Free fall

[Luigi Fortunati]

The elevator in free fall in the gravitational field is an inertial
reference system.

Is the elevator in free fall in the magnetic or electric field also an
inertial reference system?

Luigi Fortunati

[Mikko]

That depends on the magnetic and electric properties of the elevator.

Mikko

[Luigi Fortunati]

Mikko il 21/01/2024 14:42:49 ha scritto:
>> The elevator in free fall in the gravitational field is an inertial
>> reference system.
>>
>> Is the elevator in free fall in the magnetic or electric field also an
>> inertial reference system?
>
> That depends on the magnetic and electric properties of the elevator.

The elevator is metallic, the body in the elevator is the same metal as
the elevator.

The field is generated by an electromagnet.

Luigi Fortunati

[[Mod. note --

The general principal is that "free fall" means no non-gravitational
forces are acting. So there are two basic ways to have "free fall":
either (1) there are no electric and magnetic fields present, and/or
(2) the free-falling bodies have no electric charges or magnetic
dipoles/quadrupoles/etc (either "native" or induced).

In particular, if there's a magnetic field present, then (2) implies
the absence of any diamagnetic or ferromagnetic materials. That means,
for example, no iron, steel, copper, or a bunch of other common metals.
-- jt]]

[Luigi Fortunati]

Luigi Fortunati il 23/01/2024 12:46:45 ha scritto:
> Mikko il 21/01/2024 14:42:49 ha scritto:
>>> The elevator in free fall in the gravitational field is an inertial reference system.
>>> Is the elevator in free fall in the magnetic or electric field also an inertial reference system?
>>
>> That depends on the magnetic and electric properties of the elevator.
>
> The elevator is metallic, the body in the elevator is the same metal as the elevator.
>
> The field is generated by an electromagnet.
>
> Luigi Fortunati
>
> [[Mod. note --
> The general principal is that "free fall" means no non-gravitational
> forces are acting.

An even more general principal is that "free fall" means "fall without obstacles".

No one can deny that, in the most remote space far from all gravity, a metal elevator falling towards an electromagnet is in "free fall".

And a metal robot in a metal elevator resting on an electromagnet (without being able to look outside) cannot know if there is a motor that is accelerating the elevator or if there is an electromagnet that is attracting it towards the floor (principle of equivalence).

Luigi Fortunati

[Jonathan Thornburg [remove -color to reply]]

Luigi Fortunati <fortuna...@gmail.com> wrote:
> An even more general principal is that "free fall" means "fall without
> obstacles".
>
> No one can deny that, in the most remote space far from all gravity,
> a metal elevator falling towards an electromagnet is in "free fall".

This is mistaken: In this context a magnetic field counts as an "obstacle",
and a magnetic material interacting with a magnetic field is *not* in
free fall.

> And a metal robot in a metal elevator resting on an electromagnet
> (without being able to look outside) cannot know if there is a motor
> that is accelerating the elevator or if there is an electromagnet that
> is attracting it towards the floor (principle of equivalence).

A metal elevator will partially, but not fully, screen the magnetic field,
so there will still be a nonzero magnetic field inside the elevator.
This means that the robot can easily tell the difference: just see if,
inside the elevator, there's a differential acceleration between test
masses inside made of iron (ferromagnetic) vs oxygen (diamagnetic) vs
helium (non-magnetic).

--
-- "Jonathan Thornburg [remove -color to reply]" <dr.j.th...@gmail-pink.com>
currently on the west coast of Canada
"what I still don't understand to this day is why a suicide bomber is
cowardly and deceitful, and the bomber pilot who throws bombs at innocent
people from a height of five kilometers is courageous and brave."
-- Volker Pispers (German comedian)

[Mikko]

> Luigi Fortunati il 23/01/2024 12:46:45 ha scritto:
>> Mikko il 21/01/2024 14:42:49 ha scritto:
>>>> The elevator in free fall in the gravitational field is an inertial
>>>> reference system.
>>>> Is the elevator in free fall in the magnetic or electric field also an
>>>> inertial reference system?
>>>
>>> That depends on the magnetic and electric properties of the elevator.
>>
>> The elevator is metallic, the body in the elevator is the same metal as
>> the elevator.
>>
>> The field is generated by an electromagnet.
>>
>> Luigi Fortunati
>>
>> [[Mod. note --
>> The general principal is that "free fall" means no non-gravitational
>> forces are acting.
>
> An even more general principal is that "free fall" means "fall without
> obstacles".

