From: dr.j.thornburg@gmail-pink.com   
      
   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.   
      
      
      
   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.   
   (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).)   
      
   --   
   -- "Jonathan Thornburg [remove -color to reply]"    
      on the west coast of Canada   
      "Now back when I worked in banking, if someone went to Barclays,   
       pretended to be me, borrowed UKP10,000 and legged it, that was   
       `impersonation', and it was the bank's money that had been stolen,   
       not my identity.  How did things change?" -- Ross Anderson   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)