# Propagation of waves

## Main Question or Discussion Point

1-In a bottle of water,transverse waves propagte on the surface of water,while lognitudinal waves propagate in depth..what explains this?
What makes different kinds of waves propagate through water not the same kind?
You may tell me that is not true because surface waves propagates on the surface of water which are neither transverse nor longitudinal waves
see this picture

http://www.physicsclassroom.com/Class/waves/u10l1a.cfm
a rock thrown into the water, a duck shaking its tail in the water or a boat moving through the water. The water wave has a crest and a trough and travels from one location to another. One crest is often followed by a second crest that is often followed by a third crest. Every crest is separated by a trough to create an alternating pattern of crests and troughs.
According to textbook the reason for this is that"there's no intermolecular forces inside water,while intermoloecular forces are relatively large on the surface of water"

What does this mean?
2-why do longitudinal waves propagate only in fluids ?I can't imagine this?does this refers to the ability of fluids to be compressed?so compressions and rarefactions are formed?

3-In fluids:the speed of sound is inversely proportional to the square root of density,while in solids ,speed of sound is directly proportional to density
could you explain this in terms of elasticity?
Thanks

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Drakkith
Staff Emeritus
1-In a bottle of water,transverse waves propagte on the surface of water,while lognitudinal waves propagate in depth..what explains this?
What makes different kinds of waves propagate through water not the same kind?
The difference lies in the definition of each type of wave and how it acts. A transverse wave oscillates perpendicular to the direction of travel, while a longitudal wave oscillates in the direction of travel. A transverse wave on the surface occurs because it is the boundary of the medium. The water is pushed up because the density of the air is much less than water and gravity pulls the water back down. I believe this is only the case in a fluid, as a solid can have transverse waves inside it as well. Solids tend to hold their shapes and will pull the material back to it's original shape, causing the propagation of the transverse wave.

According to textbook the reason for this is that"there's no intermolecular forces inside water,while intermoloecular forces are relatively large on the surface of water"

What does this mean?
My guess is that underwater the intermolecular forces are equalized on all sides. At the surface the intermolecular forces of water molecules are much greater than of air. Hence you have surface tension and other related effects. These forces keep the water "together" instead of just spraying up everywhere into the air in a fine mist. This allows the wave to travel without losing much energy. (Otherwise the ejected water molecules would carry the energy away when they left) It makes sense to me, but I've never actually read anything about this so don't just take my word for it.

2-why do longitudinal waves propagate only in fluids ?I can't imagine this?does this refers to the ability of fluids to be compressed?so compressions and rarefactions are formed?
To my knowledge they do not travel only in fluids. A P-Wave is a longitudal wave that travels through solids, liquids, and gases and are caused by earthquakes. See here:http://en.wikipedia.org/wiki/P-wave

3-In fluids:the speed of sound is inversely proportional to the square root of density,while in solids ,speed of sound is directly proportional to density
could you explain this in terms of elasticity?
Thanks
I'm not sure really. I did find this article that explains it a little bit: http://en.wikipedia.org/wiki/Speed_of_sound#Dependence_on_the_properties_of_the_medium

Note that I'm not an expert on waves, and my knowledge comes mostly from self study. But I hope I made sense and got all that mostly correct. For more info see the following.
http://en.wikipedia.org/wiki/Transverse_wave
http://en.wikipedia.org/wiki/Longitudinal_wave

2-why do longitudinal waves propagate only in fluids ?I can't imagine this?does this refers to the ability of fluids to be compressed?so compressions and rarefactions are formed?
They don't. Longitudinal waves can propagate in solids as well as in fluids.
Transverse waves cannot usually propagate in the bulk of a fluid. The surface waves have a transverse component. You can imagine the surface as an elastic membrane.
So in solids you can have longitudinal, transverse and mixed waves.

3-In fluids:the speed of sound is inversely proportional to the square root of density,while in solids ,speed of sound is directly proportional to density
could you explain this in terms of elasticity?
Thanks
This is not so in the usual cases. The speed for both transverse and longitudinal waves in solids is inversely proportional to the square root of density. Where did you get this, from a textbook? Are you talking about some composite material, maybe?

The water is pushed up because the density of the air is much less than water and gravity pulls the water back down.
why?does air pushes water up :(?

This is not so in the usual cases. The speed for both transverse and longitudinal waves in solids is inversely proportional to the square root of density. Where did you get this, from a textbook?
You are right ..and yes I got this from textbook
anyways I already fixed this misconception

thanks

Drakkith
Staff Emeritus
why?does air pushes water up :(?
The water is pushed up initially by whatever is causing the wave. For example, dropping a rock into a pond causes the waves that propagate outward. Wind does create waves on bodies of water, but doesn't cause them to continue to move. (Though the wind can "reinforce" the waves as they travel, causing them to move much further than they would have otherwise without losing strength) Water and air are both fluids, and at the boundary between them (the sufrace of the water) interactions with wind causes waves to form.

My guess is that underwater the intermolecular forces are equalized on all sides. At the surface the intermolecular forces of water molecules are much greater than of air. Hence you have surface tension and other related effects. These forces keep the water "together" instead of just spraying up everywhere into the air in a fine mist
What do you mean by tension here?and why does this tension allow waves to propagate?

instead of just spraying up everywhere into the air in a fine mist
like a longitudinal wave?

