On 2024-01-21 09:04:19 +0000, Luigi Fortunati said:

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.

Mikko

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Luigi Fortunati <fortunati.luigi@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.thornburg@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)

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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

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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]]

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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

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On 2024-02-05 08:10:44 +0000, Luigi Fortunati said:

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

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

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

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

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

--
Mikko

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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

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On 2024-03-15 01:11:39 +0000, Luigi Fortunati said:

In free fall, can you go anywhere freely or are there constraints that prevent this?

There are constraints. A free fall requires empty space.
A free fall to the end of the empty space ends there.

Of course you can't fall straight up and you can't fall sideways.

You can if you have initial motion in that direction.

--
Mikko

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On 3/15/24 3:11 AM, Luigi Fortunati wrote:

In free fall, can you go anywhere freely or are there constraints
that prevent this? Of course you can't fall straight up and you
can't fall sideways.

As I keep saying, this depends on the meanings of the words you use.
Your wishy-washy words are a major part of your failure to
understand very basic physics.

If by the "direction of fall" you mean the 3-velocity relative to
ground-based coordinates, that can be pointed in any direction. If you
mean the 3-acceleration relative to ground-based coordinates, that can
only be pointed straight down. Note the former is the usual meaning for "direction" of any motion, including falling.

Hint: throw a ball straight up. It is moving upward, it is going upward,
and its 3-velocity (relative to ground-based coordinates) is directed
upward. It is, of course, in free fall (neglecting air resistance). So
one COULD say "it is falling upward", but that is such poor terminology
that no physicist would way that.

In free fall you can only go in one direction (the vertical one) and
in only one versus (downward).

This is just plain not true (here you use words with more definite
meanings). You can be GOING up or sideways -- that depends on the
initial conditions of your trajectory. Because "going" explicitly refers
to velocity, not acceleration. Your acceleration (relative to
ground-based coordinates) is always downward.

The elevator (in free fall) and everything inside it are forced to
fall (always) vertically and (always) downwards.

Nope. See above.

So there is a constraint.

There is a constraint on the 3-acceleration (relative to ground-based coordinates): downward. There is no constraint on the direction of
3-velocity (relative to ground-based coordinates), because this depends
on the initial conditions; of course its direction will point
increasingly downward as time goes on.

And, in free fall, can one move in a straight and uniform motion?

Yes, RELATIVE TO COORDINATES ACCELERATING DOWNWARD WITH YOU. No,
relative to ground-based coordinates.

[This opens the door to the "local vs. global"
distinction in GR. You have no hope of appreciating
the subtleties involved until you STUDY.]

No, in free fall the motion is always accelerated.

Again, this depends on what you mean by those words. In Newtonian
mechanics this is true. But we live in a post-GR world, and presuming
Newtonian mechanics is not appropriate. In GR, of course, an object in
free fall follows a geodesic through spacetime, with ZERO proper
acceleration.

So why call it "free fall" and not "forced fall"?

Because as the moderator said, it means that no NON-GRAVITATIONAL force
acts on the object. "It's a statement about what forces are (not) acting
on the body, not about the uniqueness or non-uniqueness of the resulting motion."

As I keep telling you, your approach of making false statements in this newsgroup is utterly failing to teach you basic physics. You MUST get
some good textbooks and STUDY. Better yet, take a college or university
course in physics so you'll have an instructor with whom to discuss your confusions.

Tom Roberts

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On 3/24/24 3:29 PM, Luigi Fortunati wrote:

[Moderator] That is, the gravitational force on a body with
inertial mass 2 kg is (a) precisely twice that on a body with
inertial mass 1 kg,

But (if I'm not mistaken) (a) also applies to the electromagnetic
force which, on a 2-gram body of any material, is exactly double
that on a 1-gram body of any material. Is that so?

Not at all! -- If one is charged and one is not, the electromagnetic
forces on them will be VERY different.

I repeat: your approach of making false statements here and expecting to
be corrected is not working, and you are CLEARLY not learning much
physics, if any. You NEED to get some good textbooks and STUDY. Better
yet, enroll in a college or university physics course, so you will have
an instructor with whom you can discuss your many misconceptions and confusions.

Tom Roberts

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Luigi Fortunati wrote:

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

That will depend upon the corresponding momentum of the gravitons
involved in the free fall. The fabric of spacetime will bend accordingly,
to account for the apparent distortions to the so-called magnetic and
electric fields.

bt

[[Mod. note -- We don't need the concept of "graviton" here -- a
classical picture is fine.
-- jt]]

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