From: dr.j.thornburg@gmail-pink.com   
      
   In article , I wrote   
   > the momentum transfer during the collision is actually   
   > *two-way*, i.e., *each* body transfers some momentum to the other body.   
   > That is, during the collision A's momentum changes (because B transfers   
   > some momentum to A), AND B's momentum changes (because A transfers some   
   > momentum to B).   
   >   
   > [[...]]   
      
   > In other words, during the collison B transfers momentum -3.75 to A,   
   > so that A's momentum changes by Delta_p_A=-3.75.  AND, during the   
   > collision A transfers momentum +3.75 to B, so that B's momentum changes   
   > by Delta_p_B=+3.75.   
      
   In article , Luigi Fortunati replied   
   > Body B has a momentum -3 and, therefore, cannot transfer -3.75 to body   
   > A because it does not have it.   
      
   This is mistaken.  (Linear) momentum doesn't have an inherent zero point,   
   so there's never a case where one body doesn't have enough momentum to   
   transfer some to another body.  Rather, momentum is analogous to position   
   on a number line, where being at position -3 doesn't prevent you from moving   
   a distance 3.75 either to the right or to the left.   
      
   One way to "see this in action" is to consider what the collision would   
   look like if analyzed in a different inertial reference frame (IRF).  For   
   example, let's consider an IRF which is moving to with a velocity v=-10   
   (i.e., moving the left at a speed of 10) with respect to Luigi's original   
   IRF.  In this new IRF, each velocity is the velocity in Luigi's original   
   IRF + 10.   
      
   In this new IRF, the speeds and momenta before the collision are   
      v_A_before = +11 --> p_A_before = +55   
      v_B_before =  +9 --> p_B_before = +27   
      p_total_before = p_A_before+p_B_before = +82   
   so that after the collision, the total momentum must also be p=+82.  Hence   
   the common body of mass 8 must be moving at a speed of p/m = +10.25 after   
   the collision, and A and B's speeds and momenta after the collision must be   
      v_A_after = +10.25 --> p_A_after = +51.25   
      v_B_after = +10.25 --> p_B_after = +30.75   
      p_total_after = p_A_after+p_B_after = +82   
   The velocity changes during the collision are thyus   
      Delta_v_A = v_A_after - v_A_before = +10.25 - +11 = -0.75   
      Delta_v_B = v_B_after - v_B_before = +10.25 -  +9 = +1.25   
   and the momentum changes during the collision are   
      Delta_p_A = p_A_after-p_A_before = +51.25 - +55 = -3.75   
      Delta_p_B = p_B_after-p_B_before = +30.75 - +27 = +3.75   
      
   Notice how the velocity changes during the collision, AND the momentum   
   changes and A <--> B transfers during the collision, are exactly the same   
   as when we analyzed the collision in Luigi's original IRF.   
      
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