A rod that continuously lengthens and shortens without deforming or
breaking is elastic.

If it were rigid, it could not help but warp or break.

In my animation
https://www.geogebra.org/m/mqejqtkj
the traveling twin rotates the rod AB.

In the terrestrial reference, that rod lengthens and shortens
continuously.

And it's a *rigid* auction.

Since no rigid rod can stretch and contract with impunity (without
bending or breaking), evidently those stretches and contractions are
apparent.

Or is there another explanation?

[[Mod. note -- Yes, there is another explanation: "rigid" only applies
in the rod's own inertial reference frame. Observers in other inertial reference frames may (will) observe non-ridity, but that's an artifact
of their motion and doesn't affect the rod.

ALSO, note that here we're ignoring all the logical difficulties and
paradox associated with the concept of "rigid rotation" in special
relativity, e.g., those discussed in
https://en.wikipedia.org/wiki/Ehrenfest_paradox
https://en.wikipedia.org/wiki/Born_rigidity
-- jt]]

--- SoupGate-Win32 v1.05
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Op 1/09/2023 om 21:06 schreef Luigi Fortunati:

A rod that continuously lengthens and shortens without deforming or
breaking is elastic.

If it were rigid, it could not help but warp or break.

In my animation
https://www.geogebra.org/m/mqejqtkj
the traveling twin rotates the rod AB.

In the terrestrial reference, that rod lengthens and shortens
continuously.

And it's a *rigid* auction.

Since no rigid rod can stretch and contract with impunity (without
bending or breaking), evidently those stretches and contractions are apparent.

Or is there another explanation?

After all the examples discussed here, you still struggle with simple features like length contraction (and the reality of it:)

Your animation is giving a false impression of a length contracted wheel, or rotating rod:

It seems to operate in a rest system, which of course it isn't: the whole rotating apparatus ought to be moving across the screen. No inertial system will see a length contracted object "belonging to it", ie, not "moving along".

You should moreover keep in mind (which obviously you don't) that the length contracted object is described by events (at its inner and outer extremities) that are simultaneous in the observing (moving) system, but that are not simultaneous in the
rotating rod's (or wheel's) own system!

So there is no "conflict of rigidity" between the two systems.
As moderator said, conflicts of rigidity arise when swapping "relativistically fast" between different inertial states, ie when accelerating relativistically.

So the rotating rod, or wheel, may feel rigidity problems because it is rotating relativistically, but certainly *not* because another party is looking at it while passing by at relativistic speeds!!

And as always, don't forget the difference between *calculating Lorentz objects* and *looking at Doppler/Einstein objects*. When looking at relativistic wheels you might see this:
https://www.tempolimit-lichtgeschwindigkeit.de/rad

--
guido wugi

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wugi il 03/09/2023 09:21:35 ha scritto:

And as always, don't forget the difference between *calculating Lorentz objects* and *looking at Doppler/Einstein objects*.

I really want to talk about *calculating Lorentz objects* and,
therefore, let's leave aside what is seen (or not seen).

You should moreover keep in mind (which obviously you don't) that the length contracted object is described by events (at its inner and outer extremities) that are simultaneous in the observing (moving) system, but that are not simultaneous in the rotating rod's (or wheel's) own system!

See my animation
https://www.geogebra.org/m/xzsf765h

There are 2 types of contraction.

One is the contraction of the "space" between D and E (which is what
you measure with your method and about which I have nothing to dispute
about) and another is the contraction of the "body" AC, about which I
have so much to say.

The contracting space DE has no consequences, because the space between
D and E is empty.

But the AC rod is not empty and if it contracts there are consequences
(if the contraction is real).

And this is exactly what I want to get to: does the AC body really
contract or is it just an apparent contraction?

[[Mod. note -- Yes, there is another explanation: "rigid" only applies
in the rod's own inertial reference frame. Observers in other inertial reference frames may (will) observe non-ridity, but that's an artifact
of their motion and doesn't affect the rod.

