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Density Deep vs Shallow water

  1. May 6, 2009 #1
    We were doing questions with surface waves on water and as they moved from deep to shallow water it would slow down. So the teacher said the reason is shallow water is more dense than deep water. If this is true, why so.

    Does measuring density have anything to do with horizontal or vertical?

    i think that:
    density mass divided by volume, so it like a ratio. so it should be almost the same except when there is a lot of water where the the weight causes water to compress water on the bottom very little making the volume a bit less and so its density might be a bit more.

    so i was researching on the internet and came across reason of its height increasing as wave moves to shallow water and it gets a shorter wavelength. since the freq is the same so the speed slows down. v= f [tex]\lambda[/tex]

    all this is room temperature, tap water.
  2. jcsd
  3. May 6, 2009 #2


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    Staff: Mentor

    You are correct and your teacher is wrong: it is the interference of the land, not the density of the water (which is greater at depth, but not by much). Now why it slows down in shallower water, i'm not exactly sure...It seems like it just may be a matter of exchanging translational motion for rotational.
  4. May 7, 2009 #3
    Ocean density does vary, depending on the temperature and admixture of fresh water in the salt water. The well known North Atlantic thermohaline circulation pattern depends on this. In particular, the warmer northerly Gulf Stream in the North Atlantic overflows the southerly cold water from the Artic. Melting glaciers near Greenland have been reducing the density of the cold salty water, and threatens to upset this important circulation pattern. See

    Tsunami velocities are faster in deeper water. Here is some info from NOAA:
    How fast do tsunamis travel?

    Tsunami velocity depends on the depth of water through which it travels (velocity equals the square root of the product of the water depth times the acceleration of gravity).
    Tsunamis travel approximately 475 mph in 15,000 feet of water. In 100 feet of water the velocity drops to about 40 mph.
    A tsunami travels from the central Aleutian Is. to Hawaii in about 5 hours and to California in about 6 hours, or from the Portugal coast to North Carolina in about 8.5 hours.
  5. May 7, 2009 #4
    Here's a little ditty I did about waves:
    http://img23.imageshack.us/img23/8373/wavesx.jpg [Broken]
    Last edited by a moderator: May 4, 2017
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