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Jul 13, 2022, 5:50:42 AM7/13/22

to

When Newton's bucket starts to rotate, the water slowly starts to

rotate as well and accelerates outwards due to the centrifugal force.

But the centrifugal force is ONLY in the rotating reference and not in

the inertial one.

So, how is the centrifugal acceleration of water justified EVEN in the

inertial reference where the centrifugal force is not there?

rotate as well and accelerates outwards due to the centrifugal force.

But the centrifugal force is ONLY in the rotating reference and not in

the inertial one.

So, how is the centrifugal acceleration of water justified EVEN in the

inertial reference where the centrifugal force is not there?

Jul 13, 2022, 3:13:27 PM7/13/22

to

real force is whatever is causing the object (water molecules in this

case) to follow a curved path. The object is not at rest in any

inertial frame.

Rich L.

Jul 14, 2022, 1:29:46 AM7/14/22

to

On Wednesday, 13 July 2022 at 11:50:42 UTC+2, Luigi Fortunati wrote:

> When Newton's bucket starts to rotate, the water slowly starts to

> rotate as well and accelerates outwards due to the centrifugal force.

What matters is not really how we get there, just the steady state is
> When Newton's bucket starts to rotate, the water slowly starts to

> rotate as well and accelerates outwards due to the centrifugal force.

of interest, by which I mean the water is at rest relative to the bucket.

> But the centrifugal force is ONLY in the rotating reference and not in

> the inertial one.

in which the bucket is rotating, which amounts to a combination of the

pressure forces ultimately sustained by the walls of the bucket, and

of course the gravitational force: hence it's obliquus.

> So, how is the centrifugal acceleration of water justified EVEN in the

> inertial reference where the centrifugal force is not there?

(as I get it) is that, in the reference frame *of the bucket*, where does

the apparent *centrifugal* force come from? Since, by relativity, the

situation should be totally equivalent to the universe rotating around

a bucket at rest...

Julio

Jul 14, 2022, 9:58:45 AM7/14/22

to

or one of the great mysteries of physics,

depending on your philosophical inclinations.

I'll recapitulate the history:

1) For Newton all this was completely trivial,

the bucket rotates or not, wrt to his 'absolute space'.

2) Then Ernst Mach came along, who said that 'absolute space'

has no basis in empirical fact.

It is nothing but an unwaranted philosophical abomination

that has no place in physics. (by his philosophy of positivism)

All that matters, according to Mach, is relative motion.

So Mach said that the centrifugal and Coriolis forces

must be asumed to be -caused- by all those 'Ferne Sterne'

rotating around the stationary bucket at enormous speeds.

This is known as a form of "Mach's principle".

Einstein has said that Mach served as his inspiration

for getting started on relativity.

But working things out the Einsteinian way led to a great problem.

For Mach all was fine, because Newtonian gravity,

and hence also his 'Machian forces' propagated at infinite speed.

Finite propagation speed at c spoils it.

So now the mystery: empirically we can derive what is non-rotating

by observing motions in the Solar system to great accuracy.

(or in principle, but not in practice, also with a Foucult pendulum)

A frame without centrifugal and Coriolis doesn't rotate, by definition.

OTOH we can also determine what is, or isn't rotating

by looking at Mach's 'Ferne Sterne'.

(nowadays quasars at bilions of lightyears)

And indeed, those two differently defined frames local vs global,

do not rotate wrt each other,

to one of those ludicrous accuracies hat are the rule nowadays.

(would have to look up, think micro-arcseconds/century)

So there you are. [1]

You can shrug your shoulders, and say:

yes of course, how could it be otherwise?

Or you can say:

this is a deep mystery that needs a physical explanation.

Your choice,

Jan

(who hasn't kept up)

[1] This a veritable 'mer a boire'. There is a huge literature

on various forms of Mach's principle, weak, or strong,

or something else, and on how these should be understood.

Nevertheless, the hard empirical core of it has remained,

despite observable distances growing at least a millionfold.

Jul 14, 2022, 7:57:10 PM7/14/22

to

Julio Di Egidio alle ore 07:29:41 di gioved=EC 14/07/2022 ha scritto:

>

> There is indeed a corresponding centripetal force in the inertial frame in

> which the bucket rotates ...

The centripetal force exerted by the walls of the bucket on the water

is already present before the bucket starts to rotate, because it must

counteract the centrifugal thrust of the water which, even when

stationary, would be set in motion centrifugally outwards if the walls

of the bucket did not oppose.

All of this continues to be there even when the bucket starts spinning

and the water still doesn't.

This ratio between the centrifugal and centripetal forces changes when

even the water starts to spin!

And what happens in this case? Is it the water accelerating

(centrifugally) outward or is the bucket walls accelerating

(centripetally) inward?

Is it the centrifugal force that pushes the water to accumulate against

the walls of the bucket or is it the centripetal force that pushes the

walls of the bucket to tighten against the water?

[[Mod. note -- It appears that you're confusing two quite different

forces:

(a) The outward force the water exerts on the walls of the bucket, and

the corresponding Newton's-3rd-law inward force the walls of the

bucket exert on the water, due to the water's *weight* and Pascal's

law:

... this force is described in

https://en.wikipedia.org/wiki/Vertical_pressure_variation

https://en.wikipedia.org/wiki/Pascal%27s_law

... this force is ONLY present if there's an ambient (vertical)

gravitational field (or an equivalent vertical acceleration);

this force is proportional to the vertical Newtonian "little g"

and is ABSENT if the bucket is in free-fall ("weightless"),

e.g., in a space station

... this force varies with vertical position along the bucket's

walls, i.e., this force goes to zero at the water surface,

and is at a maximum at the bottom of the bucket

... for a given volume/shape filled with water, this force is

INDEPENDENT of the water's spin (or the bucket's spin), so

it's "just" an irrelevant distraction in the context of Newton's

bucket

(b) The outward force the water exerts on the walls of the bucket, and

the corresponding Newton's-3rd-law inward force the walls of the

bucket exert on the water, due to the water's *mass* moving on

an accelerated (spinning) trajectory:

