From: dr.j.thornburg@gmail-pink.com   
      
   In article , Luigi Fortunati asked   
   > Are tidal forces real?   
      
   In article , I replied   
   > They can do work, so they must be real.   
   >   
   > [[...]]   
   >   
   > See   
   >   https://en.wikipedia.org/wiki/Tidal_power   
   > for some real-world examples of work done by tidal forces, with (in some   
   > cases) power output measured in the hundreds of megawatts.  These systems   
   > all ultimately exploit the fact that g_wrt_Earth is NOT constant from one   
   > edge of the tidal power basin to the other edge, i.e., that the (entire)   
   > tidal power basin as NOT a local inertial reference frame.   
      
   Oops, that last sentence of mine was quite wrong. :(   
      
   The variation in g_wrt_Earth over the tidal power basin is generally   
   negligible for a tidal power system.  I should have written this instead:   
      
     These systems   
     all ultimately exploit the fact that g_wrt_Earth varies with time   
     AND varies with position on the Earth's surface.  The combination   
     of these variations causes ocean tides (which certainly do mechanical   
     work).   
      
   I should also point out this GREAT animation of worldwide tides at one   
   particularly-powerful frequency:   
      
     https://en.wikipedia.org/wiki/File:Global_surface_elevation_of   
   M2_ocean_tide.webm   
      
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      currently on the west coast of Canada   
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