Re: Inertia and third principle

Force compresses and accelerates.

Compression depends on the reaction because there can be no compression if there is no reaction.

Instead, the acceleration depends precisely on the lack of the reaction that makes the force "net", i.e. not counteracted by an opposite force (it is precisely the net force F=ma of the 2nd law).

The force, therefore, is divided into force that compresses and force that accelerates (which is the excess, i.e. what remains of the total force diminished by the reaction).

If I push against the wall I only generate compression because there is no excess of the action over the reaction.

If I push a cart, with part of my force I generate compression (at the point of contact against the reaction of the cart) and with the remaining part I generate acceleration of the cart (without reaction).

At instant 3 of my animation https://www.geogebra.org/m/qterew9m the impulsive force of body A against body B (+9) and that of body B against body A (-9) generate compression and no acceleration.

At instant 5, the force +15 of body A generates an opposite reaction -13.75 of body B which compresses the two bodies (+13.75 and -13.75) and with the remaining part +2.25 (+15-13.75) accelerates body B , without reaction.

At the end of the collision, the +6 sum of the momentum of body A (+3.75) and body B (+2.25) is perfectly equal to the total momentum before the +6 collision: the conservation of momentum hi is safe.

But the total +15 impulsive force exerted by body A on body B is not at all equal to the -13.75 impulsive reaction force of body B on body A.

Therefore, the equality of only the opposite compressive forces (+13.75 and -13.75) is true and is correct but the equality of all the forces exchanged between body A and body B is not true (+15 and -13.75) .

Luigi Fortunati