... this force depends on the water's spin (NOT the bucket's spin);

this force is ONLY present if the water is spinning; this force

is ABSENT if the water is not spinning

... this force is INDEPENDENT of vertical position along the bucket's

walls: this force is IDENTICAL at the water surface and at the

bottom of the bucket

... for a given volume/shape filled with water, this force is

INDEPENDENT of the ambient gravitational field (or equivalent

vertical acceleration); notably, this force would be IDENTICAL

if the bucket were in free-fall ("weightless"), e.g., in a

space station

... this force is the one we usually discuss in the context of

Newton's bucket

Now to your specific statements & questions:

> The centripetal force exerted by the walls of the bucket on the water

> is already present before the bucket starts to rotate, because it must

> counteract the centrifugal thrust of the water which, even when

> stationary, would be set in motion centrifugally outwards if the walls

> of the bucket did not oppose.

You're referring to (a) here, which (since it doesn't vary with the water's

spin) is not relevant to a discussion of Newton's bucket.

> All of this continues to be there even when the bucket starts spinning

> and the water still doesn't.

The bucket's spin doesn't matter (for the dynamics of the water); only

the water's spin matters. [The bucket's spin does matter for calculating

the mechanical stresses on the bucket itself, due to the bucket's own

mass moving on an accelerated (spinning) trajectory.]

> This ratio between the centrifugal and centripetal forces changes when

> even the water starts to spin!

Yes, the statement "the water is spinning" implies the statement that

"the water is accelerated inwards (with respect to an inertial reference

frame)" and hence (by Newton's 2nd law) there must be net inwards forces

acting on the water. Those forces are the ones I described in (b) above.

> And what happens in this case? Is it the water accelerating

> (centrifugally) outward or is the bucket walls accelerating

> (centripetally) inward?

For simplicity let's focus on what happens once the bucket has been

spinning at a constant angular velocity for a long time, so that the water

is in uniform rotation at that same angular velocity. [I.e., let's ignore

the transient "startup" phase where the water's rotation is not yet uniform,

since the motion then is very complicated and hard to analyze.]

Then the answer to your first question is "no, the water is not accelerating

outward with respect to an inertial reference frame", and the answer to your

second question is "yes, the bucket walls (and the water) are accelerating

inward with respect to an inertial reference frame".

-- jt]]

>

> There is indeed a corresponding centripetal force in the inertial frame in

> which the bucket rotates ...

The centripetal force exerted by the walls of the bucket on the water

is already present before the bucket starts to rotate, because it must

counteract the centrifugal thrust of the water which, even when

stationary, would be set in motion centrifugally outwards if the walls

of the bucket did not oppose.

All of this continues to be there even when the bucket starts spinning

and the water still doesn't.

This ratio between the centrifugal and centripetal forces changes when

even the water starts to spin!

And what happens in this case? Is it the water accelerating

(centrifugally) outward or is the bucket walls accelerating

(centripetally) inward?

Is it the centrifugal force that pushes the water to accumulate against

the walls of the bucket or is it the centripetal force that pushes the

walls of the bucket to tighten against the water?

[[Mod. note -- It appears that you're confusing two quite different

forces:

(a) The outward force the water exerts on the walls of the bucket, and

the corresponding Newton's-3rd-law inward force the walls of the

bucket exert on the water, due to the water's *weight* and Pascal's

law:

... this force is described in

https://en.wikipedia.org/wiki/Vertical_pressure_variation

https://en.wikipedia.org/wiki/Pascal%27s_law

... this force is ONLY present if there's an ambient (vertical)

gravitational field (or an equivalent vertical acceleration);

this force is proportional to the vertical Newtonian "little g"

and is ABSENT if the bucket is in free-fall ("weightless"),

e.g., in a space station

... this force varies with vertical position along the bucket's

walls, i.e., this force goes to zero at the water surface,

and is at a maximum at the bottom of the bucket

... for a given volume/shape filled with water, this force is

INDEPENDENT of the water's spin (or the bucket's spin), so

it's "just" an irrelevant distraction in the context of Newton's

bucket

(b) The outward force the water exerts on the walls of the bucket, and

the corresponding Newton's-3rd-law inward force the walls of the

bucket exert on the water, due to the water's *mass* moving on

an accelerated (spinning) trajectory:

... this force depends on the water's spin (NOT the bucket's spin);

this force is ONLY present if the water is spinning; this force

is ABSENT if the water is not spinning

... this force is INDEPENDENT of vertical position along the bucket's

walls: this force is IDENTICAL at the water surface and at the

bottom of the bucket

... for a given volume/shape filled with water, this force is

INDEPENDENT of the ambient gravitational field (or equivalent

vertical acceleration); notably, this force would be IDENTICAL

if the bucket were in free-fall ("weightless"), e.g., in a

space station

... this force is the one we usually discuss in the context of

Newton's bucket

Now to your specific statements & questions:

> The centripetal force exerted by the walls of the bucket on the water

> is already present before the bucket starts to rotate, because it must

> counteract the centrifugal thrust of the water which, even when

> stationary, would be set in motion centrifugally outwards if the walls

> of the bucket did not oppose.

You're referring to (a) here, which (since it doesn't vary with the water's

spin) is not relevant to a discussion of Newton's bucket.

> All of this continues to be there even when the bucket starts spinning

> and the water still doesn't.

The bucket's spin doesn't matter (for the dynamics of the water); only

the water's spin matters. [The bucket's spin does matter for calculating

the mechanical stresses on the bucket itself, due to the bucket's own

mass moving on an accelerated (spinning) trajectory.]

> This ratio between the centrifugal and centripetal forces changes when

> even the water starts to spin!

Yes, the statement "the water is spinning" implies the statement that

"the water is accelerated inwards (with respect to an inertial reference

frame)" and hence (by Newton's 2nd law) there must be net inwards forces

acting on the water. Those forces are the ones I described in (b) above.

