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From: Luigi Fortunati <fortunati.luigi@gmail.com>
Newsgroups: sci.physics.research
Subject: Inertia and third principle
Date: Sat, 22 Jun 2024 02:16:01 PDT
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Approved: Jonathan Thornburg [remove -color to reply]" <dr.j.thornburg@gmail-pink.com (sci.physics.research)
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In my animation https://www.geogebra.org/m/gxqwmxah at the moment of 
the collision the F1 and F2 forces of action and reaction between the 
two bodies A and B are activated.

Is it true that force F1 is the sum of the blue forces of the particles 
of body A and force F2 is the sum of the red forces of the particles of 
body B?

Could the forces F1 and F2 exist if the blue and red forces of the 
particles of bodies A and B did not exist?

Luigi Fortunati

[[Mod. note -- At the moment of collision, the only impact forces are
those on the parts of the bodies which are in contact (labelled as points
A and B in your very nice animamation).  These then propagate pressure
waves into the bodies, which apply forces on the rest of the bodies.
So, the small red and blue arrows which your (very nice!) animation
shows as being applied to each mass point of bodies A and B at the
moment of collision are wrong -- the actual forces are very non-uniform
across each body (as well as being highly time-dependent).

Here's an article with actual data of this sort:
  Ross, Patel, and Wenzel
  "Vehicle Design and the Physics of Traffic Safety"
  Physics Today vol 49 (Jan 2006), pages 49-54
The /Physics Today/ article is behind a paywall, but as of a few minutes
ago google scholar says there's a free copy at
  https://people.cas.uab.edu/~nordlund/Courses/PH201/Links/vol59no1p49_54.pdf
The article has two graphs showing velocity as a function of time for
various parts of cars during car crashes.  You can clearly see how
different parts of a car have different velocity profiles, and hence
different accelerations.

But, I think the answer to your first question may still be "yes".  After
all, ultimately the net force on body A is the sum of all the forces on
A's individual mass points, and similarly for B.

As for your second question, suppose that instead of two solid bodies,
you just had a pair of clouds of dust moving towards each other.  Then
they'd interpenetrate and basically go right through each other, so
the "impact" forces F1 and F2 would both be zero.
-- jt]]