If it is an artifact that does not affect the rod, it means that the non-rigidity is only an appearance.

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Op 4/09/2023 om 9:14 schreef Luigi Fortunati:

wugi il 03/09/2023 09:21:35 ha scritto:

And as always, don't forget the difference between *calculating Lorentz
objects* and *looking at Doppler/Einstein objects*.

I really want to talk about *calculating Lorentz objects* and,
therefore, let's leave aside what is seen (or not seen).

You should moreover keep in mind (which obviously you don't) that the length >> contracted object is described by events (at its inner and outer extremities)
that are simultaneous in the observing (moving) system, but that are not
simultaneous in the rotating rod's (or wheel's) own system!

See my animation
https://www.geogebra.org/m/xzsf765h

There are 2 types of contraction.

Of course not.

One is the contraction of the "space" between D and E (which is what
you measure with your method and about which I have nothing to dispute
about) and another is the contraction of the "body" AC, about which I
have so much to say.

The contracting space DE has no consequences, because the space between
D and E is empty.

Space is space, it doesn't care who measures what in it, you can't
pinpoint anything in it in an absolute manner.
Contraction applies to sizes and distances of and between material objects.

But the AC rod is not empty and if it contracts there are consequences
(if the contraction is real).

Which it is.

And this is exactly what I want to get to: does the AC body really
contract or is it just an apparent contraction?

It's real, better get used to the idea.
And, bis repetita placent, you keep forgetting, or omitting, some
meaningful details, like this one:
that "your" events A and C are simultaneous in the observing (rest)
system, but they aren't in the moving system!

AB is simultaneous in both rest and moving systems, but AC isn't.
At least two other events C' and C" exist along the AC axis, in the
order A,C,C'C", so that

AC'= AB : proper lengths in *rest* system (with ABC' simultaneous in
it), and
AC = AC'/gamma(v) : length contraction in rest system;

and
AC" = AB : proper lengths in *moving* system (with ABC" simultaneous in
it), and
AC'= AC"/gamma(v) : length contraction (of rest system lengths!) in
moving system.

The contractions are real and reciprocal(!) <==> they correspond to relativistic lengths and distances, and time intervals do the same: C,
C' and C" are different events at different times.

These are real properties of real matter distribution in space and time:
empty space and time don't *care* about units, directions etc. defined
upon them.

So you can draw three "ellipses" now, instead of only one:
ABC : ellips of length contraction of moving system in rest system
ABC' : circle of rest lengths in rest system
ABC" : elongated ellips in rest system ~ "circle of rest lengths" in
moving system.
Here you can see the three in a proper setting with proper events
defining them:
https://www.desmos.com/calculator/7h1h4jvzno?lang=nl

[[Mod. note -- Yes, there is another explanation: "rigid" only applies
in the rod's own inertial reference frame. Observers in other inertial
reference frames may (will) observe non-ridity, but that's an artifact
of their motion and doesn't affect the rod.

If it is an artifact that does not affect the rod, it means that the non-rigidity is only an appearance.

Rigidity supposes checking lengths and times anywhere anytime. Now
what's the basic mechanism for checking and calibrating those? ... light clocks! So then, matter ought to "distribute" its lengths and times in accordance with light clock behaviour. It's that simple to understand
the basics of relativity, in an intuitive way at that.

Eg, light clocks and why there must be time dilation: https://www.desmos.com/calculator/zlplep7g5k?lang=nl
And light clocks and why there must be length contraction: https://www.desmos.com/calculator/ytyyijmgno?lang=nl
More at
https://www.wugi.be/srtinterac.html

Rigidity problems arise, as said earlier, when swapping among different inertial states at relativistic acceleration speeds.

--
guido wugi

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On Monday, September 4, 2023 at 2:14:38=E2=80=AFAM UTC-5, Luigi Fortunati wrote:

wugi il 03/09/2023 09:21:35 ha scritto:

...

There are 2 types of contraction.

One is the contraction of the "space" between D and E (which is what
you measure with your method and about which I have nothing to dispute
about) and another is the contraction of the "body" AC, about which I
have so much to say.