> And what happens in this case? Is it the water accelerating

> (centrifugally) outward or is the bucket walls accelerating

> (centripetally) inward?

For simplicity let's focus on what happens once the bucket has been

spinning at a constant angular velocity for a long time, so that the water

is in uniform rotation at that same angular velocity. [I.e., let's ignore

the transient "startup" phase where the water's rotation is not yet uniform,

since the motion then is very complicated and hard to analyze.]

Then the answer to your first question is "no, the water is not accelerating

outward with respect to an inertial reference frame", and the answer to your

second question is "yes, the bucket walls (and the water) are accelerating

inward with respect to an inertial reference frame".

-- jt]]

Jul 15, 2022, 10:37:46 AM7/15/22

to

In a moderator's note earlier in this thread, I referred to

> (b) The outward force the water exerts on the walls of the bucket, and

> the corresponding Newton's-3rd-law inward force the walls of the

> bucket exert on the water, due to the water's *mass* moving on

> an accelerated (spinning) trajectory:

Oops, I made two mistakes there and in the following text.

First, my text above suggests the wrong causality. What I should have

written was/is more like this:

(b) The inward force the walls of the bucket must exert on the water

in order to (by virtue of Newton's 2nd law) cause the water to

move along an accelerated trajectory; by Newton's 3rd law the

water exerts an equal and opposite (outward) force on the walls

of the bucket.

I then went on to write (something which was ok):

> ... this force depends on the water's spin (NOT the bucket's spin);

> this force is ONLY present if the water is spinning; this force

> is ABSENT if the water is not spinning

It *is* true if the bucket has a tight-fitting (flat) lid so that the

water is constrained to be in a cylindrical shape and to stay in that

shape even when the water is rotating.

But in the more common case where the bucket has an open top and is in

am ambient gravitational field with the Newtonian "little g" pointing

down (so that the water surface forms a concave parabolic surface when

the water is rotating), then I think the force (b) does in fact vary

with vertical position along the bucket's walls. To see this, consider

the following crude ASCII-art diagram (best viewed in a monopitch font)

showing a side view of some uniformly-rotating water in the bucket,

where I've labelled various parts of the water with letters/numbers

denoting their distance from the spin axis:

:

| : |

z=9 |B : B|

z=8 |BA : AB|

z=7 |BA98 : 89AB|

z=6 |BA98765 : 56789AB|

z=5 |BA987654321:123456789AB|

z=4 |BA987654321:123456789AB|

z=3 |BA987654321:123456789AB|

z=2 |BA987654321:123456789AB|

z=1 |BA987654321:123456789AB|

z=0 +-----------:-----------+

:

In the top layer of water (z=9), only the fluid labelled "B" is present

and so the force (b) I described above is only that necessary to accelerate

the water "B".

But in any of the "complete" layers of water (vertical positions z=1

through z=5 inclusive), the force (b) I described above has to accelerate

the larger mass of fluid "1", "2", ..., "B".

This argues that the inwards force (b) I described above is larger in

the z=1 through z=5 vertical positions than it is in the z=9 layer vertical

position.

Working out the precise variation of the force with vertical position

is left as an exercise for the reader.

--

-- "Jonathan Thornburg [remove -color to reply]" <dr.j.th...@gmail-pink.com>

Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA

currently on the west coast of Canada

"The question of whether machines can think is about as relevant

as the question of whether submarines can swim." - Edsger Dijkstra

> (b) The outward force the water exerts on the walls of the bucket, and

> the corresponding Newton's-3rd-law inward force the walls of the

> bucket exert on the water, due to the water's *mass* moving on

> an accelerated (spinning) trajectory:

First, my text above suggests the wrong causality. What I should have

written was/is more like this:

(b) The inward force the walls of the bucket must exert on the water

in order to (by virtue of Newton's 2nd law) cause the water to

move along an accelerated trajectory; by Newton's 3rd law the

water exerts an equal and opposite (outward) force on the walls

of the bucket.

I then went on to write (something which was ok):

> ... this force depends on the water's spin (NOT the bucket's spin);

> this force is ONLY present if the water is spinning; this force

> is ABSENT if the water is not spinning

But then I wrote:

> ... this force is INDEPENDENT of vertical position along the bucket's

> walls: this force is IDENTICAL at the water surface and at the

> bottom of the bucket

Oops, on further thought I don't think that last statement is true.
> ... this force is INDEPENDENT of vertical position along the bucket's

> walls: this force is IDENTICAL at the water surface and at the

> bottom of the bucket

It *is* true if the bucket has a tight-fitting (flat) lid so that the

water is constrained to be in a cylindrical shape and to stay in that

shape even when the water is rotating.

But in the more common case where the bucket has an open top and is in

am ambient gravitational field with the Newtonian "little g" pointing

down (so that the water surface forms a concave parabolic surface when

the water is rotating), then I think the force (b) does in fact vary

with vertical position along the bucket's walls. To see this, consider

the following crude ASCII-art diagram (best viewed in a monopitch font)

showing a side view of some uniformly-rotating water in the bucket,

where I've labelled various parts of the water with letters/numbers

denoting their distance from the spin axis:

:

| : |

z=9 |B : B|

z=8 |BA : AB|

z=7 |BA98 : 89AB|

z=6 |BA98765 : 56789AB|

z=5 |BA987654321:123456789AB|

z=4 |BA987654321:123456789AB|

z=3 |BA987654321:123456789AB|

z=2 |BA987654321:123456789AB|

z=1 |BA987654321:123456789AB|

z=0 +-----------:-----------+

:

In the top layer of water (z=9), only the fluid labelled "B" is present

and so the force (b) I described above is only that necessary to accelerate

the water "B".

But in any of the "complete" layers of water (vertical positions z=1

through z=5 inclusive), the force (b) I described above has to accelerate

the larger mass of fluid "1", "2", ..., "B".

This argues that the inwards force (b) I described above is larger in

the z=1 through z=5 vertical positions than it is in the z=9 layer vertical

position.

Working out the precise variation of the force with vertical position

is left as an exercise for the reader.

