From: niklas.holsti@tidorum.invalid   
      
   On 2020-10-24 9:37, JF Mezei wrote:   
   > On 2020-10-23 21:40, Alain Fournier wrote:   
   >   
   >> Before you make the splash in the pool, you had to go up one way or   
   >> another, and you had to give energy to your body to go up.   
   >   
   > OK, instead of climbing a ladder and jumping into a pool,. how about   
   > some object going round and round the Earth at the same altitude. Its   
   > potential energy does not change.   
   >   
   > Yet, Earth's gravity constantly acts on the object to change its   
   > direction of travel.   
      
   A force does work only when it causes movement along the force vector.   
      
   If the orbiting object is in a circular orbit, the force acts   
   perpendicularly to the motion and so does no work. The speed of the   
   object is constant, so its kinetic energy is constant. Only the   
   direction of motion changes. Total momentum is preserved by opposite   
   changes in the Earth's motion.   
      
   If the object is in an elliptical orbit, the force is not always   
   perpendicular to the motion, and there is a cyclic exchange between the   
   potential and kinetic energies of the object. But the total is constant   
   (ignoring tidal dissipation).   
      
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    * Origin: you cannot sedate... all the things you hate (1:229/2)