From: tkoenig@netcologne.de   
      
   Jonathan Thornburg [remove -color to reply]    
   schrieb:   
   > In article ,   
   > Luigi Fortunati  asks   
   >> Up to this point, I've been talking about the forces F1 and F2 between   
   >> the car and the pin, and between the towbar and the pin.   
   >>   
   >> And not about the action and reaction between car 1 and the towbar .   
   >>   
   >> So I ask: is the force F1 of car 1 on the pin equal to the action of   
   >> car 1 on the towbar ?   
   >   
   > There is no direct "action of car 1 on the towbar", because car 1 does   
   > not apply any forces to the towbar.  Rather, car 1 applies a force (F1)   
   > to the pin, and the pin applies a force (let's call it F3) to the towbar.   
      
   This point touches a general principle which is important in   
   classical mechanics.   
      
   It is always possible to subdivide a body, or a system of bodies,   
   into several parts, and replace the action of the part that was cut   
   away by a force and a torque, and apply the same force and torque,   
   with all signs reversed.   
      
   In German, this is called "Freischneiden", literally "cut free".   
   Wikipedia tells me the English equivalent is called "free-body   
   principle".  Apparently, it was first invented/discovered/developed   
   (whatever the correct term is) by Francis Bacon.   
      
   Students of mechanical engineering or related subjects who fail   
   to understand this principle usually fail in their studies, too.   
      
   In this concrete example, it is a matter of definition where to   
   make a cut or cuts.  It is entirely reasonable to include the pin   
   with the car, or not, but the important thing is to stick to the   
   cuts that were made, and not change them afterwards.  Otherwise,   
   errors will result.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)