--

-- "Jonathan Thornburg [remove -color to reply]" <dr.j.th...@gmail-pink.com>

Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA

currently on the west coast of Canada

"The question of whether machines can think is about as relevant

as the question of whether submarines can swim." - Edsger Dijkstra

Jul 15, 2022, 10:44:55 AM7/15/22

to

Luigi Fortunati alle ore 11:57:05 di giovedě 14/07/2022 ha scritto:

> [[Mod. note

Why do you say that the motion of the transitional phase is very

complicated and hard to analyze? Where do you see all this difficulty?

In this phase, one can easily observe the water which, initially in

equilibrium, progressively begins to accelerate towards the outside,

where it ends up accumulating against the walls of the bucket.

If the water accelerates outward, it means that there is a force

directed outward.

If the walls of the bucket do not accelerate inwards, it means that

there are no forces accelerating the walls of the bucket inwards.

It is the water that is set in motion by pushing towards the outside,

not the walls of the bucket which (remaining still) simply block that

centrifugal thrust!

> [[Mod. note

>> And what happens in this case? Is it the water accelerating

>> (centrifugally) outward or is the bucket walls accelerating

>> (centripetally) inward?

>

> For simplicity let's focus on what happens once the bucket has been

> spinning at a constant angular velocity for a long time, so that the water

> is in uniform rotation at that same angular velocity. [I.e., let's ignore

> the transient "startup" phase where the water's rotation is not yet uniform,

> since the motion then is very complicated and hard to analyze.]

> -- jt]]
>> (centrifugally) outward or is the bucket walls accelerating

>> (centripetally) inward?

>

> For simplicity let's focus on what happens once the bucket has been

> spinning at a constant angular velocity for a long time, so that the water

> is in uniform rotation at that same angular velocity. [I.e., let's ignore

> the transient "startup" phase where the water's rotation is not yet uniform,

> since the motion then is very complicated and hard to analyze.]

Why do you say that the motion of the transitional phase is very

complicated and hard to analyze? Where do you see all this difficulty?

In this phase, one can easily observe the water which, initially in

equilibrium, progressively begins to accelerate towards the outside,

where it ends up accumulating against the walls of the bucket.

If the water accelerates outward, it means that there is a force

directed outward.

If the walls of the bucket do not accelerate inwards, it means that

there are no forces accelerating the walls of the bucket inwards.

It is the water that is set in motion by pushing towards the outside,

not the walls of the bucket which (remaining still) simply block that

centrifugal thrust!

Jul 15, 2022, 10:45:58 AM7/15/22

to

In a moderator's note earlier in this thread, I referred to

> (b) The outward force the water exerts on the walls of the bucket, and

> the corresponding Newton's-3rd-law inward force the walls of the

> bucket exert on the water, due to the water's *mass* moving on

> an accelerated (spinning) trajectory:

> the corresponding Newton's-3rd-law inward force the walls of the

> bucket exert on the water, due to the water's *mass* moving on

> an accelerated (spinning) trajectory:

Oops, I made two mistakes there and in the following text.

First, my text above suggests the wrong causality. What I should have

written was/is more like this:

(b) The inward force the walls of the bucket must exert on the water

in order to (by virtue of Newton's 2nd law) cause the water to

move along an accelerated trajectory; by Newton's 3rd law the

water exerts an equal and opposite (outward) force on the walls

of the bucket.

I then went on to write (something which was ok):

First, my text above suggests the wrong causality. What I should have

written was/is more like this:

(b) The inward force the walls of the bucket must exert on the water

in order to (by virtue of Newton's 2nd law) cause the water to

move along an accelerated trajectory; by Newton's 3rd law the

water exerts an equal and opposite (outward) force on the walls

of the bucket.

I then went on to write (something which was ok):

> ... this force depends on the water's spin (NOT the bucket's spin);

> this force is ONLY present if the water is spinning; this force

> is ABSENT if the water is not spinning

> this force is ONLY present if the water is spinning; this force

> is ABSENT if the water is not spinning

But then I wrote:

> ... this force is INDEPENDENT of vertical position along the bucket's

> walls: this force is IDENTICAL at the water surface and at the

> bottom of the bucket

> ... this force is INDEPENDENT of vertical position along the bucket's

> walls: this force is IDENTICAL at the water surface and at the

> bottom of the bucket

Jul 15, 2022, 7:11:23 PM7/15/22

to

On Friday, 15 July 2022 at 01:57:10 UTC+2, Luigi Fortunati wrote:

> Julio Di Egidio alle ore 07:29:41 di gioved=EC 14/07/2022 ha scritto:

> >

> > There is indeed a corresponding centripetal force in the inertial frame in

> > which the bucket rotates ...

>

> The centripetal force exerted by the walls of the bucket on the water

> is already present before the bucket starts to rotate, because it must

> counteract the centrifugal thrust of the water which, even when

> stationary, would be set in motion centrifugally outwards if the walls
> Julio Di Egidio alle ore 07:29:41 di gioved=EC 14/07/2022 ha scritto:

> >

> > There is indeed a corresponding centripetal force in the inertial frame in

> > which the bucket rotates ...

>

> The centripetal force exerted by the walls of the bucket on the water

> is already present before the bucket starts to rotate, because it must

> counteract the centrifugal thrust of the water which, even when

> of the bucket did not oppose.

But that is not "Newton's bucket", it's just something else. Moreover,

and more basically, centripetal/centrifugal is not just any old force,

in fact has not even to do with shapes and "containers", it is

specifically how we call forces that derive from *rotational motion*.

<snip>

> This ratio between the centrifugal and centripetal forces changes when

> even the water starts to spin!

That just cannot be: those two forces are just two different descriptions
> even the water starts to spin!

of the same physics, typically associated with the two distinct frames

of reference, the one inertial in which the thing (bucket, spinning top,

whatever) is rotating, and the one rotating with the thing.

In fact, more concretely, whichever the frame of reference we

choose, we can draw vectors representing both the centripetal and

the centrifugal force (as measured in their respective frames) and

those two vectors, unless I am badly mistaken, stay identically

equal and opposite.