Not really. It is the same or less general depending on the interpretation
of the word "obstacle".

If there is a non-gravitational interaction that does not prevent the fall
the fall is not a free fall but one may say that there is no obstacle.

Mikko

[Tom Roberts]

On 1/25/24 2:32 AM, Luigi Fortunati wrote:
>> [[Mod. note -- The general principal is that "free fall" means no
>> non-gravitational forces are acting.

Yes.

> An even more general principal is that "free fall" means "fall
> without obstacles".

No. Any "obstacle" would necessarily exert a force on the object, but
some forces are not obstacles. The moderator's statement is more
general, and is indeed the accepted meaning of "freefall" in physics.

Do not attempt to redefine the meanings of common words and phrases
-- that will prevent you from communicating with other people,
especially physicists.

> No one can deny that, in the most remote space far from all gravity,
> a metal elevator falling towards an electromagnet is in "free fall".

Not true: every physicist would deny that, because the electromagnet
exerts an electromagnetic force on the elevator, making it NOT be
in freefall.

You REALLY need to get a book on basic physics and STUDY it. Your
repeated questions around here are not effective in your learning
physics. Better yet, enroll in a physics course at a local college or
university, so you will have an instructor to discuss these issues.

Tom Roberts

[[Mod. note -- I second these recommendations. A few of the (many)
excellent introductory physics books available are those described in:
https://en.wikipedia.org/wiki/Physical_Science_Study_Committee
https://en.wikipedia.org/wiki/Harvard_Project_Physics
https://en.wikipedia.org/wiki/Fundamentals_of_Physics
-- jt]]

[Luigi Fortunati]

Tom Roberts il 27/01/2024 09:23:06 ha scritto:
>> No one can deny that, in the most remote space far from all gravity, a metal elevator falling towards an electromagnet is in "free fall".
>
> Not true: every physicist would deny that, because the electromagnet
> exerts an electromagnetic force on the elevator, making it NOT be
> in freefall.

In the gravitational field the force of gravity acts on the elevator in the same way as, in the electromagnetic field, the electromagnetic force acts on the metal elevator.

Force that accelerates is one and force that accelerates is the other!

Luigi Fortunati

[Luigi Fortunati]

Tom Roberts il 27/01/2024 09:23:06 ha scritto:

>> No one can deny that, in the most remote space far from all gravity, a metal elevator falling towards an electromagnet is in "free fall".
>
> Not true: every physicist would deny that, because the electromagnet
> exerts an electromagnetic force on the elevator, making it NOT be
> in freefall.

Gravity also exerts a force on the elevator.

Force is electromagnetic and force is gravitational.

Luigi Fortunati

[Tom Roberts]

On 1/30/24 2:27 AM, Luigi Fortunati wrote:
> Tom Roberts il 27/01/2024 09:23:06 ha scritto:
>>> No one can deny that, in the most remote space far from all
>>> gravity, a metal elevator falling towards an electromagnet is in
>>> "free fall".
>> Not true: every physicist would deny that, because the
>> electromagnet exerts an electromagnetic force on the elevator,
>> making it NOT be in freefall.
>
> Gravity also exerts a force on the elevator.

Not in GR, which is now the generally-accepted theory of gravitation.

But to forestall your (baseless) objection, the standard definition of
freefall says no NON-GRAVITATIONAL forces are acting on the object.

I repeat: posting zillions of questions here is not helping you learn
very basic physics. Get a good textbook and STUDY, or better, enroll in
a physics course at a college or university.

Tom Roberts

[Luigi Fortunati]

Tom Roberts il 31/01/2024 08:59:59 ha scritto:
>> Gravity also exerts a force on the elevator.
>
> Not in GR, which is now the generally-accepted theory of gravitation.

The fact that it is "generally" accepted does not make it an absolute
truth (absolute truths do not exist).

