From: ram@zedat.fu-berlin.de   
      
   Luigi Fortunati  writes:   
   >If you are blindfolded on a bus and ask you if it is accelerating or   
   >not, you do not need to ask "accelerating in relation to whom?": Answer   
   >that it is accelerating if you feel the force and answer that you are   
   >not accelerating if you do not perceive it.   
      
     Presuming you know that the street (bus) is not sloped.   
     (When the front of the bus is lower than its back, this   
     might feel similar to a situation where the front and   
     back have the same height, and the bus brakes.)   
      
     When one sits in a resting bus, one's body is pressed   
     against the sitting surface. This is because the bus is   
     constantly accelerated away from it geodesic by the surface   
     of the earth. But kinetically the normal acceleration of the   
     bus relative to the center of the Earth is 0, because the   
     distance to the center of the Earth is constant.   
      
     With closed eyes one cannot tell this from a situation where   
     there is a kinematic acceleration of g (9.81 m/s^2) of the   
     distance to the center of the Earth because the bus is in a   
     space far away from the Earth and actually has a kinematic   
     acceleration of g (9.81 m/s^2) relative to the center of the   
     Earth.   
      
     OTOH, when the bus is falling freely in the gravitational   
     field of the Earth, there clearly is an acceleration of the   
     bus (as the second derivative of its height), but inside the   
     bus one is floating freely and does not feel any   
     acceleration whatsoever (in the sense that it just feels as   
     if at rest in a space with no gravitation). This is the   
     situation of the folks in the ISS.   
      
     The ISS is always accelerated, since it's on a circular path,   
     but its inhabitants do not feel any acceleration at all, which   
     also is called "weightlessness". This is so, because the   
     ISS is moving on its geodesic.   
      
     Summarized, whenever one is living in a container and is   
     pressed against any wall of that container, this means that   
     the container is accelerated away from it geodesic, even if   
     its kinematic acceleration might be zero when measured as   
     the second derivative of the distance to some reference point.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)