How Do You Calculate Group Velocity from Phase Velocity?

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Homework Help Overview

The discussion revolves around calculating the group velocity of ripples on a liquid surface, given the phase velocity formula involving surface tension and density. Participants explore the relationship between phase velocity and group velocity, questioning the definitions and variables involved.

Discussion Character

  • Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss the definitions of phase and group velocity, questioning how to derive group velocity from the given phase velocity. There are inquiries about the role of surface tension and wavenumber in the calculations, as well as the implications of differentiating the phase velocity.

Discussion Status

The discussion is active, with participants providing insights and questioning each other's assumptions. Some guidance has been offered regarding the differentiation process, but there is still uncertainty about the relationships between the variables involved.

Contextual Notes

Participants are navigating the definitions of variables such as surface tension and wavenumber, with some confusion about their roles in the equations. There is a focus on the need for additional information to fully resolve the problem.

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If a question says: The phase velocity of ripples on the liquid surface is (2πS / λp)^(1/2), where S is the surface tension and p is the density of the liquid. Find the group velocity of the ripples. I know that the phase velocity = omega/k, and group velocity = delta omega/delta k. Do I just take the deriviative of the phase velocity with respect to S?
 
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1. Is S k?
2.[tex]\frac{d}{dk}(\frac{\omega}{k})=\frac{d\omega}{dk}[/tex]?
is that what you're saying?
Then think again.
 
S is the surface tension of the liquid. Is there another way to find the group velocity?
 
You have the definition; wherever have you gotten the idea that the surface tension S is the wavenumber "k"?
 
I realize that S is not k. Yet how do I do the problem?
 
Since you have the phase velocity, you may find the frequency [tex]\omega[/tex]
The group velocity is then, by your definition, the derivative of [tex]\omega[/tex] with respect to "k".
 
How can we find omega if we don't know what k is?
 
Multiply your phase velocity with k.
 
then isn't (2πS / λp)^(1/2) a constant?
 
  • #10
No, because your wavelength satisfies identically the relation:
[tex]\lambda{k}=2\pi[/tex]
since your expression for ph.vel. is proportional to the square root of the wavelength, your frequency will be proportional to the square root of the wavenumber
 
  • #11
this probably sounds really idiotic on my part. I just need to take (dw/dk) of k(2πS / λp)^(1/2), right?
 
  • #12
And that will give me the group velocity?
 
  • #13
Yes, it will
Differentiate, if you dare..:wink:
 
  • #14
product rule?
 

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