> But to forestall your (baseless) objection, the standard definition of
> freefall says no NON-GRAVITATIONAL forces are acting on the object.

This standard definition is as legitimate as the one that says: a
metallic body that "falls" towards an electromagnetic pole accelerates
in the same way as the body that "falls" towards a similar center of
gravity.

The two accelerations are indistinguishable from each other (one
acceleration is as good as the other).

Are you able to demonstrate that it is possible to distinguish one
acceleration from another?

Luigi Fortunati


[[Mod. note --

The problem with the "magnetic free-fall" definition is that the
acceleration differs from one metallic body to another. For example,
a copper body will experience different magnetic forces -- and hence
a different acceleration -- than an iron body.

In contrast, with the standard definition of free-fall, free-fall is
*universal*, i.e., *all* (non-spinning test) bodies have the same
(zero) non-gravitational forces, and (experimentally) they all have
the same free-fall acceleration.
-- jt]]

[Luigi Fortunati]

Luigi Fortunati il 01/02/2024 01:44:37 ha scritto:
> [[Mod. note --
> The problem with the "magnetic free-fall" definition is that the
> acceleration differs from one metallic body to another. For example,
> a copper body will experience different magnetic forces -- and hence
> a different acceleration -- than an iron body.
>
> In contrast, with the standard definition of free-fall, free-fall is
> *universal*, i.e., *all* (non-spinning test) bodies have the same
> (zero) non-gravitational forces, and (experimentally) they all have
> the same free-fall acceleration.
> -- jt]]

Ok, let's call "free fall" only the gravitational one which is universal (the same for everyone) and "non-free fall" the one with the addition of electromagnetic acceleration.

On the scaffolding there are the painter and the metal robot (with the same mass) and underneath there is an electromagnet with a force equal to the gravitational one.

Obviously, the electromagnet only acts on the robot and not on the painter.

When they are on the scaffolding, the weight force of the robot is double (despite having the same mass as the painter) because it is subject to two forces: gravitational and electromagnetic.

When they fall, the robot's acceleration is double that of the painter because it is generated by two forces instead of just one.

Therefore, let's call that of the painter "free fall" (without electromagnetic acceleration) and that of the robot "non-free fall " (because there is the addition of electromagnetic acceleration).

And now we see the effects of "free fall" and " non-free fall ".

In the "free fall" of the painter, Einstein says that the gravitational force disappears (because it is not a real force) and I ask: does the gravitational acceleration of the painter in free fall also disappear like the gravitational force or does the force disappear and not the acceleration ?

And during the robot's "non-free fall", which force (gravitational or electromagnetic) disappears and which does not? And which acceleration disappears and which does not?

Luigi Fortunati

[Luigi Fortunati]

There is a well that reaches exactly the center of the Earth and there
are two elevators: one stationary at the bottom of the well (elevator A)
and the other in free fall (elevator B).

Are the two elevators inertial reference systems of the same type or is
one reference system more inertial than the other?

Luigi Fortunati

[Mikko]

The elevator B is moving so it is affected by air resistance and other
friction effect. It also meets the fast varying tidal effects from
density variations in the matter around the well.

If these effects can be kept small enough that they cannot be detected
in B then B is in free fall.

If the well is deep enough that the elevator A does not touch its
bottom the elevator A is in free fall.

If both elevators are in free fall in the above sense then there is
no detectable difference in the conditions inside of the elevator.

When the two elevators collide neither of the elevators is no longer
in free fall.

--
Mikko

[Luigi Fortunati]

Mikko il 06/02/2024 09:26:51 ha scritto:
>> There is a well that reaches exactly the center of the Earth and there
>> are two elevators: one stationary at the bottom of the well (elevator A)
>> and the other in free fall (elevator B).
>>
>> Are the two elevators inertial reference systems of the same type or is
>> one reference system more inertial than the other?

> The elevator B is moving so it is affected by air resistance and other
> friction effect. It also meets the fast varying tidal effects from
> density variations in the matter around the well.
>
> If these effects can be kept small enough that they cannot be detected
> in B then B is in free fall.
>
> If the well is deep enough that the elevator A does not touch its
> bottom the elevator A is in free fall.
>
> If both elevators are in free fall in the above sense then there is
> no detectable difference in the conditions inside of the elevator.
>
> When the two elevators collide neither of the elevators is no longer
> in free fall.