Incidentally, this is not Newton's third law, though it looks analogous

since it's another case of equal and opposite: the two forces in the

third law actually both exist, are not simply two sides (descriptions)

of the same coin...

> Is it the centrifugal force that pushes the water to accumulate against

> the walls of the bucket or is it the centripetal force that pushes the

> walls of the bucket to tighten against the water?

>

> [[Mod. note -- It appears that you're confusing two quite different

> forces:

I think that more basic and to the point here was to note that

centrifugal/centripetal are, as said, just two sides of the same

one coin.

Julio

Jul 15, 2022, 7:11:33 PM7/15/22

to

(This entire discussion is in the context of Newtonian mechanics.)

On 7/15/22 9:44 AM, Luigi Fortunati wrote:

> If the water accelerates outward, it means that there is a force

> directed outward.

In the rotating-bucket coordinates:

As the bucket starts spinning you are correct -- the increasing

"centrifugal force" induces an increasing pressure gradient that causes

the fluid to increasingly rise higher for increasing radius. In a steady

state there is no acceleration anywhere and the net force is zero on

each small portion of the water -- the "centrifugal force" exactly

balances the horizontal fluid force induced by gravity and the

surrounding fluid; the radial pressure gradient causes the surface to be

higher for increasing radius.

In the inertial frame in which the bucket axis is at rest:

As the bucket starts spinning the acceleration of each small portion of

water is rather complicated (nonzero radial and tangential components).

In a steady state there is a centripetal force (directed radially

inward) that is different for each small portion of the water -- this

maintains each portion's "orbit" around the axis. For small portions of

the water against the wall it comes from the wall; for other portions it

comes from neighboring portions of the water. All other components of

force sum to zero for each small portion of the water; the radial

pressure gradient causes the surface to be higher for increasing radius.

> If the walls of the bucket do not accelerate inwards, it means that

> there are no forces accelerating the walls of the bucket inwards.

In the rotating-bucket coordinates:

in the steady state, the centripetal force on each small portion of the

wall equals the "centrifugal force" on it. All components of force sum

to zero for each small portion of the water. The centripetal force of

the wall is canceled by the "centrifugal force" on it. No portion of

bucket or water accelerates in any direction.

In the inertial frame in which the bucket axis is at rest:

in the steady state, the centripetal force on each small portion of the

wall accelerates it radially inward, maintaining its "orbit" around the

axis. Ditto for the wall. There is, of course, no "centrifugal force".

You should see from the above discussion that it is ESSENTIAL that you

specify which coordinates or frame you are discussing. Your repeated

failure to do that turns what you say into nonsense.

Tom Roberts

On 7/15/22 9:44 AM, Luigi Fortunati wrote:

> If the water accelerates outward, it means that there is a force

> directed outward.

As the bucket starts spinning you are correct -- the increasing

"centrifugal force" induces an increasing pressure gradient that causes

the fluid to increasingly rise higher for increasing radius. In a steady

state there is no acceleration anywhere and the net force is zero on

each small portion of the water -- the "centrifugal force" exactly

balances the horizontal fluid force induced by gravity and the

surrounding fluid; the radial pressure gradient causes the surface to be

higher for increasing radius.

In the inertial frame in which the bucket axis is at rest:

As the bucket starts spinning the acceleration of each small portion of

water is rather complicated (nonzero radial and tangential components).

In a steady state there is a centripetal force (directed radially

inward) that is different for each small portion of the water -- this

maintains each portion's "orbit" around the axis. For small portions of

the water against the wall it comes from the wall; for other portions it

comes from neighboring portions of the water. All other components of

force sum to zero for each small portion of the water; the radial

pressure gradient causes the surface to be higher for increasing radius.

> If the walls of the bucket do not accelerate inwards, it means that

In the rotating-bucket coordinates:

in the steady state, the centripetal force on each small portion of the

wall equals the "centrifugal force" on it. All components of force sum

to zero for each small portion of the water. The centripetal force of

the wall is canceled by the "centrifugal force" on it. No portion of

bucket or water accelerates in any direction.

In the inertial frame in which the bucket axis is at rest:

in the steady state, the centripetal force on each small portion of the

wall accelerates it radially inward, maintaining its "orbit" around the

axis. Ditto for the wall. There is, of course, no "centrifugal force".

You should see from the above discussion that it is ESSENTIAL that you

specify which coordinates or frame you are discussing. Your repeated

failure to do that turns what you say into nonsense.

Tom Roberts

Jul 17, 2022, 4:04:06 PM7/17/22

to

Tom Roberts alle ore 11:11:21 di venerdě 15/07/2022 ha scritto:

> You should see from the above discussion that it is ESSENTIAL that you

> specify which coordinates or frame you are discussing. Your repeated

> failure to do that turns what you say into nonsense.

None of the things I said happen in one reference yes and in the other
> You should see from the above discussion that it is ESSENTIAL that you

> specify which coordinates or frame you are discussing. Your repeated

> failure to do that turns what you say into nonsense.

no.

The accumulation of water on the walls of the bucket occurs in ALL

references.

The transition from initially still water particles and then moving

outwards (radial acceleration) occurs in ALL references.

Jul 17, 2022, 4:04:06 PM7/17/22

to

On 7/15/22 6:11 PM, Julio Di Egidio wrote:

> I think that more basic and to the point here was to note that

> centrifugal/centripetal are, as said, just two sides of the same one

> coin.

Not at all! They are VERY different: centripetal force is a real force,
> I think that more basic and to the point here was to note that

> centrifugal/centripetal are, as said, just two sides of the same one

> coin.

usually one that keeps one object in orbit around another object;

"centrifugal force" is a fictitious "force" used in Newtonian mechanics

to permit one to act as if rotating coordinates were inertial, so one

can apply Newton's laws -- in general that is not sufficient and one

also needs "Coriolis and Euler forces" (which are also fictitious).

[I put fictitious "forces" in scare quotes, because

they are not really forces.]

