From: titang78@gmail.com   
      
   Le samedi 7 septembre 2019 10:15:40 UTC+2, richali...@gmail.com a écrit :   
   >   
   > It all depends on the reference frame of the observer.  Let's be a   
   > little more precise about how the ruler starts moving.  Start from a   
   > frame at rest with the ruler, and at t=0, in that frame, accelerate the   
   > ruler suddenly by applying an imulse uniformly along the entire length   
   > of the ruler, so that all parts of the ruler start moving   
   > simultaneously, in that frame.   
   >   
   > For t>0, the ruler is now in a different rest frame moving at speed V,   
   > assumed close to C.  In this frame the normal, unstressed length of the   
   > ruler is L, just like it was in the original ruler rest frame, but the   
   > ruler is now stressed.  We have to be very careful about how the ruler   
   > arrived in this frame, from the point of view of an observer that was in   
   > this frame all along.  What was simultaneous in the original frame is   
   > not simultaneous this frame.  In this frame the observer will see the   
   > forward end of the ruler approach at speed V and then suddenly stop.   
   > Progressively more and more of the ruler come to a stop until finally   
   > the trailing end starts moving.  As a result, in this frame, the ruler   
   > will appear to compress during this impulse acceleration.  Immediately   
   > after the acceleration event the ruler will be compressed and its length   
   > will be shorter than L.  The ruler would then expand to its normal,   
   > unstressed length L in this frame in a manner based on the material   
   > properties of the ruler.   
   >   
   > So yes, in this frame, for a brief period of time, the two ends of the   
   > ruler will appear to move towards each other.   
   >   
   > Rich L.   
      
   When the ruler arrives at x=s, it is in constant motion. So, Lorentz   
   transform applies. Because L/gamma is smaller than L, we can always find   
   a x2 such that s+x2/gamma < x2 . x2 is a point in the frame of the   
   ruler, but not of the ruler. This mathematical, not mechanical. There is   
   not stress to compress the abscissa axis of the frame.   
      
   See my reply to Sylvia   
      
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