Re: Inertia and third principle

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.
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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?
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Could the forces F1 and F2 exist if the blue and red forces of the particles of bodies A and B did not exist?
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Luigi Fortunati
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[[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).
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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.
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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