Longitudinal and transverse wave propagation

[Misr]

Why do transverse waves propagate through solids , while logitudinal waves can only propagate through fluids??
I'm still confused about this

 

[grzz]

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

 

[Misr]

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

 

[Hootenanny]

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.

 

[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.

 

[Misr]

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.

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

 

[Hootenanny]

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

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.

 

[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?

 

[Hootenanny]

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?

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.

 

[Edi]

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

 

[Hootenanny]

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

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.

 

[Misr]

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

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 :(

 

[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.

 

[Misr]

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.

This is elasticity I suppose

 

[sophiecentaur]

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

 

[davenn]

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

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

 

[Misr]

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

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

 

[davenn]

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

because of the higher density

D

 

[davenn]

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

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

 

[Hootenanny]

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.

 

[sophiecentaur]

A fluid gas (ideal) has no "springs involved. There will be no lateral force at all. The net force on a small volume of gas will be due to an imbalance in the number of molecules arriving from one side an the other. I.e. the pressure. Gas pressure doesn't work sideways.

 

[Misr]

you will feel a force pulling your hand back toward the spring

what does this indicate?Indicates that it can resist the longitudinal waves,while can't resist transverse waves,so only longitudinal waves can propagate through??

A fluid gas (ideal) has no "springs involved. There will be no lateral force at all. The net force on a small volume of gas will be due to an imbalance in the number of molecules arriving from one side an the other. I.e. the pressure. Gas pressure doesn't work sideways

can't imagine this :(

 

[olivermsun]

... it can resist the longitudinal waves,while can't resist transverse waves,so only longitudinal waves can propagate through??

Think of it this way. If you take one of the simplest examples of propagating waves, say a wave traveling down a tensioned string, the oscillation only happens because the string "fights" against the up and down motion. The string is under tension, so it wants to return to center. If the string were under basically no tension, you could lift up a section and it would just flop down again limply -- there would be no propagating wave.

Same thing goes for solids and fluids. If you push a section of rock to the side it wants to spring back (unless it breaks!), hence it can support this side-to-side (transverse) wave. If you try to push some water (or gas) it flows (mostly) limply aside.

Obviously, gravity waves (waves which depend on gravity and continuity), which are neither pure transverse nor pure longitudinal waves, exist. But these are not due to any material "strength" or springiness.

 

[Studiot]

can't imagine this :(

When you are trying to understand wave motion it is important to realize that
there are not one but two motions involved.

Motion of the wave itself. This is always in the forwards direction ie the direction of propagation.

Motion of the particles in the medium through which the wave is travelling.
Particle motion may be up and down at right angles to the wave motion. This is called a transverse wave.
Or the particle motion may be back and for parallellto the wave motion.
This is callled a longitudinal wave.

Notice in both cases I gave the motion a 'to an fro' . This is because the individual particles do not leave their mean position.

Incidentally sea waves have a bit of both characteristics in them. The particles actually describe circles or ellipses around a mean point.

go well

 

[sophiecentaur]

what does this indicate?Indicates that it can resist the longitudinal waves,while can't resist transverse waves,so only longitudinal waves can propagate through??

What do you mean by "can resist"? It is the resilience of the springs that causes energy to propagate as a wave 'resistance' doesn't come into it.. The springs transfer energy from one region to the next. In a gas there are no springs - only pressure due to molecules moving and hitting each other.