From: helbig@asclothestro.multivax.de   
      
   In article , Luigi Fortunati   
    writes:   
      
   > In the formula F=ma force is related to acceleration.   
   >   
   > If acceleration exists in all references, the force is real, otherwise,   
   > if it exists in one reference and "disappears" in the other, then the   
   > force is "apparent".   
      
   You can always transform an acceleration away by using an accelerating   
   reference frame   
      
   > On the other hand, in the formula F=m1*m2/d^2 there is NO acceleration   
   > and this is the difference between the second formula and the previous   
   > one.   
      
   It looks like you forgot the gravitational constant; that gives the   
   right-hand side the dimensions of ma.   
      
   > In F=m1*m2/d^2 the force depends exclusively on the presence of the two   
   > masses m1 and m2 (which always exist) and on the square of their   
   > distance (which always exists).   
      
   And on the gravitational constant.   
      
   > Therefore, if the masses m1 and m2 and the distance d^2 never disappear   
   > (in any reference) the force F=m1*m2/d^2 is always real.   
      
   Acceleration is always "real".  (That is actually still a puzzle:   
   acceleration relative to what?  As far as I know, no-one has ever shown   
   quantitatively that Mach's principle can explain that.)   
      
   > And what is the force F=m1*m2/d^2? It's gravity.   
      
   Right, which is why the expression usually includes the gravitational   
   constant.   
      
   > So gravity is not "apparent" because (the formula says so and not me)   
   > its force does not disappear by changing the SDR.   
      
   An acceleration is real; you can feel it.  You can find an accelerating   
   reference frame and transform it away (at least locally).   
      
   The essence of general relativity is that gravitation, in some sense, IS   
   acceleration, but as far as I know no-one knows why the gravitational   
   constant has a non-zero value or has the value it has (apart from   
   weak-anthropic explanations).   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)