From: fortunati.luigi@gmail.com   
      
   Il 28/02/2026 19:43, Mikko ha scritto:   
   > On 28/02/2026 10:48, Luigi Fortunati wrote:   
   >> I wasn't very happy with the way this discussion ended.   
   >>   
   >> The case involved the elastic collision in the animation   
   >> https://youtu.be/wl0c6NMysY4 where the two bodies A and B collide at   
   >> equal and opposite velocities v and -v.   
   >>   
   >> In the end, we concluded the discussion by discussing velocities v_A =   
   >> +1 m/s and v_B = -1000 m/s, which have nothing to do with what I had   
   >> asked, nor with the animation, where the two velocities are equal and   
   >> opposite.   
   >>   
   >> I didn't have the presence of mind to clarify this, but now I'm ready to   
   >> do so by discussing the interesting clarifications that other comments   
   >> have highlighted.   
   >>   
   >> We all agreed that (under the conditions of the animation) with the mass   
   >> of body A between "m" and less than "3m" both bodies recoil after the   
   >> collision, while with the mass of A greater than "3m," body A slows down   
   >> but continues to move forward without stopping.   
   >>   
   >> What if the mass of body A is exactly equal to "3m"?   
   >>   
   >> In this case, it doesn't go back or move forward.   
   >>   
   >> So it stops.   
   >>   
   >> But where does it stop?   
   >   
   > It stops at the last place where somthing can change its motion.   
      
   No, it only stops when the opposing force is strong enough to stop it.   
      
   The opposing force that stops the two bodies is elasticity, which we can   
   legitimately imagine as the reaction of a spring that, during the   
   collision, is compressed between the two bodies and reacts accordingly.   
      
   Obviously, the spring's reaction is not immediate because its force   
   starts from zero and progressively increases with compression, which   
   lasts a certain interval of time during which its elastic push increases   
   both to the left against body "3m" (body A) and to the right against   
   body "m" (body B).   
      
   If the force at both ends of the spring is equal, it stops body B BEFORE   
   it can stop the more massive body A.   
      
   Therefore, there will be a residual interval of time during which the   
   spring continues to be pushed to the right by body A (which has not yet   
   stopped) but is no longer pushed to the left by body B (which has   
   already stopped).   
      
   Luigi Fortunati   
      
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