# Is Wave Velocity Always Double the Group Velocity in Deep Water?

• pt176900
In summary, the problem involves showing that in deep water, the wave velocity is twice the group velocity. The equations for wave velocity (Vw) and group velocity (Vg) are provided, where w is the angular frequency and k is the wave number. To solve this problem, a functional relationship between w and k is needed, which can be found in Griffiths. A helpful resource for understanding the concept of group velocity is also provided.
pt176900
Problem 9.22 from Griffiths:

Show that in deep water (where the depth is greater than the wavelength) the wave velocity is twice that of the group velocity. where Vw = w/k and Vg = dw/dk

where w is the angular frequency, and k is the wave number.

I'm really not certain how to proceed. Can someone give me a hint?

pt176900 said:
Problem 9.22 from Griffiths:

Show that in deep water (where the depth is greater than the wavelength) the wave velocity is twice that of the group velocity. where Vw = w/k and Vg = dw/dk

where w is the angular frequency, and k is the wave number.

I'm really not certain how to proceed. Can someone give me a hint?

This is pretty cool. Check this out for starters

http://galileo.phys.virginia.edu/classes/109N/more_stuff/Applets/sines/GroupVelocity.html

I think you need a functional relationship between $\omega$ and k to do this. That relationship for deep water is known, so I assume you can find it in Griffiths.

Last edited by a moderator:

Sure, I can provide you with some guidance on how to approach this problem. First, let's define the terms wave velocity and group velocity in more detail.

Wave velocity (Vw) is the speed at which the wave travels through the medium, and it is given by the ratio of the angular frequency (w) to the wave number (k), as stated in the problem. This can also be written as Vw = w/k.

Group velocity (Vg) is the speed at which the energy of the wave travels, and it is given by the derivative of the angular frequency with respect to the wave number, or Vg = dw/dk.

Now, let's consider the case of deep water where the depth is greater than the wavelength of the wave. In this scenario, the dispersion relation for deep water waves is given by w^2 = gk, where g is the acceleration due to gravity.

To show that the wave velocity is twice the group velocity, we need to compare the expressions for Vw and Vg. We can start by taking the derivative of the dispersion relation with respect to k:

2w(dw/dk) = g

Then, we can substitute in the expression for Vg:

2wVg = g

Next, we can rearrange the equation to solve for Vg:

Vg = g/2w

Finally, we can substitute in the expression for Vw, which we know is equal to w/k, to get:

Vg = g/2(w/k)

Now, we can simplify this expression by multiplying both the numerator and denominator by 2:

Vg = (2g)/(2w/k)

And since 2w/k is equal to Vw, we can replace it in the equation to get:

Vg = (2g)/Vw

This shows that the group velocity (Vg) is indeed half the wave velocity (Vw), as stated in the problem.

I hope this helps to guide you in solving this problem. Remember to always start by defining the terms and equations involved, and then use algebraic manipulation to simplify and solve the problem. Good luck!

## What is the difference between wave velocity and group velocity?

Wave velocity refers to the speed at which a single wave propagates through a medium, while group velocity refers to the speed at which a group of waves travels through a medium. This means that while wave velocity is specific to one wave, group velocity takes into account the collective behavior of multiple waves.

## Can wave velocity be greater than the speed of light?

No, according to the laws of physics, the speed of light is the ultimate speed limit for anything in the universe. Therefore, wave velocity cannot exceed the speed of light.

## How does the medium affect wave velocity and group velocity?

The velocity of a wave is affected by the properties of the medium it travels through, such as density, elasticity, and temperature. In contrast, the medium does not have a direct effect on group velocity. Instead, the interaction between waves and the medium can cause changes in the group velocity.

## What is the relationship between wave frequency and group velocity?

The relationship between wave frequency and group velocity is inverse. This means that as the frequency of a wave increases, the group velocity decreases and vice versa. This is known as the frequency dispersion effect.

## Why is group velocity important in the study of wave mechanics?

Group velocity is important because it helps us understand the behavior of multiple waves in a medium and how they interact with each other. It is also crucial in the study of wave propagation and the transmission of information through different mediums, such as in telecommunications and signal processing.

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