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Propagation of waves

  1. Sep 28, 2011 #1
    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
    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?
  2. jcsd
  3. Sep 28, 2011 #2


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    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.

    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.

    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

    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.
  4. Sep 28, 2011 #3
    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.

    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?
  5. Oct 5, 2011 #4
    why?does air pushes water up :(?
  6. Oct 5, 2011 #5
    You are right ..and yes I got this from textbook
    anyways I already fixed this misconception

  7. Oct 5, 2011 #6


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    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.
  8. Oct 5, 2011 #7
    What do you mean by tension here?and why does this tension allow waves to propagate?

    like a longitudinal wave?
  9. Oct 5, 2011 #8


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    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.

    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.
  10. Oct 11, 2011 #9
    Acutally,I'm totally confused,I can't have a clear picture for this
  11. Oct 11, 2011 #10


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    Sorry. What exactly are you confused about?
  12. Nov 5, 2011 #11
    what is the importance of tension in the propagation of transverse waves?

    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?
  13. Nov 5, 2011 #12


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    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.
  14. Nov 8, 2011 #13
    do longitudinal waves propagate through gases beacause gases are compressible?
  15. Nov 8, 2011 #14
    longitudinal waves can propagate through water,although water is incompressible,how?
  16. Nov 8, 2011 #15


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    Water is not incompressible, it is simply MUCH less compressible that something like air.
  17. Nov 8, 2011 #16
    yep,okay..how about the first question:
  18. Nov 8, 2011 #17


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    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.
  19. Nov 8, 2011 #18
    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?
  20. Nov 8, 2011 #19
    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.

  21. Nov 8, 2011 #20
    what is wrong with that :(

    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?
    I couldn't ignore tension too
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