No, there is only one. Relative to the coordinates of the (inertial) observer, the moving coordinates (i.e. rulers moving with the rod) are contracted.
This is real. The rod is contracted, the rulers are contracted, the space
is contracted.

The contracting space DE has no consequences, because the space between
D and E is empty.

But the AC rod is not empty and if it contracts there are consequences
(if the contraction is real).

The rod is contracted, as viewed by the stationary observer, but that doesn't
mean that there is any stress in the rod. As far as the rod is concerned everything is normal.

Rich L

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wugi il 05/09/2023 09:21:30 ha scritto:

...

Richard Livingston il 05/09/2023 06:01:20 ha scritto:

...

We obviously live in different worlds.

In my world, a body that contracts, compresses.

And a rigid body doesn't become flexible just because it's in motion.

[[Mod. note --
You are taking it for granted that the body does in fact contract
or become flexible. But those observations are made by an observer
who is not at rest with respect to the body!

If observer A (at rest with body X) observes X to be uncontracted,
and observer B1 (moving at velocity v1 with respect to X) observes X
to be contracted by some amount R1, and observer B2 (moving at some
different velocity v2 with respect to X) observes X to be contracted
by some different amount R2, what should we infer? More generally,
there are infinitely many possible observers B1, B2, B3, B4, ...,
each of who will observe a different contraction of X.

In special relativity, the answer is that we privilege observations
made in the rest frame of the body being observed (in this case X).

To put it another way, can you explain why you think it's paradoxical
that observer B1 observes something different from observer A?
-- jt]]

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Op 7/09/2023 om 7:52 schreef Luigi Fortunati:

wugi il 05/09/2023 09:21:30 ha scritto:

...

Richard Livingston il 05/09/2023 06:01:20 ha scritto:

...

We obviously live in different worlds.

In my world, a body that contracts, compresses.

And a rigid body doesn't become flexible just because it's in motion.

You live in a still Newtonian world, not understanding relativity of simultaneity and the length contraction and time dilation properties
that go with it. Absolute rigidity implies absolute simultaneity and
time (for a measurement to be "absolute"), which don't exist in SRT.

And as Mod. said, what with all those different observers who observe
all those different length contractions? Which "absolute" length
contraction is disturbing your rigidity considerations?

[[Mod. note --
You are taking it for granted that the body does in fact contract
or become flexible. But those observations are made by an observer
who is not at rest with respect to the body!

If observer A (at rest with body X) observes X to be uncontracted,
and observer B1 (moving at velocity v1 with respect to X) observes X
to be contracted by some amount R1, and observer B2 (moving at some
different velocity v2 with respect to X) observes X to be contracted
by some different amount R2, what should we infer? More generally,
there are infinitely many possible observers B1, B2, B3, B4, ...,
each of who will observe a different contraction of X.

In special relativity, the answer is that we privilege observations
made in the rest frame of the body being observed (in this case X).

To put it another way, can you explain why you think it's paradoxical
that observer B1 observes something different from observer A?
-- jt]]

--
guido wugi

--- SoupGate-Win32 v1.05
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[[Mod. note --
To put it another way, can you explain why you think it's paradoxical
that observer B1 observes something different from observer A?
-- jt]]

I do not consider it paradoxical that observer B1 observes something
different from observer A.

I think it is paradoxical that observers B1 and A see different things
both real: if one thing is real the other must be apparent and vice
versa.

This is why I speak of reality and appearance.

A simple and clear example is the following.

If observer B1 stands on the carousel, he sees the earth rotate a full
360 degrees in 5 seconds and sees the carousel stationary.

Instead, observer A on the ground sees the carousel rotate 360 degrees
in 5 seconds and sees dry land.

One sees one thing, the other sees another: can both be real? Obviously
not.

If what observer A sees is real, what observer B1 sees can only be
apparent.

In fact, if the Earth really rotated at that angular velocity, it would shatter!

