p_f-p_i[/tex]
    Therefore, the impulse is simply the change in momentum.
     
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