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Water waves and motion of bodies.

  1. Apr 11, 2012 #1
    My question is quite a simple one, but one that is confusing me quite a bit. I was think why a ship needs an anchor? This sounds silly but this is why I ask. When you but a ball in the middled of water for example and you are a meter away and create waves in the water the ball just move up and down, it does not move. So basically the same principle as a transverse wave, where the medium through which the energy travels, is perpendicular to the wave direction. So surly the is no need for a anchor because the boat will not move, it will only bob up and down.
  2. jcsd
  3. Apr 11, 2012 #2
    Your idea is correct, but in an ocean the currents, tides and drag due to wind resistance of the boat will all mean it can move around significantly.
  4. Apr 11, 2012 #3


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    There's two things in your experiment that will change your results depending on how you do them:

    1) driving function: how did you drive the waves when you created them? What if you would have used a more lateral driving motion?

    2) the shape of the contact point between water and floating object. You used a ball, which is radially symmetric. A boat is only symmetric in one dimension.

    And finally, the direct action of wind as it pushes directly on the boat can cause significant drift in otherwise stable conditions (i.e. when the wave are flat calm).
  5. Apr 11, 2012 #4


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    You are basically right about the effect of water waves. Whatever the details of how the water moves and the shape of the boat, the only effect of "small" waves would be to make the boat oscillate about one point. But if the wave was large compared with the boat, the boat could be carried along like a surfer.

    But as #2 said, the anchor also restrains the boat against movements caused by water curremts, tides, and wind forces, not just against wave motion.
  6. Apr 16, 2012 #5
    I would just like to say thanks for clearing my problem up for me. For some reason, i never took wind direction or even wind in my problem. Once again many thanks.
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