[[Mod. note -- What do you mean when say that something is or isn't
"real"? In special relativity we don't really have such a concept.
Rather, we have the concept of what some specified (inertial) observer observes,

ALSO, note that B1, B2, B3, ... observing the body X to be Lorentz-
-contracted does *not* imply any special internal stresses in X.
There are several two different ways of obtaining ("proving") the
statement in my previous sentence. For example:
(a) We can introduce the concept of "stress tensor", note (show)
that it is in fact a tensor, note that Lorentz transformations
can be viewed as tensor transformations of coordinates, and make
use of the elementary tensor calculus result that a tensor that's
zero in one coordinate system (basis) is zero in any other coordinate
system (basis), so that the stress tensor vanishing in A's inertial
reference frame implies it must also vanish in any other inertial
reference frame.
(b) More generally, to reason about internal stresses in objects, we
need a theory of (relativistic) continuum mechanics, and we can
use this to work out the internal stresses in the body X, computed
in A's intertial reference frame, B1's inertial reference frame,
B2's intertial reference frame, etc etc.
(c) At a microscopic level, the size of an object is determined by the
lengths of the chemical bonds between the object's constituent atoms.
These lengths are in turn determined by the Schroedinger equation
for the atoms' valence electrons. So, we could work out how
solutions of the Schroedinger equation change under Lorentz
transformations.

(a), (b), and (c) should all give the same answer, namely, the usual
Lorentz contraction does *not* imply any special internal stresses in
the object.
-- jt]]

--- SoupGate-Win32 v1.05
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Op 10/09/2023 om 10:30 schreef Luigi Fortunati:

[[Mod. note --
To put it another way, can you explain why you think it's paradoxical
that observer B1 observes something different from observer A?
-- jt]]

I do not consider it paradoxical that observer B1 observes something different from observer A.

I think it is paradoxical that observers B1 and A see different things
both real: if one thing is real the other must be apparent and vice
versa.

What a poor understanding of reality.

This is why I speak of reality and appearance.

It's more common sense then to deny any form of reality. All observers
see appearances.

A simple and clear example is the following.

If observer B1 stands on the carousel, he sees the earth rotate a full
360 degrees in 5 seconds and sees the carousel stationary.

Instead, observer A on the ground sees the carousel rotate 360 degrees
in 5 seconds and sees dry land.

One sees one thing, the other sees another: can both be real? Obviously
not.

No indeed, "obviously" there are only appearances.

If what observer A sees is real, what observer B1 sees can only be
apparent.

In fact, if the Earth really rotated at that angular velocity, it would shatter!

Would it? You forget that with Earth, the whole universe is rotating
around B1, so that ultimately it is only B1 who is feeling the "real" rotational force acting upon them, from Earth + Universe.

When you look askew at an object, you will see it kind-of contracted, so
is that real or not? Do you only see the object "really" when looking at
it from a perpendicular view? In all other cases, will you infer stress
forces in the "contracted" objects?

Now try this kind of "observations" when time and movement come into
play, remembering for once relativity of simultaneity.

--
guido wugi

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wugi il 10/09/2023 08:37:40 ha scritto:

When you look askew at an object, you will see it kind-of contracted, so is that real or not?

The object is real, the contraction is not.

--- SoupGate-Win32 v1.05
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On Thursday, September 7, 2023 at 1:52:45 AM UTC-4, Luigi Fortunati wrote:

wugi il 05/09/2023 09:21:30 ha scritto:

...

Richard Livingston il 05/09/2023 06:01:20 ha scritto:

...

We obviously live in different worlds.

In my world, a body that contracts, compresses.

And a rigid body doesn't become flexible just because it's in motion.

You fail to understand that there are no "rigid" bodies.

A wooden ruler or metal rod both are mainly empty space
containing matter constrained by various forces, mainly
electromagnetic forces. The space contracts without
any special consideration of the atoms that exist at the
many points between point A and point C. Just as you agree
that there is nothing special about the space between points D and E.