The difference is: a real force cannot be made to vanish by changing

coordinates, while a fictitious "force" will vanish in inertial

coordinates. As nature uses no coordinates, all natural phenomena must

be independent of coordinates; contrariwise, all coordinate-dependent

quantities are purely human inventions. Note this distinction is theory

dependent: in Newtonian mechanics the force of gravity is real, while in

General Relativity it is fictitious.

Ultimately all fictitious "forces" can be traced to geometry: in GR they

are directly related to specific components of the connection.

Tom Roberts

Jul 18, 2022, 2:39:00 AM7/18/22

to

On Sunday, 17 July 2022 at 22:04:06 UTC+2, Tom Roberts wrote:

> On 7/15/22 6:11 PM, Julio Di Egidio wrote:

>

> > I think that more basic and to the point here was to note that

> > centrifugal/centripetal are, as said, just two sides of the same one

> > coin.

>

> Not at all! They are VERY different: centripetal force is a real force,

> usually one that keeps one object in orbit around another object;

> "centrifugal force" is a fictitious "force" used in Newtonian mechanics

> to permit one to act as if rotating coordinates were inertial, so one

No, "apparent" is indeed a technical term referring specifically to the fact
> On 7/15/22 6:11 PM, Julio Di Egidio wrote:

>

> > I think that more basic and to the point here was to note that

> > centrifugal/centripetal are, as said, just two sides of the same one

> > coin.

>

> Not at all! They are VERY different: centripetal force is a real force,

> usually one that keeps one object in orbit around another object;

> "centrifugal force" is a fictitious "force" used in Newtonian mechanics

> to permit one to act as if rotating coordinates were inertial, so one

that the description in the inertial frame is the privileged one, the one for

which the laws of mechanics hold: but that doesn't mean that if you hop

on a merry-go-round you mustn't hold yourself to prevent falling off...

So, the two sides, i.e. descriptions, of the same one coin: and then one

should also note that too much emphasis on just inertial frames and

motion is also and in itself an aberration...

Julio

Jul 18, 2022, 5:50:31 AM7/18/22

to

Tom Roberts alle ore 07:54:37 di domenica 17/07/2022 ha scritto:

> The difference is: a real force cannot be made to vanish by changing

> coordinates, while a fictitious "force" will vanish in inertial

> coordinates.

Exactly.
> The difference is: a real force cannot be made to vanish by changing

> coordinates, while a fictitious "force" will vanish in inertial

> coordinates.

I totally agree with you, the fictitious force disappears in the inertial reference.

But if it doesn't go away, obviously it's not fictitious!

So, look at my animation

https://www.geogebra.org/m/mrthyefq

We are in an inertial reference where the centrifugal force should not be there.

Still, the rope gets longer!

Can you explain to me how it stretches if there is no centrifugal force?

Jul 18, 2022, 3:14:25 PM7/18/22

to

On 7/18/22 4:50 AM, Luigi Fortunati wrote:

> Tom Roberts alle ore 07:54:37 di domenica 17/07/2022 ha scritto:

>> The difference is: a real force cannot be made to vanish by

>> changing coordinates, while a fictitious "force" will vanish in

>> inertial coordinates.

>

> Tom Roberts alle ore 07:54:37 di domenica 17/07/2022 ha scritto:

>> The difference is: a real force cannot be made to vanish by

>> changing coordinates, while a fictitious "force" will vanish in

>> inertial coordinates.

>

> I totally agree with you, the fictitious force disappears in the

> inertial reference. But if it doesn't go away, obviously it's not

> fictitious!

In inertial coordinates, "fictitious forces" DO go away, as you agreed.
> inertial reference. But if it doesn't go away, obviously it's not

> fictitious!

> So, look at my animation https://www.geogebra.org/m/mrthyefq We are

> in an inertial reference where the centrifugal force should not be

> there. Still, the rope gets longer!

> Can you explain to me how it stretches if there is no centrifugal

> force?

object an impulse [#] to the right; your drawing also starts the object

at the (pre-computed) radius with which it will orbit [@]. To keep the

object orbiting in a circle, the rope must pull it off its inertial

straight-line path -- that pull is the centripetal force that keeps the

object in circular orbit, and is what stretches the rope. No

"centrifugal force" is involved.

[#] Large force of very short duration.

[@] Given the elasticity of the rope. There is an initial

radially-outward force on the object to stretch the rope

appropriately; it vanishes as soon as the object starts

to move, as the rope then provides the centripetal force.

This initial outward force, the initial position of the

object, and the magnitude of the initial impulse, must

all be coordinated to make the object's path a circle.

Tom Roberts

Jul 18, 2022, 3:14:37 PM7/18/22

to

On 7/17/22 12:54 AM, Luigi Fortunati wrote:

description/explanation will often depend on coordinates, and in this

thread it does.

Tom Roberts

> Tom Roberts alle ore 11:11:21 di venerdė 15/07/2022 ha scritto:

>> You should see from the above discussion that it is ESSENTIAL that

>> you specify which coordinates or frame you are discussing. Your

>> repeated failure to do that turns what you say into nonsense.

>

> None of the things I said happen in one reference yes and in the

> other no.

Physical phenomena are necessarily independent of coordinates. But the
>> You should see from the above discussion that it is ESSENTIAL that

>> you specify which coordinates or frame you are discussing. Your

>> repeated failure to do that turns what you say into nonsense.

>

> None of the things I said happen in one reference yes and in the

> other no.

description/explanation will often depend on coordinates, and in this

thread it does.

Tom Roberts

Jul 19, 2022, 4:18:44 AM7/19/22

to

Jonathan Thornburg [remove -color to reply] <dr.j.th...@gmail-pink.com>

wrote:
[ASCII parabola]

> But in any of the "complete" layers of water (vertical positions z=1 a

> through z=5 inclusive), the force (b) I described above has to accelerate

> the larger mass of fluid "1", "2", ..., "B".

>

> This argues that the inwards force (b) I described above is larger in

> the z=1 through z=5 vertical positions than it is in the z=9 layer vertical

> position.

