From: alain245@videotron.ca   
      
   Le Oct/25/2020 à 08:32, Niklas Holsti a écrit :   
   > 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.   
      
   Yes, but the fact that the bulge is eastward from the sub-lunar point   
   isn't really important here. Even if the bulge was at the sub-lunar   
   point we would still have tides. The important point is that Earth's   
   rotation makes the bulge move around Earth. And that is what allows us   
   to get energy from tidal power generation. If Earth rotated at the same   
   speed as the Moon orbital motion, like if the Moon was in geostationary   
   orbit, then the tidal bulge would not move on Earth's surface and we   
   would not be able to generate power from the static tide. There would   
   still be a tidal bulge, but it wouldn't move and tidal power generation   
   wouldn't work.   
      
   The energy from tidal power generation comes from the potential energy   
   stored in Earth's rotation. When we generate energy from tides, we slow   
   down the flow of the tides, and this in turn slows down the rotation of   
   Earth. (It also raises the orbit of the Moon a tiny fraction, but that   
   is a second order phenomenon which only means that Earth's rotation is   
   slowed down a little more than by the amount of Energy that we extracted   
   for power generation.) There is a *lot* of Energy stored in Earth's   
   rotation, so we can extract some for a very long time before that would   
   have a noticeable effect on Earth's rotation.   
      
      
   Alain Fournier   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)