Re: Newton's Gravity

Luigi Fortunati il 08/01/2025 03:11:43 ha scritto:

Jonathan Thornburg [remove -color to reply] il 08/01/2025 08:49:57 ha
scritto:

...
    Let's now imagine that M1 and M2 are held apart by a light stick
    so as be at a fixed distance from each other, forming a "dumbbell"
    (still at reset in a Newtonian IRF, and let's say floating out in
    space far from any other masses).   Then (if the force of M1 on M2
    were NOT equal-in-magnitude-and-opposite-in-direction to the force
    of M2 on M1), that nonzero "total gravitational force on M1+M2"
    would accelerate the dumbbell with respect to the Newtonian IRF,
    violating the law of conservation of momentum.

>
This reasoning of yours is very interesting; after reading it I went
for a walk, which is my favorite way to think.
>
It was an intense and very pleasant half hour.
>
If there is a stick that keeps the two bodies at a fixed distance, it
means that there are other forces besides the gravitational ones.
>
Without the stick there is only the gravitational force of body A on
body B and the opposite gravitational force of B on A.
>
These two forces add together because they both make up the overall
gravitational force that attracts the two bodies towards each other.
>
The stick has another function completely opposite to the attractive
force of gravity and, in fact, the stick repels instead of attracting,
it opposes, both on the side of body A and on the other side, opposing
the approach of the two bodies.
>
We therefore have two attractive forces and two repelling forces.
>
The set of these four forces is absolutely balanced: the sum of the
forces directed toward the approach is exactly equal and opposite to
the sum of the forces directed toward the separation.
>
If the stick is not there, bodies A and B approach each other by
accelerating (attractive gravitational forces), if the stick is there,
the two bodies A and B stop approaching each other (repelling reaction
forces).
>
These are two distinct conditions: free motion (without the stick)
constrained motion (with the stick).
>
If I jump from the wall, while I am in the air the Earth attracts me
towards it and I also attract the Earth towards me: we both exert an
attractive force.
>
When I land on the floor, the floor repels me and I repel the floor (we
both repel each other preventing the approach).
>
I am against very long posts that are dispersive, so I will stop here.
>
In the next post I will focus on the equality or otherwise of the two
distinct pairs of forces in relation to the conservation of momentum.

In my animation https://www.geogebra.org/m/ntefhssz I have visualized
the two bodies A and B with their respective decreasing gravitational
fields.

Body B (smaller) is entirely immersed in the strong red ring of force
10 of the gravitational field of A, while body A (whose center of
gravity is far from body B) is only marginally touched by the weak
gravitational force of body B.

If we reduce body B even further to the minimum of its mass (with the
appropriate slider), the gravitational force that B experiences from
body A is at its maximum, while (on the contrary) the gravitational
force of body B becomes practically non-existent and also acts only in
one point and not on the whole of body A (more or less like the
gravitational force of my body acts very weakly on only one point of
the Earth (my room at most) and not on the whole Earth.

Instead, if we increase the mass of body B to the maximum, it becomes
equal to that of body A and, only at this point, the two opposing
gravitational forces (A towards B and B towards A) become totally
equal, adding their mutual attractive effects.

Luigi Fortunati