>

> Working out the precise variation of the force with vertical position

> is left as an exercise for the reader.

Which is again made trivial by noting that the centrifugal force can be
> the larger mass of fluid "1", "2", ..., "B".

>

> This argues that the inwards force (b) I described above is larger in

> the z=1 through z=5 vertical positions than it is in the z=9 layer vertical

> position.

>

> Working out the precise variation of the force with vertical position

> is left as an exercise for the reader.

derived from the centrifugal potential. The parabolic shape is an

equipotential surface, [1] when everything is stationary in co-rotating

coordinates,

Jan

[1] So the surface is given by g z = 1/2 \Omega^2 (x^2 + y^2) if the

origin is chosen suitably.

[added] So the centrifugal `force' is not just a force that acts

somewhere, it is actually a force field. It appears as such in for

example meteorological models, which are of course done on a rotating

Earth. (but Coriolis is more important)

Jul 19, 2022, 6:28:21 AM7/19/22

to

[-]

> But in the more common case where the bucket has an open top and is in

> am ambient gravitational field with the Newtonian "little g" pointing

> down (so that the water surface forms a concave parabolic surface when

> the water is rotating), then I think the force (b) does in fact vary

> with vertical position along the bucket's walls. To see this, consider

> the following crude ASCII-art diagram (best viewed in a monopitch font)

> showing a side view of some uniformly-rotating water in the bucket,

> where I've labelled various parts of the water with letters/numbers

> denoting their distance from the spin axis:

>

> :

> | : |

> z=9 |B : B|

> z=8 |BA : AB|

> z=7 |BA98 : 89AB|

> z=6 |BA98765 : 56789AB|

> z=5 |BA987654321:123456789AB|

> z=4 |BA987654321:123456789AB|

> z=3 |BA987654321:123456789AB|

> z=2 |BA987654321:123456789AB|

> z=1 |BA987654321:123456789AB|

> z=0 +-----------:-----------+

> :

>

> In the top layer of water (z=9), only the fluid labelled "B" is present

> and so the force (b) I described above is only that necessary to accelerate

> the water "B".

>

> am ambient gravitational field with the Newtonian "little g" pointing

> down (so that the water surface forms a concave parabolic surface when

> the water is rotating), then I think the force (b) does in fact vary

> with vertical position along the bucket's walls. To see this, consider

> the following crude ASCII-art diagram (best viewed in a monopitch font)

> showing a side view of some uniformly-rotating water in the bucket,

> where I've labelled various parts of the water with letters/numbers

> denoting their distance from the spin axis:

>

> :

> | : |

> z=9 |B : B|

> z=8 |BA : AB|

> z=7 |BA98 : 89AB|

> z=6 |BA98765 : 56789AB|

> z=5 |BA987654321:123456789AB|

> z=4 |BA987654321:123456789AB|

> z=3 |BA987654321:123456789AB|

> z=2 |BA987654321:123456789AB|

> z=1 |BA987654321:123456789AB|

> z=0 +-----------:-----------+

> :

>

> In the top layer of water (z=9), only the fluid labelled "B" is present

> and so the force (b) I described above is only that necessary to accelerate

> the water "B".

>

> But in any of the "complete" layers of water (vertical positions z=1

Jul 20, 2022, 2:39:29 AM7/20/22

to

of my animation

https://www.geogebra.org/m/mrthyefq

but it is also true that, at the same time, object B provides the rope

with centrifugal force, otherwise the rope could not maintain its

elongation over time (elongation present in all references).

Centripetal and centrifugal forces act together and never separately:

one exists only by virtue of the fact that the other also exists (and

vice versa).

The centripetal force of A on B could not exist without the

corresponding (and opposite) centrifugal force of B on A.

Therefore the centrifugal force of A on B cannot disappear until the

centripetal force of B on A also disappears.

Luigi Fortunati

Jul 20, 2022, 7:32:05 AM7/20/22

to

by the use of non-standard terminology,

such as calling the reaction to the centripetal force

a centrifugal force.

'Centrifugal force' has a welll defined technical meaning,

(as the pseudo-force in rotating coordinates)

and you shouldn't use the term for anything else.

(like a force the happens to point towards the centre)

Perhaps the method employed by my high school teacher,

long ago, can help you.

At the start of dealing with the subject he declared:

---- CENTRIFUGAL FORCES DO NO EXIST ----

and any pupil who dared to mention the word

got A BIG FAT RED CROSS through his work.

(so no talk about rotating coordinates)

He correctly saw that mixing centrifugal force from rotating coordinates

with centripetal force from stationary coordinates can only end

with pupils getting thoroughly confused.

(like thinking that the two can, or sould balance each other)

So a BIG FAT RED CROSS for you, for what you wrote above,

Jan

Jul 21, 2022, 7:17:26 AM7/21/22

to

J. J. Lodder alle ore 13:32:02 di mercoledì 20/07/2022 ha scritto:

> ...

There is, for example, the force that (in the driver's eyes) accelerates the lighter on the dashboard of the car when cornering.

This acceleration is truly fictitious because it disappears in the inertial reference and does not stretch any elastic cord.

But the case with my animation

https://www.geogebra.org/m/mrthyefq

it is completely different because the elastic cord that stretches is there.

So how could the elastic cord stretch (in all references) if (in this specific case) there is no centrifugal force?

> ...

> Perhaps the method employed by my high school teacher,

> long ago, can help you.

> At the start of dealing with the subject he declared:

> ---- CENTRIFUGAL FORCES DO NO EXIST ----

It is undoubtedly true that there are centrifugal forces ("apparent" or "fictitious") that do not exist.
> long ago, can help you.

> At the start of dealing with the subject he declared:

> ---- CENTRIFUGAL FORCES DO NO EXIST ----

There is, for example, the force that (in the driver's eyes) accelerates the lighter on the dashboard of the car when cornering.

This acceleration is truly fictitious because it disappears in the inertial reference and does not stretch any elastic cord.

