Group Velocity Derivation: Understanding the Role of Ignored Terms

AI Thread Summary
The discussion centers on the derivation of group velocity and the reasoning behind ignoring certain terms in the calculation. It highlights a common confusion regarding the significance of terms before the integral sign and their potential impact on phase shifts. The consensus is that group velocity focuses on the speed of amplitude rather than phase, which justifies the omission of specific terms. Participants emphasize that while these terms may affect phase, they do not influence the overall group velocity calculation. Ultimately, understanding this distinction is crucial for accurately interpreting wave behavior.
chingel
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I was reading the derivation on Wikipedia:

http://en.wikipedia.org/wiki/Group_velocity#Derivation

Why is the first part before the integral sign ignored when calculating the velocity? Surely it would also cause a phase shift in some time interval and make the waves move forward (or backward)?
 
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Group velocity cares about the speed of the amplitude, phases are not relevant for the group velocity.
 
Ok, but when calculating the velocity by dividing the terms in front of t by those in front of k, why can some of the terms in front of t be ignored?
 

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