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Water waves

  1. Aug 11, 2009 #1
    From dimensional analysis, the speed of water waves can be determined as follows:

    Only the gravitational constant 'g=10 m/s^2' matters. g has units of L/T^2, so the only way to get a velocity is [tex]\frac{1}{(2\pi)^{\frac{1}{2}}}g^{\frac{1}{2}}*\lambda^{\frac{1}{2}} [/tex] where [tex]\lambda [/tex] is the wavelength.

    The problem is that I looked at a small brook which is normally still, but was moving a little from a breeze, and estimated the wavelength (no longer than a foot), and applied the formula. The result is that the speed of the wave comes out a lot faster than what I observed.

    So my question is has anyone else observed something similar? Or does this formula only work for big ocean waves?
     
  2. jcsd
  3. Aug 12, 2009 #2
    The equation you quoted in an approximation for deep water.

    I think it is due to the depth of the water. In shallow water the wave speed is v ~ sqrtgd where d is the depth of water. Depth < lambda/20 in this case.

    The formula of all depth is a little more complicated.

    See http://hyperphysics.phy-astr.gsu.edu/Hbase/watwav.html#hwav
     
  4. Aug 12, 2009 #3
    I guess dimensional analysis can only take you so far.

    At least it makes sense that for shallow water, the speed is less. I guess this is due to friction from the floor under the sea. It's interesting that it doesn't depend on the type of floor. For solids, the type of material determines a coefficient of friction. For liquids, the layer touching the ground doesn't move at all (does anyone know why?), and the layers above it have a friction that's the viscosity of the liquid. However, the viscosity doesn't even enter the equation, so maybe it's not because of friction from the floor of the sea!
     
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