# A question on group velocity

1. Jan 1, 2014

### ShayanJ

You people know that group velocity of a wave packet is calculated with the formula $v_g=\frac{d \omega}{d k}$.But this gives an expression which,in general,is a function of k.My problem is,I can't think of an interpretation for it.What is that wave-number appearing in the expression for group velocity?
Thanks

2. Jan 1, 2014

### Redbelly98

Staff Emeritus
I'm not sure what is confusing about this. As long as ω and k are not related linearly, the slope dω/dk will change with ω or k.

3. Jan 1, 2014

### ShayanJ

I know that different points of the wave packet move with different velocities,but I wanna know how can I find the wave number associated to each point of the wave packet.

4. Jan 2, 2014

### Redbelly98

Staff Emeritus
The wave number is associated with the entire packet, not with different parts of the packet.

A couple of ways to get the wave number are:

1. Use the wavelength, or perhaps the average wavelength, for the oscillations within the wave packet.

2. Look at the frequency spectrum of the wave packet, and use the peak or average frequency in that spectrum to determine the wave number.

5. Jan 3, 2014

### ShayanJ

I can't accept what you say!
In the presence of dispersion,any wave packet,in general,will lose shape which means different parts of it move with different velocities.
Also computing group velocity with a wave number which can be chosen between some alternatives seems a little arbitrary!