From: fortunati.luigi@gmail.com   
      
   On Fri, 07 Nov 2025 12:49:08 PST, Luigi Fortunati   
    wrote:   
   >Based on your own numbers, wall X experiences two forces: the external   
   >force F = +1N directed to the right and the force F3 = -0.9N directed   
   >to the left.   
   >   
   >The difference between the two opposing forces (F and F3) is the net   
   >force +0.1N that accelerates wall X without compressing it, while the   
   >remaining opposing forces (+0.9 and -0.9) compress it without   
   >accelerating it.   
   >   
   >One force accelerates it and the other two compress it.   
   >   
   >How do we detect the net force +0.1N? We measure the acceleration of   
   >wall X.   
   >   
   >How do we detect opposing forces on the same body? We measure its   
   >compression.   
   >   
   >And now let's move on to the block of ice.   
   >   
   >Based on your analysis, the block of ice experiences only one force,   
   >the external force F4 = +0.9N, and nothing else.   
   >   
   >If this were indeed the case, the block of ice should accelerate   
   >without experiencing any compression.   
   >   
   >And yet, for the entire duration of the acceleration, the block of ice   
   >undergoes a compression that can be measured by highly precise   
   >instruments.   
   >   
   >How could the block of ice (or any other material) compress if, as you   
   >say, there is no opposing force?   
   >   
   >Luigi Fortunati   
   >   
   >[[Mod. note --   
   >Consider an imaginary vertical line dividing the block of ice into a   
   >part /IL/ to the left of the line and a part /IR/ to the right of the   
   >line.  The position of the vertical line is such that /IL/ contains a   
   >fraction /f/ of the ice, where /f/ is some as-yet-unspecified number   
   >in the range [0,1], i.e., /IL/ has a mass /f m_ice/ and IR has a mass   
   >/(1-f) m_ice/.   
   >   
   >/IR/ is accelerating to the right, so by Newton's 2nd law there must be   
   >a net force /F5/ to the right acting on IR.  This force can only come   
   >from /IL/ being compressed and pushing right on /IR/.  By Newton's 3rd   
   >law, /IR/ also pushes left on /IL/ with a force /F6 = -F5/   
      
   Yes, but only if the third law is correct.   
      
   And mind you, I'm not saying the two opposing forces don't exist, I'm   
   just saying they're not equal.   
      
   >Since uniform acceleration is equivalent to a gravitational field, this   
   >system is very analogous to a rope hanging vertically, where the tension   
   >varies along the rope.   
      
   Exactly, acceleration is (almost perfectly) equivalent to the   
   gravitational field (equivalence principle).   
      
   But the variation in tension along the rope indicates a direction and   
   a sense: it is a tension that is also a vector!   
      
   >In a following posting I'll work out this analysis in a bit more detail.   
      
   I look forward to your next post with great interest.   
      
   Luigi Fortunati   
      
   P.S. You haven't answered my question: if the block of ice, as you   
   say, is subject only to the external force F4 = +0.9N, why is it under   
   tension throughout the acceleration? If there is tension, it means   
   that there are also opposing forces, not just net forces.   
      
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