From: fortunati.luigi@gmail.com   
      
   Il 22/01/2026 07:20, Jonathan Thornburg [remove -color to reply] ha scritto:   
   > In article <10kn92v$10o8l$1@dont-email.me>, Luigi Fortunati asks   
   >> 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?   
   >   
   > The answer is that the father's feet are (we've assumed) firmly   
   > planted on the ground, i.e., there's no relative motion between the   
   > father's feet and the ground.  So the only thing that could actually   
   > be accelerated by the force the father is applying to push the ground   
   > to the right is the entire Earth, and the acceleration of the entire   
   > earth (mass ~ 6e24 kg) under a force of 630N is so small (around 1e-22   
   > m/s^2) that we can neglect it.   
   >   
   > In contrast, the effective mass that's being accelerated by the   
   > father-son action-reaction is just the father's and son's hands and   
   > part of their arms, with an effective mass of (we assumed) only 15kg,   
   > so the acceleration is non-trivial.   
      
   All correct, but I was talking about the acceleration of the   
   dynamometers and not of the feet, hands, and ground.   
      
   Why does the dynamometer in their hands remain stationary until the   
   father pushes it to the left with a force of -600N?   
      
   Evidently because, on the other side, the son is pushing it to the right   
   with the same force of +600N.   
      
   And why does the dynamometer accelerate to the left when the father   
   increases his force to -610N?   
      
   Evidently because the son does NOT oppose the same opposing force of   
   +610N, otherwise the dynamometer would remain stationary and not accelerate!   
      
   If the dynamometer accelerates to the left, it means that the force of   
   the father (who pushes it to the left) is greater than and not equal to,   
   that of the son (who pushes it to the right).   
      
   > Luigi also wrote (about part of the father-son action-reaction force):   
   >>> 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.   
   >   
   > On the contrary, inertial forces can be real forces.   
   >   
   > As a simpler example, consider a block of mass 10kg sitting on a   
   > sheet of ice (so that it can move horizontally with very little friction).   
   > If I stand at the edge of the ice and push on the block with a horizontal   
   > force of 1N, causing the block to accelerate at 0.1 m/s^2, the block's   
   > reaction force on my hand is an inertial force, and it is a real force.   
      
   Exactly.   
      
   The inertia of the block on the ice, which *reacts* to your push, exerts   
   a real, not an apparent, force.   
      
   Therefore, the inertia of bodies is an apparent force only when there   
   are no external forces attempting to change their state of motion, but   
   it becomes a real force when the bodies react to someone attempting to   
   accelerate them.   
      
   > (This may be easier to imagine if we instead make the block be of mass   
   > 1000 kg, so that the block's acceleration is very small (0.001 m/s^2).)   
      
   This is also correct: the greater the mass of the bodies, the greater   
   their actual inertial reaction force.   
      
   Luigi Fortunati   
      
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