From: fortunati.luigi@gmail.com   
      
   Il 12/02/2026 07:30, Luigi Fortunati ha scritto:   
   > The Wikipedia entry for "Elastic collision"   
   > https://en.wikipedia.org/wiki/Elastic_collision   
   > contains the following animation   
   > https://youtu.be/wl0c6NMysY4   
   > where the two bodies collide at point x and instantly reverse direction.   
   >   
   > Does this seem correct?   
   >   
   > Can the 2m mass body be stopped at point X of the collision and pushed   
   > back by the smaller body?   
   >   
   > Luigi Fortunati   
   >   
   > [[Mod. note --   
   > The Wikipedia animations assume (1) Newtonian mechanics, (2) 1-D motion   
   > with no other forces acting, and (3) the elastic collisions occur very   
   > quickly (i.e., each body's acceleration is nonzero for only a short time).   
   > And saying that the collisions are *elastic* implies that there's no   
   > permanent deformation of either body after the collision.   
   >   
   > Within these assumptions, yes, the Wikipedia animations look correct.   
   >   
   > The answer to your question "Can the 2m mass body be stopped at point X   
   > of the collision and pushed back by the smaller body?" is yes, that's how   
   > Newtonian mechanics works.   
   >   
   > The Wikipedia article includes a section "Derivation of solution" which   
   > nicely explains how to derive the solution from conservation of momentum   
   > (which always holds) and conservation of energy (which holds in an elastic   
   > collision).   
   > -- jt]]   
      
   I dispute what the moderator wrote.   
      
   A body of mass 2m cannot bounce back (in place!) when it collides with a   
   body of mass m, otherwise a body of mass 3m, 10m, or 100m would also   
   bounce back.   
      
   It's obvious that a body of mass 100m, colliding with a body of mass m,   
   can only slow down but not stop in place and bounce back!   
      
   So, there should be a mass limit within which a larger body bounces off   
   a smaller body and beyond which it slows down but does not stop and does   
   not come back.   
      
   Does this limit exist? I don't think so.   
      
   Luigi Fortunati   
      
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