Re: Inertia and third principle

Luigi Fortunati il 30/07/2024 19:05:34 ha scritto:

In my animation https://www.geogebra.org/m/qterew9m ... 

If body A passes some momentum to body B, body B must politely return  to body A the *same* momentum it received, no more and no less.
[[Mod. note -- Newton's laws say that (assuming that there are no external forces acting) the *total* momentum of the system remains constant. 

Sure! The *total* momentum of the system remains (undoubtedly) constant, this is established by the law of conservation of momentum which I have never disputed.

If we look at the momentum *changes* during the collision, we have   p_A_after - p_A_before = 3.75 - 15 = -11.25 , while B's momentum change during the collision is   p_B_after - p_B_before = 2.25 - -9 = +11.25 so the momentum *changes* are indeed precisely opposite. -- jt]]

Yes, even the *changes* in momentum during the collision are equal and opposite, otherwise the law of conservation of momentum which I have never disputed would not be correct.

My objection concerns something else, it concerns the equality between action and reaction (the third principle).

In the case of my animation, the action of body A is worth +15 because there are 15 particles and their impulses directed towards the right are worth +15 (during the collision the momentum p=mv becomes impulsive force F=ma because the velocity changes and the change in speed is acceleration).

Instead, the reaction of body B is worth -13.75 because it is formed by the impulses of the 9 particles (directed towards the left) which are worth -9 and by the inertial reaction of body B (which opposes the external force of body A) which is worth -3.75.

The residue +2.25 (+15-13.75) is the net force that accelerates body B to the right.

Therefore, the +15 action of body A is greater (and not equal) than the -13.75 reaction of body B.

Luigi Fortunati