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Homework Help: Wave dispersion and the bandwidth theorem

  1. Apr 14, 2015 #1
    1. The problem statement, all variables and given/known data
    Consider a propagating wave packet with initial length L0.
    Use the bandwidth theorem to show that the minimum range of angular frequencies present in the wave packet is approximately:
    \Delta \omega = \frac{v_{g}}{L_{0}}
    where vg is the group velocity.
    2. Relevant equations
    The dispersion relationship for the wave is:
    \omega ^{2} = gk

    3. The attempt at a solution
    attached as photo along with original problem sheet. For some reason I get the answer as:
    \Delta \omega = 2\pi \frac{v_{g}}{L_{0}}
    see method attached.

    Attached Files:

    Last edited: Apr 14, 2015
  2. jcsd
  3. Apr 15, 2015 #2


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    Homework Helper
    Gold Member

    Yes i agree with your result. but how exactly have you been taught the bandwidth theorem? I just know it as [itex]\Delta k\Delta x\approx 2\pi[/itex] where [itex]\Delta k , \Delta x [/itex] are defined properly.
    Last edited: Apr 15, 2015
  4. Apr 15, 2015 #3
    Well, yeah we defined it as \begin{equation} \Delta k \Delta x = 2\pi \end{equation} and then the rest can be derived from there.
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