From: niklas.holsti@tidorum.invalid   
      
   On 2020-10-25 11:23, JF Mezei wrote:   
      
   > If you look at wave system to generate electricity, floating devices go   
   > up and down wth the waves. Going down, they generate electricity with   
   > whatever gravity dow to the floating device (if not a force, whatever).   
   >   
   > Similarly tidal power generation relies on gravity's effects of the moon   
   > causing oceasn to move around and rise on one side while gong down on   
   > the other.  So gravity from the moon causes massive movement of water.   
      
   The cyclic up-down movement comes from the rotation of the Earth. If the   
   day were one month long (and if the Moon's orbit were in the plane of   
   the equator), the lunar tides would be stationary with respect to the   
   Earth -- the sea level would be a bit higher directly under the Moon,   
   and on the opposite side of the Earth, and lower in between.   
      
   The energy from tidal power generation comes from the Earth's rotational   
   energy. The interaction (friction) between the lunar tides and the Earth   
   slows down the Earth's rotation. A tidal power station increases that   
   interaction and adds to the slowing down of the Earth's rotation.   
      
   > IOf Gravity isn't a force, what forces causes such massive movement of   
   > water every 6 hours ?   
      
   Asking "is gravity a force" is the wrong question. In Newtonian   
   mechanics it is _modelled_as a force; in general relativity it is   
   modelled by geodesics, and forces come into play only when something   
   forces some object _not_ to move along a geodesic. For example, if a   
   bottle stands on a table, it is the table that is pushing up on the   
   bottle to keep the bottle from following a geodesic, that is, from falling.   
      
   In other words, there are movements that do not have to be "caused by a   
   force". Movement along geodesics comes naturally.   
      
   The water bulge that forms a lunar high tide, on the Moon-facing side of   
   the Earth, would like, geodesically, to stay directly under the Moon.   
   The Earth's rotation, and its drag on the water, tries to force the   
   water to rotate with the Earth. This displaces the bulge eastwards from   
   the sub-lunar point.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)