Enjoy,
Ed

--- SoupGate-Win32 v1.05
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wugi il 10/09/2023 08:37:40 ha scritto:

A simple and clear example is the following.

If observer B1 stands on the carousel, he sees the earth rotate a full
360 degrees in 5 seconds and sees the carousel stationary.

Instead, observer A on the ground sees the carousel rotate 360 degrees
in 5 seconds and sees dry land.

One sees one thing, the other sees another: can both be real? Obviously
not.

No indeed, "obviously" there are only appearances.

If what observer A sees is real, what observer B1 sees can only be
apparent.

In fact, if the Earth really rotated at that angular velocity, it would
shatter!

Would it? You forget that with Earth, the whole universe is rotating
around B1, so that ultimately it is only B1 who is feeling the "real" rotational force acting upon them, from Earth + Universe.

I am so naive that I have always believed that the man on the in
movement carousel feels the rotational force of the carousel and not
that of the Earth + Universe.

Luigi

--- SoupGate-Win32 v1.05
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Edward Prochak il 13/09/2023 09:03:51 ha scritto:

We obviously live in different worlds.

In my world, a body that contracts, compresses.

And a rigid body doesn't become flexible just because it's in motion.

You fail to understand that there are no "rigid" bodies.

I'll tell you what I understand clearly and what I don't.

Of course, I understand that in my world the rigid body remains rigid
and does not become flexible just because it moves at some speed,
because speed does not act either on the size of bodies or on their
rigidity.

Having said this, I am also ready to accept that at very high speeds
(close to that of light) exceptional phenomena can occur, such as the contraction of the length of the body or the loss of its rigidity.

However, before I really believe it, I would like there to be at least
some proof.

So I ask you: has the contraction or loss of rigidity of a body in
relativistic motion ever been measured?

If such a measure never existed, I would have to believe in this
phenomenon only by faith and I am not a believer.

A wooden ruler or metal rod both are mainly empty space
containing matter constrained by various forces, mainly
electromagnetic forces. The space contracts without
any special consideration of the atoms that exist at the
many points between point A and point C. Just as you agree
that there is nothing special about the space between points D and E.

This is not a demonstration or even proof.

Luigi

--- SoupGate-Win32 v1.05
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Luigi Fortunati <fortunati.luigi@gmail.com> wrote:

[[...]] I am also ready to accept that at very high speeds
(close to that of light) exceptional phenomena can occur, such as the contraction of the length of the body or the loss of its rigidity.

However, before I really believe it, I would like there to be at least
some proof.

So I ask you: has the contraction or loss of rigidity of a body in relativistic motion ever been measured?

This is a good opportunity to mention Tom Roberts' nice web page from
2007, "What is the experimental basis of Special Relativity?", now at
https://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html
which summarizes many experimental measurements/tests of special
relativity.

To answer Luigi Fortunati's question, I don't know of any such measurement, although Tom Roberts' web page (section 7) describes a strong indirect confirmation from the observed behavior of current-carying wires.

I'd also like to mention a beautiful paper (cited in that web page),
Carroll O. Alley,
"Proper Time Experiments in Gravitational Fields with Atomic Clocks,
Aircraft, and Laser Light Pulses",
pages 363-427 in
"Quantum Optics, Experimental Gravity, and Measurement Theory",
eds. Pierre Meystre and Marlan O. Scully,
Proceedings Conf. Bad Windsheim 1981,
1983 Plenum Press New York, ISBN 0-306-41354-X.
Alley's paper describes a direct experimental realisation of the twin
paradox, using atomic clocks flown in airplanes. In these experiments
the airplane was flown in a "racetrack" pattern over Chesapeake Bay, illuminated by a ground-based pulsed laser so that the stationary and
moving clocks could be compared *in real time* while the airplane was
in flight. The results (particularly figures 44-47) clearly show both
special- and general-relativistic (gravitational) time effects.

Unfortunately I don't know of any open-access copy of Alley's paper
online, but I have a pdf of it and can email it to anyone who's
interested.

