From: fortunati.luigi@gmail.com   
      
   In my animation   
   https://www.geogebra.org/m/abg3ewgy   
   there is a blue force 10 directed to the right which generates an acceleration   
   10.   
      
   If I want to reduce the acceleration from 10 to 5, I need to apply a red   
   force –5 (directed to the left) on mass A, so that the net force on mass   
   A becomes equal to 5 and the acceleration also drops from 10 to 5 .   
      
   However, there is also another way to obtain the same result (reduce the   
   acceleration of A) without applying any force, just add the mass B (with   
   the appropriate box).   
      
   In this way, mass B generates a red force –5 on mass A, halving its   
   acceleration.   
      
   How is it possible that adding mass B (which is a scalar) acts *exactly*   
   like the force F=-5 (which is a scalar)?   
      
   Luigi Fortunati   
      
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