From: mikko.levanto@iki.fi   
      
   On 13/02/2026 09:26, Luigi Fortunati wrote:   
   > 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.   
      
   That is not always true. If the smaller body is not stationary before   
   the collision it may have enough momentum to stop the larger body and   
   make it bounce back. As long as there are no interaction with any   
   third body the speed of the center of mass of the pair is unchanged.   
      
   --   
   Mikko   
      
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