Inclined plane

[Luigi Fortunati]

In my animation
https://www.geogebra.org/m/thsyz5me
there is a body on a frictionless plane.

Each inclination of the plane corresponds to precisely identified
forces.

If the plane is horizontal (angle=0 degrees) the body stands still
because there are two opposite and equal forces (black and blue).

If the plane is inclined (0<angle<90 degrees) there are 3 forces:
2 (black and blue) opposite and equal (but of decreasing intensity)
and 1 (red and unopposed) of increasing intensity.

If the plane is vertical (angle=90 degrees) there is only one red force.

Am I wrong if I say that these forces transform into each other (and
vice versa) but are not created or destroyed?

Luigi Fortunati

[[Mod. note -- I would say that that's somewhat confusing terminology.
Forces are never "created" or "destroyed", i.e., there's no active agent
which "creates" or "destroys" a force, and there's no differential
equation governing such a process.

I find the following conceptualization to be more useful: There are
certain forces acting in a given physical system, and if the physical
system changes (such as changing the plane's inclination angle), the
forces may also change.
-- jt]]

[Luigi Fortunati]

Luigi Fortunati il 23/01/2024 12:38:54 ha scritto:
> [[Mod. note -- I would say that that's somewhat confusing terminology.

> Forces are never "created" or "destroyed"...

That's exactly what I say too.

It is Einstein who talks about forces that "disappear": see the case of
the painter when he falls from the scaffolding (and no longer perceives
any force) and the case of the elevator when the cables break (and
whoever is inside floats while he falls) .

In this regard, I prepared my animation
https://www.geogebra.org/m/bqvyp3wk
where there is the Earth (which is worth as much as the painter's
scaffolding and the elevator cables), there is the body A resting on
the ground (like the painter on the scaffolding and the lift stopped at
the floor) and there is body B in free fall (like the painter falling
and like the lift with broken cables).

What disappears in body B compared to body A? The two real forces (blue
and black) disappear and not the red gravitational force which first
*compressed* the body and the Earth against each other and, afterwards,
*accelerates* the body towards the center of the Earth (and the center
of the Earth towards the body).

What disappears are the two real forces of action and reaction, and not
the force of gravity!

Luigi Fortunati

[Mikko]

On 2024-01-28 22:58:55 +0000, Luigi Fortunati said:

> Luigi Fortunati il 23/01/2024 12:38:54 ha scritto:
>> [[Mod. note -- I would say that that's somewhat confusing terminology.
>> Forces are never "created" or "destroyed"...
>
> That's exactly what I say too.
>
> It is Einstein who talks about forces that "disappear": see the case of
> the painter when he falls from the scaffolding (and no longer perceives
> any force) and the case of the elevator when the cables break (and
> whoever is inside floats while he falls) .

If you want to see a force appear, the initial position of the ball
must be above the plane, not touching it.

If you want to see a force disappear you can use a spherical surface
istead of a plane.

Mikko

[Luigi Fortunati]

In the post you are replying to, my animation is not the same as before
but it is https://www.geogebra.org/m/bqvyp3wk where the surface is
spherical.

In this animation there are red gravitational forces that have two
distinct functions:
- they compress body A against the Earth, without accelerating either
body A or the Earth (therefore, they are forces that, in this case,
compress but do not accelerate)
- and they accelerate body B in free fall towards the Earth (and vice
versa) without compressing (therefore, they are forces which, in this
case, accelerate and do not compress).

These red forces never disappear because in the gravitational field
(where bodies A and B are found) they are always present and act (as I
specified above) both on stationary bodies (like body A) and on those
in free fall (like body B).

Only the effect that the red forces generate changes: compression in
one case and acceleration in the other.

My animation therefore demonstrates that (in free fall) the only forces
that disappear are the blue and black contact forces (action and
reaction) but the red gravitational forces do not disappear, the
effects of which are evident and measurable.

Luigi Fortunati