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Waves of any sort do not transfer mass

  1. Aug 8, 2004 #1
    Ive read that waves of any sort do not transfer mass, but only transfer energy. My question is why do I then float towards shore when at a beach and in the water. :uhh:
  2. jcsd
  3. Aug 9, 2004 #2

    In theory if you have a cork bobbing up and down in some waves it stays in the same place at the same time in each wave cycle. That is, at the top of the wave it will always be at the same place, and at the bottom of the wave it will always be at the same place, but these are not necessarily the same as each other. I think that the cork actually takes an eliptical path.

    However, in practice the cork will move along with the waves. This is due to the friction between the water and cork.
  4. Aug 9, 2004 #3


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    It's not so much the friction between the water and the cork (or you) as between the water and the beach. If you were in the open ocean, you would just bob up and down on the waves, not move in the direction of the waves. Near shore, however, the bottom (the beach extended underwater) retards the movement of the water and you get a "rotary" action- the top of the water moves toward the beach, the bottom away from it.
  5. Aug 9, 2004 #4


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    Then there is the issue that some waves (especially those in the open ocean) are wind driven...
  6. Aug 10, 2004 #5
    ya, my initial thought was because of the wind. An ocean breeze might be taking me into the shore. Thanx for the help.
  7. Aug 10, 2004 #6
    Water waves are directional, and they propagate as a force-carrier.
    Show me a single experiment(minus wind) where a free-floating bouy does not also propagate, though slowly, with the direction of the wave.
    Any takers?
  8. Aug 11, 2004 #7
    I think hallsofivy told you why, its because the bouy is near shore. If you are in the deep ocean, the no mass will be moved, only force.
  9. Aug 12, 2004 #8
    Then there's the ocean currents.
  10. Aug 13, 2004 #9

    Chi Meson

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    COme to my lab. I've put a tiny amount of lycopodium powder on the surface of water in a ripple tank. No wind, no currents. The powder stays put in the same spot as the rippler creates constant parallel waves. Aftera few minutes small currents get created and the powder begins to move, but the waves themselves do not move the powder.

    I'd like to show this, but our school has not got the approriate type of camera to record it. The demonstration is easily reproduceable though; pepper will make a suitable dust. put water in a baking sheet and create waves with a ruler. Be careful not to remove the ruler completely out of the water and don't move the ruler back and forth, just up and down slightly.
  11. Aug 13, 2004 #10

    Thanks, Chi, I appreciate the experimental evidence. Respectfully, I see two factors of importance. 1) In a constrained tank environment it is reasonable to assume that forward propagation of a wave is hindered, in concept, by virtue of reflectance. Thus, a forward moving wave in a tank set-up is countered by reflectance to some actual degree.
    2) Powder used only shows what happens on the surface of the wave. This is not an accurate depiction of what happens to sub-surface object extensions; such as a human floating in waves. The cohesiveness of water molecules and "boundary effects" with the air above it makes the wave surface act as a flexible skin. Thus, a static "bob" effect is noted with surface-only objects(i.e. dust) However, this is not the case with objects which have extensions below the surface boundary, and the dynamics of movement are different.
  12. Aug 15, 2004 #11

    Chi Meson

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    Point 1 would not be a factor since the ripples are absorbed by the embankmens of the tank. The subsequent sloshing at these embankments is what causes the small currents to be created after a while.

    Point 2 is a good one and I had to think about it for a while. About all I can contribute to this discussion from here is this:

    A: No one disagrees that an object bobbing inthe ocean will soon move in the direction of wave propagation; No one disagrees that this will be primarily, if not entirely, due to the wind and currents.

    B: To my knowledge, in the pure wave motion of an ideal surface wave witout wind or currents, there is no net motion of molecules in the medium. THis implies to me that there could be no net force (over any number of complete cycles of the wave) on the floating object.

    That's my understanding. Let me know if I'm missing something.
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