You're twisting everything I wrote.

I wrote that the well ends at the center of the Earth (the bottom of the
well is at the center of the Earth).

That elevator A is STOPPED at the bottom of the well and, therefore, is
stationary at the center of the Earth.

That elevator A stays still not because the bottom of the shaft prevents
it from falling but because (at the center of the Earth) there is no
force of gravity and no space-time curvature that sets it in motion to
go somewhere: it stays still because nothing and no one pushes him
somewhere.

That elevator B is in free fall without friction and without resistance.

That elevator B is falling (in free fall) towards elevator A which it
will collide with in the future.

I would like to know *now* (before their clash) if they are both
inertial reference systems and if they are in the same way or if one is
more inertial than the other.

I hope I have been clear.

Luigi Fortunati

[Mikko]

The elevator at the bottom is not inertial. If the well were deeper the
elevator could but the bottom prevernts that.

--
Mikko

[Luigi Fortunati]

Mikko il 06/02/2024 15:26:36 ha scritto:
> The elevator at the bottom is not inertial. If the well were deeper the
> elevator could but the bottom prevernts that.

The bottom doesn't impede anything and, to prove it, in my new animation https://www.geogebra.org/m/mdymaxsb I totally eliminate it.

Now, the well goes from one part of the Earth to the other.

Initially, elevator A is constrained near the surface and elevator B is constrained to the center of the earth: neither of the 2 elevators can move.

If you remove the constraints with the appropriate button, elevator A accelerates in free fall and elevator B remains stopped in its place.

Question: Is the unconstrained elevator B that stays in its place an inertial or accelerated reference frame?

Luigi Fortunati

[Mikko]

On 2024-02-11 08:54:07 +0000, Luigi Fortunati said:

> Mikko il 06/02/2024 15:26:36 ha scritto:

>> On 2024-02-06 12:21:12 +0000, Luigi Fortunati said:

>>> Mikko il 06/02/2024 09:26:51 ha scritto:

>>>> If the well is deep enough that the elevator A does not touch its
>>>> bottom the elevator A is in free fall.

>>> I wrote that the well ends at the center of the Earth (the bottom of the
>>> well is at the center of the Earth).
>>>
>>> That elevator A is STOPPED at the bottom of the well and, therefore, is
>>> stationary at the center of the Earth.

>> The elevator at the bottom is not inertial. If the well were deeper the
>> elevator could but the bottom prevernts that.
>
> The bottom doesn't impede anything and, to prove it, in my new
> animation https://www.geogebra.org/m/mdymaxsb I totally eliminate it.

The new problem is essentially different from the one discussed above.
As can be seen, the elevator at the centre of Earth (the labels are
swapped in the new problem, so now it is B) so that its centre is at
the centre of Earth, which would be impossible in the original problem
as half of it would be under the bottom of the well.

The new problem is better if the intent was to make both elevators
as inertial as possible. The additional constraint that prevents
the collision at the centre of Earth causes a small deviation from
inertiality but its direction is sideways so it does not affect
the most important motions, which are in the vertical direction.

--
Mikko

[Luigi Fortunati]

Mikko il 12/02/2024 12:53:21 ha scritto:
> The new problem is essentially different from the one discussed above.
> As can be seen, the elevator at the centre of Earth (the labels are
> swapped in the new problem, so now it is B) so that its centre is at
> the centre of Earth, which would be impossible in the original problem
> as half of it would be under the bottom of the well.
>
> The new problem is better if the intent was to make both elevators
> as inertial as possible. The additional constraint that prevents
> the collision at the centre of Earth causes a small deviation from
> inertiality but its direction is sideways so it does not affect
> the most important motions, which are in the vertical direction.

To avoid any possible collision (so no one gets hurt), in my animation
https://www.geogebra.org/m/etm68buf I left only one elevator.

Well, is elevator A stopped in his place (before and after the cables
broke) an accelerated or inertial reference?

Luigi Fortunati