From: PointedEars@web.de   
      
   Jim Pennino wrote:   
   > Popping Mad  wrote:   
   >> If the center of the earth would have 0 force do to gravity (all the   
   >> Gravity cancels out) then why is it that the deeper you go into a plant,   
   >> like Jupiter for example, the pressure increases and the center of a   
   >> star, when it forms, ignites into nuclear fusion?   
   >   
   > Pressure and gravity are not the same force.   
      
   I appreciate your wanting to put it in simple language, but let us not   
   oversimplify.   
      
   Pressure is NOT a force.  It is the result of a force acting on a surface:   
      
     P = F/A.   
      
   "Gravity" is NOT a force.  _Gravitation_ is an interaction between objects.   
   It can be described by a force acting between those objects, and that   
   (Newton's theory) is the simplest correct explanation here that one can come   
   up with.   
      
   > Pressure is the result of mass that is above you.   
      
   This pressure is *produced* by gravitation (and is therefore fully called   
   "gravitational pressure" in astrophysics); in the theory that we chose to   
   use (see above), it is the *result* of the gravitational force acting   
   between the parts of Earth.   
      
   If we would imagine a person within Earth, this person just happens to be   
   *between* those parts, and so the gravitational force between the opposite   
   parts is pushing "down" on them (towards the center of gravitation to which   
   all the parts are mutually attracted because they are attracted to each   
   other).  This produces the pressure e.g. on the skin of that person (which   
   is their surface, with the area A_person):   
      
     P(r) = F_G(r)/A_person =~ G M(r)^2/(r^2 A_person).   
      
   [As we are approaching the center of gravitation, M(r) increases and r   
   decreases, so P(r) increases.]   
      
   > Gravity is the result of mass that is around you.   
      
   Therefore, this is wrong.  However, because gravitation is an interaction   
   between *all* objects (as I pointed out several times now), it is also an   
   interaction between Earth and the person; and, as you say correctly, in our   
   thought experiment the matter of Earth is all around the person.  So in the   
   center of Earth, the gravitational force in one direction cancels the one in   
   the opposite direction (that is how that point is *defined*).   
      
   > On the surface of the Earth all the mass above you is air   
      
   Correct.   
      
   > and the pressure because of the weight of that air is about 14 psi.   
      
   I am not well-versed in this non-SI unit, but IIRC the standard atmospheric   
   pressure is ca. 1023.25 hPa.   
      
   > As you descend into the Earth there is more and more massive stuff above you   
   > pushing you down and the pressure goes up.   
      
   Correct, in a sense.   
      
   > As you descend into the Earth the vector sum of gravity changes. You   
   > now have mass above you pulling you up. The gravity thus decreases.   
      
   Correct, in a sense.  As I point out above, there is no quantity called   
   "gravity".  What decreases here is the magnitude/norm of the net   
   gravitational force/acceleration, as calculated.  (It does not make sense to   
   speak of the increase or decrease of a vector.)   
      
   > The simplist way to put it is that as you go down, the gravity of the   
   > stuff above you cancels the gravity of the stuff below you but the   
   > weight of stuff above you is always increasing.   
      
   This wording can be confusing because the person is weightless in the center   
   of gravitation.   
      
   --   
   PointedEars   
      
   Twitter: @PointedEars2   
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