From: PointedEars@web.de   
      
   Thomas 'PointedEars' Lahn wrote:   
   > First of all, it is not good to think of gravitation as a property of an   
   > object as in "Earth *has* gravity".  That is NOT how it works.  [This is   
   > frequently taught wrong in schools.]  Instead, it is an *interaction*   
   > _between_ objects.  According to Newton's theory, objects _attract each   
   > other_ because they have non-zero mass.  So (according to Newton) it is not   
   > so that "earth does gravity" but that Terra (_Earth_) has non-zero mass,   
      
   ["non-zero X" simply means "X is not equal to zero"]   
      
   > and so do other objects (including people like you and me), and so everything   
   > is attracted to everything else.  (As the story goes, he realized that an   
   apple   
   > and Earth attract each other in the same as Earth and the Moon attract each   
                                  ^^^^^^^^^^^   
   in the same _way_   
      
   > other, and Earth and the Sun are attracted to each other: Gravitation was   
   > *universal*, not limited to Earth.  Thus he could also explain how Kepler's   
   > planetary orbits arose, and predict them.)   
   >   
   > One way to understand how the magnitude of the gravitational acceleration at   
   > the center of Terra is approximately zero is to consider that a test object   
   > with a negligible non-zero mass (a "test mass") located there will be   
   > attracted gravitationally by all the matter that surrounds it (which also   
   > has non-zero mass) in all directions of space approximately in the same way   
   > (*exactly* in the same way if the planet were spherically-symmetric and had   
   > a uniform mass density; we know that this is not so for any planet, but it   
   > is still a good approximation), so the net gravitational force on it and its   
   > net gravitational acceleration is approximately (would be *exactly*) zero.   
      
   Granted, the "incredibly unbroken" one-paragraph sentence above is far too   
   long, and reminds me of a certain Dr. Fassbinder }:-)   
      
   An attempt to rewrite it:   
      
   One way to understand how the magnitude of the gravitational acceleration   
   at the center of Terra is approximately zero is to consider a test object   
   with a negligible non-zero mass (a "test mass") there.  It will be attracted   
   gravitationally by all the matter that surrounds it (which also has non-zero   
   mass) in all directions of space approximately in the same way.  So the net   
   gravitational force on it and its net gravitational acceleration are   
   approximately zero.   
      
   [The test object would be attracted *exactly* in the same way if the planet   
   were spherically-symmetric and had a uniform mass density; we know that this   
   is not so for any planet, but it is still a good approximation.]   
      
   --   
   PointedEars   
      
   Twitter: @PointedEars2   
   Please do not cc me. / Bitte keine Kopien per E-Mail.   
      
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