--
-- "Jonathan Thornburg [remove -color to reply]" <dr.j.thornburg@gmail-pink.com>
currently on the west coast of Canada
"Dear everyone who says masks don't work:
Please get your next surgery with no one wearing masks.
Thank you for your cooperation. -- anon"

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[Moderator, this reply is a little more detailed than my recent reply. Hopefully you see this and just discard the previous one. Ed]

On Sunday, September 17, 2023 at 4:27:09 AM UTC-4, Luigi Fortunati wrote:

Edward Prochak il 13/09/2023 09:03:51 ha scritto:

We obviously live in different worlds.

In my world, a body that contracts, compresses.

And a rigid body doesn't become flexible just because it's in motion.

You fail to understand that there are no "rigid" bodies.

I'll tell you what I understand clearly and what I don't.

My apologies. I should have said that your statements assume
rigid bodies, but that such bodies do not exist in reality. They can
be useful in some problems in fundamental mechanics, but
fail miserably in problems of relativity.

Of course, I understand that in my world the rigid body remains rigid
and does not become flexible just because it moves at some speed,
because speed does not act either on the size of bodies or on their
rigidity.

Here is assumed the condition you are seeking to be explained.
And the explanation I gave is that it is not the "rigid" body that
is stretched or compressed.

Having said this, I am also ready to accept that at very high speeds
(close to that of light) exceptional phenomena can occur, such as the contraction of the length of the body or the loss of its rigidity.

It has nothing to do with the lost of rigidity. It has to do with the fact that the space-time itself is bent.

However, before I really believe it, I would like there to be at least
some proof.

So I ask you: has the contraction or loss of rigidity of a body in relativistic motion ever been measured?

Jonathan's post has some good references. I'll just suggest one
easy to follow explanation from Fermi Labs on youtube https://youtu.be/d29cETVUk-0?si=67T9L64zxnCVeGCr
The description includes some other helpful references also.

If such a measure never existed, I would have to believe in this
phenomenon only by faith and I am not a believer.

A wooden ruler or metal rod both are mainly empty space
containing matter constrained by various forces, mainly
electromagnetic forces. The space contracts without
any special consideration of the atoms that exist at the
many points between point A and point C. Just as you agree
that there is nothing special about the space between points D and E.

This is not a demonstration or even proof.

It is simply a follow-up to this comment from you earlier in this thread:
On Monday, September 4, 2023 at 3:14:38 AM UTC-4, Luigi Fortunati wrote:

One is the contraction of the "space" between D and E (which is what
you measure with your method and about which I have nothing to dispute
about) and another is the contraction of the "body" AC, about which I
have so much to say.

My point was that there is NO difference between
'the contraction of the "space" between D and E'
and
'the contraction of the "body" AC'

Until you understand that, you will continue to struggle
with understanding SR and GR.

Hope that helps.
Ed

--- SoupGate-Win32 v1.05
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On Sunday, September 17, 2023 at 4:27:09=E2=80=AFAM UTC-4, Luigi Fortunati wrote:

Edward Prochak il 13/09/2023 09:03:51 ha scritto:

We obviously live in different worlds.

In my world, a body that contracts, compresses.

And a rigid body doesn't become flexible just because it's in motion.

You fail to understand that there are no "rigid" bodies.

I'll tell you what I understand clearly and what I don't.

Of course, I understand that in my world the rigid body remains rigid
and does not become flexible just because it moves at some speed,
because speed does not act either on the size of bodies or on their
rigidity.

Having said this, I am also ready to accept that at very high speeds
(close to that of light) exceptional phenomena can occur, such as the contraction of the length of the body or the loss of its rigidity.

However, before I really believe it, I would like there to be at least
some proof.

So I ask you: has the contraction or loss of rigidity of a body in relativistic motion ever been measured?

Actually I just came across a nice video that may help https://youtu.be/d29cETVUk-0?si=oJqx8bBQ8l7EBq1K
The video describes how length contraction explains how
magnetism really works.

If such a measure never existed, I would have to believe in this
phenomenon only by faith and I am not a believer.

