From: fortunati.luigi@gmail.com   
      
   Il 18/01/2026 03:00, Luigi Fortunati ha scritto:   
   >> How can the son increase his force on his father to +610N if the force   
   >> he's exerting (+600N) is already equal to his maximum capacity?   
   >>   
   >> [[Mod. note --   
   >> The son can't increase /F_son_arms_on_son_hands/.  But the son doesn't   
   >> directly control /F_son_hands_on_F_father_hands/: the additional 10N by   
   >> which /F_son_hands_on_F_father_hands/ exceeds /F_son_arms_on_son_hands/   
   >> is essentially due to the inertia of the son's hands (which are accelerating   
   >> to the left with an acceleration of /a_hands/).   
   >>   
   >> This is similar to how /F_father_arms_on_father_hands/ is 630N, but   
   >> /F_father_hands_on_son_hands/ is only 610N -- the 20N difference is due   
   >> to the inerta of the father's hands (which are also accelerating to the   
   >> left with an acceleration of /a_hands/).   
   >> -- jt]]   
   >   
   > Is the additional force +10N included in F_son_hands_on_F_father_hands   
   > of +610N a real or apparent force?   
   >   
   > Luigi Fortunati   
   >   
   > [[Mod. note -- It's a real force, measured in an inertial reference frame.   
   > -- jt]]   
      
   I agree: that force is certainly real.   
      
   But it contradicts what you wrote, namely that this greater force of +10   
   N is "due to the inertia of the son's hands."   
      
   If it is inertia, it should be an apparent force.   
      
   In any case, since you mentioned measurements, I tried to imagine how   
   these opposing forces could be measured.   
      
   The most obvious way to do this is to use a dynamometer.   
      
   If we place one between the father's feet and the ground, and another   
   between the father and son's hands while they are still pushing with an   
   equal force of 600 N, we'll notice that the dynamometers are all   
   stationary and not accelerating.   
      
   This should convince us that they are receiving the same force from the   
   right and left.   
      
   Instead, when the father increases his force against the ground from   
   +600N to +630N and the force of his hands against those of his son from   
   -600N to -610N, the dynamometer at his feet remains stationary and the   
   one at his hands accelerates to the left from zero velocity to a   
   non-zero velocity of -v.   
      
   Why do the two dynamometers behave differently?   
      
   Why does the action-reaction between the father and the ground only   
   cause compression of the dynamometer at his feet, while the   
   action-reaction between the father and son's hands also causes   
   acceleration in addition to compression?   
      
   Luigi Fortunati   
      
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