From: fortunati.luigi@gmail.com   
      
   Luigi Fortunati alle ore 06:14:21 del 15/08/2017 ha scritto:   
   > 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?   
      
   Il moderatore ha scritto:   
   > [[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.   
      
   The force does not drop to zero because of the speed that reaches a   
   certain value but due to the lack of resistance that occurs when the   
   speeds are equal!   
      
   The punch of the boxer exerts a force on the opponent's face not by its   
   speed but by the resistance of the face.   
      
   And even the fastest fist does not exert any strength if the face is   
   too far away at the same speed!   
      
   A certain speed, rather than another, only serves to create a   
   resistance!   
      
   It's the resistance that realizes force.   
      
   The third principle is clear on the subject: if there is an action   
   there is also an equal and opposite reaction.   
      
   But if there is no reaction, there is no action either.   
      
   Action and reaction are the two sides of the same medal.   
      
   Einstein's bleacher exercises a force to the scaffolding if the   
   scaffold exercises a forces to the bleacher.   
      
   When the the force of the scaffold on the bleacher ceases, even the   
   force of the bleacher on the scaffold ends.   
      
   > Not all forces are of that type.   
      
   All I have written is independent of the type of force and therefore it   
   is valid for the wind, the current of the river and the Riemann   
   curvature tensor.   
      
   > The motion of a rocket moving in a vacuum without drag forces   
   > provides a counterexample.   
      
   Force accelerates masses, not speed (F=ma not v=ma).   
      
   The rocket that moves in a vacuum is of all different from my examples   
   because it does not accelerate and therefore does not need forces, as   
   opposed to my bodies that from being stopped then they take speed, that   
   is accelerate.   
      
   --   
   Luigi Fortunati   
      
   Credere e' piu' facile che pensare   
   Believing is easier than thinking   
      
   [[Mod. note --   
   1. Your examples are all contact forces.  Consider the motion of a   
      magnet attracted or repelled by a nearby magnet as an example of   
      a non-contact force which does not drop to zero at any finite speed.   
      
   2. In an earlier moderator's note in this thread, I wrote that   
        "The motion of a rocket moving in a vacuum without drag forces   
         provides a counterexample."   
      I'm sorry, I mistakenly left out a key phrase.  What I should have   
      written was   
        "The motion of a rocket accelerating under its own thrust in a   
         vacuum without drag forces provides a counterexample."   
      
   3a. If we analyze the falling bleacher in Newtonian mechanics and/or   
       special relativyt, then when the scaffolding is not exerting any   
       force on the bleacher, the bleacher does indeed accelerate   
       (at an acceleration which is in fact just the local "little g").   
   3a. If we analyze the falling bleacher in general relativity, then   
       when the scaffolding is not exerting any force on the bleacher,   
       the bleacher is in free-fall (apart from air resistance, which   
       is small in this situation).  This means that the bleacher is   
       accelerating with respect to the ground (which is not in free-fall);   
       this relative acceleration is equal to the local "little g" in   
       magnitude.   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)