A wooden ruler or metal rod both are mainly empty space
containing matter constrained by various forces, mainly
electromagnetic forces. The space contracts without
any special consideration of the atoms that exist at the
many points between point A and point C. Just as you agree
that there is nothing special about the space between points D and E.

This is not a demonstration or even proof.

It is your insistence of perfectly rigid bodies that I was addressing.
You can put them in your model, but then it is not a model of reality.

Enjoy the video.
Ed

--- SoupGate-Win32 v1.05
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Op 17/09/2023 om 10:26 schreef Luigi Fortunati:

wugi il 10/09/2023 08:37:40 ha scritto:

A simple and clear example is the following.

If observer B1 stands on the carousel, he sees the earth rotate a full
360 degrees in 5 seconds and sees the carousel stationary.

Instead, observer A on the ground sees the carousel rotate 360 degrees
in 5 seconds and sees dry land.

One sees one thing, the other sees another: can both be real? Obviously
not.

No indeed, "obviously" there are only appearances.

If what observer A sees is real, what observer B1 sees can only be
apparent.

In fact, if the Earth really rotated at that angular velocity, it would
shatter!

Would it? You forget that with Earth, the whole universe is rotating
around B1, so that ultimately it is only B1 who is feeling the "real"
rotational force acting upon them, from Earth + Universe.

I am so naive that I have always believed that the man on the in
movement carousel feels the rotational force of the carousel and not
that of the Earth + Universe.

(A late one). He feels the reaction force by Earth+Universe upon the
carousel's rotational action, transmitted through the carousel's
foundation and its own stiffness.
Compare with the water in the bucket, which doesn't feel the bucket's
swinging action, but rather the reaction upon the bucket by the rope, by
the hand, by the person swinging the bucket around, by the floor
retaining his feet, by Earth, ...

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guido wugi

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Op 8/04/2024 om 8:54 schreef Luigi Fortunati:

wugi il 04/04/2024 23:24:16 ha scritto:

I am so naive that I have always believed that the man on the in
movement carousel feels the rotational force of the carousel and not
that of the Earth + Universe.

(A late one). He feels the reaction force by Earth+Universe upon the
carousel's rotational action, transmitted through the carousel's
foundation and its own stiffness.

What does the reaction force of the Earth+Universe to the rotating
action of the carousel have to do with the man on the carousel?

The man on the spinning carousel acts-reacts only with the carousel,
not with the Earth+Universe!

Compare with the water in the bucket, which doesn't feel the bucket's
swinging action, but rather the reaction upon the bucket by the rope, by
the hand, by the person swinging the bucket around, by the floor
retaining his feet, by Earth, ...

Here too, the only force that the water in the bucket feels is the
force of friction with the walls of the bucket and not that of the
hands, the rope or the Earth+Universe.

Better back to your drawing board. Litterally.
Try to draw all intervening action/reaction pairs and check if you
haven't sent some force to "nowhere" ;)

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guido wugi

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Op 9/04/2024 om 13:42 schreef Luigi Fortunati:

wugi il 09/04/2024 09:00:40 ha scritto:

Better back to your drawing board. Litterally.
Try to draw all intervening action/reaction pairs and check if you haven't sent some force to "nowhere" ;)

Here is the drawing:
https://www.geogebra.org/m/tecb3dkw

For me, only the two blue and red forces act between man and the carousel; and nothing else.

For you?

So you've despatched the blue force to nowhere ;-)
It is not generated where you draw it, that's a transmitted reaction
force (through the carousel's structure). It's generated in the foundation/anchor point(s, or areas, or volumes) of the carousel. After
the red action force has been transmitted to those in the first place,
of course.

If you consider initially only the red centrifugal force, and no
foundations, then the carousel will tend to be accelerated along the
(moving) centrifugal force, for lack of any basis for generating a
reaction force.

So then, you have to count at least with Earth. Whether also with the
rest of the Universe, that's a problem that even Mach couldn't solve in Principle, AFAIU.

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