Path: csiph.com!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail From: pa@see.signature.invalid (Pierre Asselin) Newsgroups: sci.physics.research Subject: Re: Elastic Collision Date: Fri, 13 Feb 2026 18:05:45 PST Organization: PANIX Public Access Internet and UNIX, NYC Lines: 32 Approved: Jonathan Thornburg [remove -color to reply]" References: <10mhc4k$avqk$1@dont-email.me> <10mlemh$1pima$1@dont-email.me> X-Trace: individual.net 8NGIqIVmyjZSVDOy52wlxQ6+Cq4mxv8Qnizo2Rn/ZLVJY/OGB9OGP4WyvX Cancel-Lock: sha1:kDNb4MjvAvT/mB8O+ZIt5PNQkP8= sha256:C/BMLb3ahYgOOArLDwQudyzzgfxGAw4mHvj9crmlchE= Xref: csiph.com sci.physics.research:11845 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. > [ ... ] > 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, And yet, it bounces back, though with a reduced velocity. > otherwise a body of mass 3m, 10m, or 100m would also bounce back. No, a body of mass 3m would stop cold, and bounce the body of mass m with velocity 2*v. More massive bodies would continue forward with reduced velocity, and bounce the smaller body with increased velocity. > 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! Slows down a bit and bounces the smaller body with velocity approaching 3*v. The formulas are in the text above the animations. (Mind the signs of the vA1,vB1 and vA2,vB2.) -- pa at panix dot com