From: ram@zedat.fu-berlin.de   
      
   Luigi Fortunati  writes:   
   >Why does the force of gravity disappear in the free-falling elevator and   
   >on Earth (which is also in free-fall) does it not disappear?   
      
     What people perceive to be "the force of gravity" on the   
     surface of the Earth actually is a /combination/ of /two/   
     forces: The actual downward force of gravity pulling   
     everything towards the center of the Earth and an effective   
     force of the Earth's surface that is directed upwards and is   
     preventing things to actually fall further towards the   
     center of the Earth.   
      
     This combination of forces can be characterized by its   
     action on two test bodies: one body resting on the Earth's   
     surface and one body one meter above the first body in the   
     air. The combined action of the force of gravity and the   
     force of the Earth's surface will reduce the distance of   
     those two bodies in the course of time. And it is this   
     squeezing together of the two bodies that we perceive as the   
     "force of gravity". Let me call this force the "squeezing   
     force". People are actually shorter in the evening because   
     they have been squeezed all day while they were sitting,   
     standing, or walking vertically and they grow a bit again   
     when allowed to rest horizontally for some hourse. This   
     squeezing force is called "force of gravity" in everyday   
     language.   
      
     If you put the same two test bodies into an elevator that is   
     falling freely, one on the floor of the elevator and the   
     other one into the air one meter above the first one,   
     their distance will /not/ decrease because the force of the   
     Earth's surface and thus the squeezing force is missing here.   
     Since the squeezing force is called "force of gravity" in   
     everyday language, a layman would say, "There is no force of   
     gravity in the free-falling elevator".   
      
     From the point of view of physics, however, gravity is   
     always there; it is the force of the Earth's surface (and   
     thus the squeezing force) that is sometimes there and   
     sometimes not. If there would be no force of gravity on   
     the falling elevator, it would not be falling!   
      
     (The above has been written in the spirit of Newton's view   
     of gravitation as a force. Within general relativity [GR],   
     gravitation is not regarded to be a force anymore, but   
     I have ignored GR above since it is not required to answer   
     the question at hand. For the same reason, I have not   
     written anything about how exactly the force of the Earth's   
     surface comes about.)   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)