Re: Inertia and third principle

As was pointed out to me, what happens in my animation https://www.geogebra.org/m/gxqwmxah is rather complex because it varies over time and generates pressures in the two bodies.

All true, but the impact time is very short and the forces acting in that brief instant can be considered as a whole.

The first overall consideration is that the forces of the collision are of inertial origin, the blue forces are activated due to the inertia of the particles of body A which would like to go to the right and cannot do so because the inertia of the particles of body B opposes (red forces).

And the inertia (red forces) of the particles of body B that would like to go to the left cannot do so because the inertia of the particles of body A opposes it (blue forces).

In this way, only compression is generated and no acceleration is generated because there is no net force.

As for this animation, there is nothing else to say.

Things get complicated when the masses of the two bodies A and B are not equal, as happens in my animation https://www.geogebra.org/m/tmwxph9z where the mass of body A is double that of body B ( mA=2mB).

In this case, the compression is not due to all the blue forces versus the red forces, because the blue forces are double the red forces!

And then, half of the blue forces (on one side) and all the red forces (on the other) generate compression, while the remaining half of the blue forces (which has no reaction forces) generates acceleration towards the right of the body AB (the two bodies A and B are now unified after the collision).

All this can be seen better in my animation https://www.geogebra.org/m/bxndbjwp where I added what happens after the collision: bodies A and B remain stationary, bodies C and D go to the right (a due to the net force), the bodies E and F go towards the left.

The action and reaction forces of the third principle are those where the reaction exists.

Instead, the forces that accelerate without compressing are those where there is no reaction and, therefore, these forces must be excluded from the third law.

Luigi Fortunati

Ps. The velocity v=1/3 of the unified body CD after the collision, calculated with half the blue force (FC/2=1) divided by the unified mass of the body CD (m=3) is perfectly identical to the velocity after the collision calculated by conservation of momentum.