Water waves and motion of bodies.

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Discussion Overview

The discussion revolves around the question of why ships require anchors, particularly in the context of water waves and the motion of bodies in water. Participants explore the principles of wave motion, the effects of currents, tides, and wind on a vessel's movement, and the differences between theoretical models and real-world conditions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that since a ball in water only moves up and down when waves are created, a ship should similarly remain stationary and not require an anchor.
  • Another participant counters that ocean currents, tides, and wind resistance can cause significant movement of a ship, indicating the need for an anchor.
  • A third participant points out that the method of generating waves and the shape of the object in water can affect the results, noting that a ball's symmetry differs from that of a boat.
  • It is mentioned that while small waves may cause a boat to oscillate around a point, larger waves can carry the boat along, similar to a surfer.
  • Participants agree that anchors are necessary to counteract not just wave motion but also the effects of currents and wind forces.

Areas of Agreement / Disagreement

Participants generally agree that while small waves may not cause significant movement, larger waves and environmental factors like wind and currents necessitate the use of anchors. However, there is some debate regarding the initial premise of whether a ship would remain stationary in water due to wave motion alone.

Contextual Notes

The discussion highlights the limitations of the initial thought experiment, particularly regarding the assumptions about wave behavior, the effects of wind, and the differences in object shapes affecting movement in water.

Who May Find This Useful

This discussion may be useful for individuals interested in fluid dynamics, marine engineering, or those seeking to understand the practical implications of wave motion on floating bodies.

Taylor_1989
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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.
 
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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.
 
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).
 
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.
 
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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