From: fortunati.luigi@gmail.com   
      
   On Mon, 20 Oct 2025 19:08:24 PDT, Luigi Fortunati   
    wrote:   
   >If the only negative force applied to the system is the external force   
   >F_ext_on_Y=-3N, how does the force F_Y_on_X =-4N exist?   
   >   
   >Where does he get that extra -1 from? Who is providing it to him?   
   >   
   >   
   >[[Mod. note --   
   >For convenience of discussion, let's take the motion as being horizontal,   
   >with the + direction being to the right.   
   >   
   >The only negative *external* force applied to the system is the external   
   >force F_ext_on_Y=-3N (pushing left on Y with a force of 3N).  But because   
   >there's a larger-in-magnitude force F_ext_on_X (pushing right on X with a   
   >force of 6N), the net force on the whole system is to the right, so the   
   >whole system is accelerating to the right with an acceleration (with   
   >respect to an inertial reference frame) of   
   >  a = (F_ext_on_X + F_ext_on_Y) / (m_X + m_Y)   
   >    = +3N / 9kg   
   >    = +1/3 m/s^2   
   >   
   >The answer to Luigi's question is that X and Y are both *accelerating*   
   >(to the right), there are also internal forces acting between X and Y:   
   >X is pushing rightward on Y (this is F_X_on_Y), and Y is pushing leftward   
   >on X to the left (this is F_Y_on_X).   
   >   
   >So F_ext_on_Y is *not* the only negative force -- there is also F_Y_on_X.   
      
   I agree with you: there are also *internal* forces and they are   
   exactly the ones I want to find, clarify and highlight.   
      
   I also agree that, in addition to the negative force F_ext_on_Y=-3,   
   there is also the other negative force F_Y_on_X=-4.   
      
   But the origin of the force F_Y_on_X=-4 is not only external and not   
   even only internal.   
      
   At the origin there is certainly the external force F_ext_su_Y=-3N and   
   then there is the added force -1N which is formed inside the Y body   
   and is then discharged onto the X body.   
      
   It is the force I was talking about in my previous question and we can   
   see it more clearly in the red -1N force of the animation   
   https://www.geogebra.org/classic/ykncqakp where I composed the   
   opposite external forces +6N and -3N into the single external force   
   F_ext_su_X=+3N, without any external force F_ext_su_Y.   
      
   Is this red -1N force the force F_Y_on_X=-1N that arises inside body Y   
   in response to the external force of body X that is accelerating it?   
      
   Luigi Fortunati   
      
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