From: fortunati.luigi@gmail.com   
      
   Il moderatore alle ore 18:37:00 del 19/08/2017 ha scritto:   
   > [[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]]   
      
   When the bleacher is on the scaffold there is no acceleration because   
   they are both still.   
      
   Instead there is a force and we can be measured with the dynamometer   
   that is nothing but a spring.   
      
   Just put this spring under the bleacher's feet and the spring   
   contracts: the force is there!   
      
   Really, the forces are 2, there is the force of the bleacher against   
   the scaffolding (direct force down) and there is the force of the   
   scaffolding against the bleacher (straight upwards).   
      
   There are forces and there is no acceleration because the two forces   
   are equal and opposite and therefore the resulting is zero.   
      
   But that does not mean that the forces there are not so much that the   
   dynamometer measures it.   
      
   At this point, the fundamental question is: what appening if these two   
   equal and opposite forces disappeared simultaneously?   
      
   It would happen that the resultant of the two forces would continue to   
   be nothing as before as BUT the forces are no longer there.   
      
   And the bleacher should be suspended in the air without dropping!   
      
   Because the bleach is precipitated, the two forces can not disappear   
   both!   
      
   --   
   Luigi Fortunati   
      
   Credere e' piu' facile che pensare   
   Believing is easier than thinking   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)