From: fortunati.luigi@gmail.com   
      
   The force of the wind pushes me forward, then I run and the wind is   
   gone, it disappears along with its force.   
      
   If I have the same speed, the wind no longer exercises (it can not   
   exert) any force on me.   
      
   In the river its water pushes me (it exerts a force on me) but if I (at   
   dead body) then drift over the water, I do not feel any more push, the   
   force the river exerted on me has vanished.   
      
   Third example: if I use my force to push the car I can do it until it   
   has a speed equal to or greater than mine, then I can no longer exert   
   any force on it because it no longer opposes me.   
      
   At that point I could be the strongest man in the world but my force is   
   no longer manifest, it does not exist.   
      
   Fourth example, Einstein's bleacher.   
      
   As long as it is fixed on the scaffold, the force of gravity is there   
   but when the bleacher falls (and no longer opposes resistance) the   
   "force" of gravity disappears.   
      
   >From the previous examples, is it fair to say that force is manifested   
   exclusively when there is a resistance that opposes and never when   
   every form of resistance is lacking?   
      
   --   
   Luigi Fortunati   
      
   Credere e' piu' facile che pensare   
   Believing is easier than thinking   
      
   [[Mod. note -- No, your generalized conclusion is not valid -- your   
   first three examples are all forces which you have defined to be   
   dependent on the velocity of the moving body, with the force falling   
   to zero at a certain (finite) velocity.  Not all forces are of that   
   type.  The motion of a rocket moving in a vacuum without drag forces   
   provides a counterexample.   
      
   Your fourth example is based on an incorrect mixing of Newtonian and   
   general-relativistic arguments:   
   * In Newtonian mechanics, the force of gravity does NOT "disappear"   
     in free-fall -- the freely-falling body has a nonzero acceleration   
     ("little g") with respect to an inertial reference frame.   
   * In general relativity, gravity isn't a force in the first place,   
     so there's nothing to "disappear".  Rather, what people usually think   
     of as gravity is a *ficitious* force (analogous to centrifugal "force"   
     in Newtonian mechanics) due to being in a NON-freely-falling reference   
     frame.  Gravitational tidal forces don't go away in any reference frame.   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)