Drakkith
Staff Emeritus
What do you mean by tension here?and why does this tension allow waves to propagate?
The tension simply helps hold the water together. It is the same effect that keeps water striders from sinking into the water. It helps the wave propegate by not letting part of the energy be lost by water molecules being ejected from the wave. Honestly I would just ignore all the water tension stuff, it's not really that important to understand propagation of waves in water. I probably shouldn't have brought it up, as I'm not really sure how important it is and I'm probably confusing you.

like a longitudinal wave?
More like a landmine going off and throwing dirt into the air. If the dirt was held together then that energy would propegate along the ground instead of losing it by throwing dirt into the air. Think of it like stretching a tight rubber sheet over the mine that could withstand the blast. The energy would be captured and would ripple outwards in waves in the sheet.

Acutally,I'm totally confused,I can't have a clear picture for this

Drakkith
Staff Emeritus
Acutally,I'm totally confused,I can't have a clear picture for this
Sorry. What exactly are you confused about?

what is the importance of tension in the propagation of transverse waves?

The tension simply helps hold the water together. It is the same effect that keeps water striders from sinking into the water. It helps the wave propegate by not letting part of the energy be lost by water molecules being ejected from the wave. Honestly I would just ignore all the water tension stuff, it's not really that important to understand propagation of waves in water. I probably shouldn't have brought it up, as I'm not really sure how important it is and I'm probably confusing you.
molecules of the fluid are sliding over each other in a fluid,I guess.

another question is that how longitudinal waves propagate through a fluid?

Pythagorean
Gold Member
A wave requires that the objects waving be coupled so that the kinetic disturbance (the wave) can e transferred from one object to the next.

Tension is a description of the coupling between molecules.

do longitudinal waves propagate through gases beacause gases are compressible?

longitudinal waves can propagate through water,although water is incompressible,how?

Drakkith
Staff Emeritus
do longitudinal waves propagate through gases beacause gases are compressible?
longitudinal waves can propagate through water,although water is incompressible,how?
Water is not incompressible, it is simply MUCH less compressible that something like air.

do longitudinal waves propagate through gases beacause gases are compressible?

Drakkith
Staff Emeritus
I don't know how to answer that. If it wasn't compressible would the wave travel? I don't know. I assume that the compressibility greatly influences the wave, but I don't know how accurate it is to say that the wave travels because air is compressible.

"do longitudinal waves propagate through gases beacause gases are compressible?"
I am not sure what you have in mind with this question.
Any medium is compressible, solid, liquid, gas, etc.
The speed of propagation of the longitudinal elastic wave is related to the compressibility of the medium.
Are you trying too see what will happen if the medium were incompressible? This will be a quite unphysical situation.
Are you looking if it is possible to have longitudinal waves which are not related to compressibility?

The tension simply helps hold the water together. It is the same effect that keeps water striders from sinking into the water. It helps the wave propegate by not letting part of the energy be lost by water molecules being ejected from the wave. Honestly I would just ignore all the water tension stuff, it's not really that important to understand propagation of waves in water. I probably shouldn't have brought it up, as I'm not really sure how important it is and I'm probably confusing you.
1. Tension does not hold water within the surfaces.
2. "not letting energy...". Excuse me; are you a physics student?
3. "ignore water tension......not really that important..". I am sorry, it is really important; you don't realize.
4. "I probably....". Why did you have to even write these? You could have asked the questions instead!
5. It is surface tension, not "Water Tension".

A1. Water molecules inside the surfaces are held by intermolecular forces (Electromagnetic forces)
A2. It causes propagation of surface waves on liquids having surface tension sue to the elastic nature of the force which acts in the opposite direction of displaced particles of water in a surface wave. However, gravity force is significant and adds to the velocity of the wave.
A3. see: http://en.wikipedia.org/wiki/Gravity_wave#Quantitative_description
A4. Do not write anything that may confuse others; do not write just for the sake of writing!
A5. self explanatory.

chill!!!

A1. Water molecules inside the surfaces are held by intermolecular forces (Electromagnetic forces)
1. Tension does not hold water within the surfaces.
what is wrong with that :(

A1. Water molecules inside the surfaces are held by intermolecular forces (Electromagnetic forces)
intermolecular forces do exist between the molecules whether at the surface or at the bottom,but at the surface of water ,there's no more water molecules above that surface ,but air molecules...this causes surface tension. Is that true?
3. "ignore water tension......not really that important..". I am sorry, it is really important; you don't realize.
I couldn't ignore tension too

Drakkith
Staff Emeritus
1. Tension does not hold water within the surfaces.
I assume you mean that the hydrogen bonds between water molecules holds them together, which is also where surface tension comes from.

2. "not letting energy...". Excuse me; are you a physics student?
I am not. Why do you ask?

3. "ignore water tension......not really that important..". I am sorry, it is really important; you don't realize.
I'm sure it is. I couldn't explain it though.

4. "I probably....". Why did you have to even write these? You could have asked the questions instead!
And you could have simply corrected any mistakes without the attitude.

A4. Do not write anything that may confuse others; do not write just for the sake of writing!
I write for the sake of learning for myself and others. I'm sorry that this particular thread is one of my more confusing ones. If you don't like it then don't come to a forum where anyone can contribute, even those of us without formal education.