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From: Luigi Fortunati <fortunati.luigi@gmail.com>
Newsgroups: sci.physics.research
Subject: Re: Inertia and third principle
Date: 24 Jun 2024 12:24:36 GMT
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Luigi Fortunati il 21/06/2024 21:16:01 ha scritto:
> 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.

The net force on body A is the vector sum of the endless blue internal
forces of its own particles (which push body A to the right) and the
single external red force F2 (which pushes body A to the left).

Luigi Fortunati