Water waves accelerate in deeper water.

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Discussion Overview

The discussion revolves around the behavior of water waves as they transition between shallow and deep water, specifically focusing on the changes in speed, frequency, and wavelength. Participants explore the underlying causes of these changes and their relation to Newton's Second Law, as well as drawing analogies to other physical phenomena.

Discussion Character

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

Main Points Raised

  • One participant describes the standard demonstration of water wave refraction, noting that waves speed up when moving from shallow to deeper water and questions the forces responsible for this acceleration.
  • Another participant suggests that viscosity in shallow water affects wave speed, proposing that lower viscous forces in deeper water allow for quicker transfer of transverse vibrations.
  • A different viewpoint emphasizes that Newton's laws apply to individual parcels of water rather than the wave as a whole, drawing an analogy to gyroscopic motion and the behavior of shadows in relation to light speed.
  • One participant mentions that higher pressure in deeper water might influence the elastic modulus, potentially explaining the increased wave speeds.

Areas of Agreement / Disagreement

Participants express differing views on the mechanisms behind wave acceleration and the application of Newton's laws, indicating that multiple competing explanations are present and the discussion remains unresolved.

Contextual Notes

Limitations include the need for further exploration of how viscosity, pressure, and elastic modulus interact in this context, as well as the assumptions made regarding the applicability of Newton's laws to wave dynamics.

barfllwyd
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A standard demonstration of refraction is to show water ripples traveling from deeper to shallow water. The frequency remains constant and the wavelength is observed to be shorter and the velocity lower. BUT when the water wave travels from shollow to deeper water the opposite happens and the wave increases its speed. What causes these changes in speed and in particular what's accelerating the wave. What's happened to Newton's Second Law? A common analogy is a car traveling from tarmac to sand which would slow the car but what force would accelerate the car when traveling from sand to tarmac.
How would any explanation then be related to the refraction of light?
 
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I think it has something to do with viscosity this being greatest in the shallow water.The wave speed is determined by how quickly the transverse vibrations are carried from one layer to the next and I think this transfer occurs more quickly where viscous forces are smaller.I would like to hear other opinions,it is an interesting question.
 
Newtons laws apply to the individual parcels of water that, moving up and down, make up the wave, rather than to the wave itself. (Similarly, you might think a gyroscope behaves anomalously if you treat it as a whole rather than accounting for the dynamics of its constituents. Likewise, we can make a shadow move across a wall faster than the speed of light without violating relativity.)
 
Deeper water does have higher pressure. I'm not sure how, but this might have an effect on the elastic modulus, which could account for the higher wave speeds.
 

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