# Longitudinal and transverse wave propagation

1. Aug 24, 2011

### Misr

Why do transverse waves propagate through solids , while logitudinal waves can only propagate through fluids??

2. Aug 24, 2011

### grzz

Do not longitudinal waves propagate also through solids? Sound is a longitudinal wave and it does propagate through a solid.

Last edited: Aug 24, 2011
3. Aug 24, 2011

### Misr

Yes you are right.The speed of sound waves is even greater in solids..but I don't know why

4. Aug 24, 2011

### Hootenanny

Staff Emeritus
Roughly speaking, the speed of sound is proportional to the square root of the bulk modulus of the medium through which the wave is travelling. The bulk modulus quantifies how much the medium will compress under a given applied pressure. Obviously, in general it is harder to compress a solid than a fluid and hence solids have a higher bulk modulus and thus sound travels faster through them.

5. Aug 24, 2011

### grzz

Imagine some people scattered in a square and there is a rumour being spread around. The closer they are together the faster the rumour spreads.
The closer the atoms are in a solid the quicker they interact with each other and the faster the longitudinal wave travels.

6. Aug 24, 2011

### Misr

This is such a great answer but I need an answer to the original questions of the thread too

7. Aug 24, 2011

### Hootenanny

Staff Emeritus
I thought that we've established the original question was flawed. Longitudinal waves can travel through solids.

Inviscid fluids do not support shear stress and hence transverse waves cannot propagate through inviscid fluids. However, in general, both transverse and longitudinal waves can propagate through both fluids and solids.

8. Aug 24, 2011

### Edi

So a superfluid, like super cooled helium, would not allow transverse waves to propagate?
In that case - what would happen if I dropped something in the superfluid .. a process with would, in normal fluids, generate transverse waves on the surface?

9. Aug 24, 2011

### Hootenanny

Staff Emeritus
Actually, supercooled helium-3 does support shear waves: http://www.nature.com/nature/journal/v400/n6743/abs/400431a0.html

In this case the effective shear forces are supplied by quantum effects.

10. Aug 24, 2011

### Edi

In that case - when and what does NOT support transverse waves?

11. Aug 24, 2011

### Hootenanny

Staff Emeritus
Any inviscid fluid that is of sufficiently high temperature that quantum effects can be neglected. Sadly, these do not exist. However, most fluids are good approximations to inviscid fluids.

Last edited: Aug 24, 2011
12. Aug 24, 2011

### Misr

I don't understand.We are going to study fluids the next chapter..I think I have to wait a bit because I don't understand this :(

13. Aug 25, 2011

### olivermsun

When you try to bend or "parallelogram" a metal box (within reason), it wants to spring back. Try to deform a box of fluid, it's supposed to flow.

14. Aug 25, 2011

### Misr

This is elasticity I suppose

15. Aug 25, 2011

### sophiecentaur

With a fluid, there is no 'restoring force' for lateral displacements - so you can't get a wave to propagate.

16. Aug 25, 2011

### davenn

yup exactly, as I was taught in geophysics 101 so long ago :)

hence why when you are on a boat at sea etc during an earthquake, you only feel the arrival of the P wave never the secondary ( shear/traverse) wave
I wanted to recite the practical demonstration of this that he did using a jug of beer
but his wording is all a little dim in the past now ;) Thanks Dr P.O.K.

Dave

17. Aug 26, 2011

### Misr

but longitudinal waves can propagate through fluids so what are you trying to explain :(

18. Aug 26, 2011

### davenn

because of the higher density

D

19. Aug 26, 2011

### davenn

he, like I was telling you why traverse (shear) waves are unable to pass through liquids, which is what you originally asked.
so to repeat the answer .... liquids cannot support a shear stress (wave).

to give some maths.....

V = Velocity
Shear Modulus = the ability of a material to withstand shear deformation

Dave

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Last edited: Aug 26, 2011
20. Aug 26, 2011

### Hootenanny

Staff Emeritus
Let me try a different analogy.

If you think of a horizontal spring and think about keeping one end still but pulling the other end a little bit in the horizontal direction, you will feel a force pulling your hand back toward the spring. On the other hand, if you move one end of the spring a little bit vertically, you will feel no force (there will actually be a very small force, but it is negligible). This is what we mean when we say there is "no shear force". Moving one end of the spring horizontally represents pressure waves, whereas moving one end of the spring vertically represents shear (transverse waves).

You can think of a fluid as being a lattice of atoms connected by springs.

On the other hand, if you think of a horizontal metal bar and keep one end still, the metal bar will resist any movement, both in the horizontal direction (stretching the bar, i.e. pressure waves) and in the vertical direction (bending the bar, i.e. shear/transverse waves). This is what we mean when we say that the bar "supports shear waves".

You can think of a solid as being a lattice of atoms connected by metal bars.

Both of these are obviously simple, non-physical analogies, but hopefully they will make the distinction between waves in solids and fluids a little clearer.