But the case with my animation

https://www.geogebra.org/m/mrthyefq

it is completely different because the elastic cord that stretches is there.

So how could the elastic cord stretch (in all references) if (in this specific case) there is no centrifugal force?

Jul 21, 2022, 9:59:07 AM7/21/22

to

accellerates your ball on the cord keeping it on a circular path. If you

don't keep the rope fixed in the origine and nobody accelerates your

ball, then nothing is stretched.

So in the end, you are stretching the rope.

Jul 21, 2022, 9:59:37 AM7/21/22

to

Luigi Fortunati <fortuna...@gmail.com> wrote:

Again, you confuse yourself with your incorrect terminology.

The term 'centrifugal force' has a well defined technical meaning.

It is the universal apparent force that appears

in a rotating coordinate system, acting on any mass element,

and equal to dm grad (-1/2 \Omega^2 (x^2 + y^2)

Nothing else should be called a 'centrifugal force'.

(on pain of a BIG FAT RED CROSS through your work)

So by definition no 'centrifugal force' can exist

in a non-rotating coordinate system.

If there are forces pointing out

from what can be taken as a rotation axis

they are NOT centrifugal forces.

Calling them that is just a beginner's error,

Jan

(who doesn't look at animations)

The term 'centrifugal force' has a well defined technical meaning.

It is the universal apparent force that appears

in a rotating coordinate system, acting on any mass element,

and equal to dm grad (-1/2 \Omega^2 (x^2 + y^2)

Nothing else should be called a 'centrifugal force'.

(on pain of a BIG FAT RED CROSS through your work)

So by definition no 'centrifugal force' can exist

in a non-rotating coordinate system.

If there are forces pointing out

from what can be taken as a rotation axis

they are NOT centrifugal forces.

Calling them that is just a beginner's error,

Jan

(who doesn't look at animations)

Jul 22, 2022, 3:06:17 AM7/22/22

to

On 7/20/22 1:39 AM, Luigi Fortunati wrote:

> It is true that the rope provides its centripetal force to the

> object B of my animation https://www.geogebra.org/m/mrthyefq but it

> is also true that, at the same time, object B provides the rope with

> centrifugal force, otherwise the rope could not maintain its

> elongation over time (elongation present in all references).

That is NOT "centrifugal force". That is merely the usual force of
> It is true that the rope provides its centripetal force to the

> object B of my animation https://www.geogebra.org/m/mrthyefq but it

> is also true that, at the same time, object B provides the rope with

> centrifugal force, otherwise the rope could not maintain its

> elongation over time (elongation present in all references).

tension in the rope.

If you simply pull on a rope connected to a mass, in a straight line, no

rotation, your pull will induce tension in the rope, and the tension

force will accelerate the mass. We don't usually discuss the "force the

mass exerts on the rope", but if we did it would merely be the reaction

force of Newton's 3rd law (responding to the force of tension the rope

exerts on the mass).

As Jan has pointed out, you confuse yourself, and your readers, by using

the term "centrifugal force" for situations in which it simply does not

exist. While there can be other "center-fleeing forces", "centrifugal

force" is a technical term with a specific meaning that applies ONLY in

rotating coordinates.

Tom Roberts

Jul 22, 2022, 3:06:17 AM7/22/22

to

Torn Rumero DeBrak alle ore 15:59:04 di giovedì 21/07/2022 ha scritto:

>> But the case with my animation

>> https://www.geogebra.org/m/mrthyefq

>> it is completely different because the elastic cord that stretches is there.

>>

>> So how could the elastic cord stretch (in all references) if (in this

>> specific case) there is no centrifugal force?

>

> It is simply (for your understanding level) your force, that

> accellerates your ball on the cord keeping it on a circular path.

A single force is not enough to stretch an elastic cord, you need two of
>> But the case with my animation

>> https://www.geogebra.org/m/mrthyefq

>> it is completely different because the elastic cord that stretches is there.

>>

>> So how could the elastic cord stretch (in all references) if (in this

>> specific case) there is no centrifugal force?

>

> It is simply (for your understanding level) your force, that

> accellerates your ball on the cord keeping it on a circular path.

opposite sign.

But let's see how the forces work in this case.

The centripetal force of my hand acts on one end of the bungee cord and

the centrifugal force of the end of the bungee cord acts on my hand.

They are two opposing forces and not just one.

The centripetal force of the other end of the cord acts on the ball and

the centrifugal force of the ball acts on the other end of the string.

Again, the forces act in pairs and not alone.

There is no point on the hand, cord and ball where there is the action

of a single force without the reaction of the opposite one.

> So in the end, you are stretching the rope.

side there is the ball is pulling it from the opposite side!

Jul 22, 2022, 7:04:58 AM7/22/22

to

[Moderator's note: It seems that all has been said which can be

meaningfully said in this thread. Thus, any future posts must present

something truly new rather than just a repeat (rephrased or not) of

previous exchanges. -P.H.]

Luigi Fortunati <fortuna...@gmail.com> wrote:

> Torn Rumero DeBrak alle ore 15:59:04 di giovedÃ 21/07/2022 ha scritto:

> >> But the case with my animation

> >> https://www.geogebra.org/m/mrthyefq

> >> it is completely different because the elastic cord that stretches is

> >>there.

> >>

> >> So how could the elastic cord stretch (in all references) if (in this

> >> specific case) there is no centrifugal force?

> >

> > It is simply (for your understanding level) your force, that

> > accellerates your ball on the cord keeping it on a circular path.

>

> But let's see how the forces work in this case.

>

> The centripetal force of my hand acts on one end of the bungee cor

YES.

> and the centrifugal force of the end of the bungee cord acts on my hand.

BIG FAT RED CROSS.

> They are two opposing forces and not just one.

BIG FAT RED CROSS.

> The centripetal force of the other end of the cord acts on the ball and

YES.

> the centrifugal force of the ball acts on the other end of the string.

BIG FAT RED CROSS.

> Again, the forces act in pairs and not alone.

Just action = reaction, otherwise

there could be no momentum conservation.

> There is no point on the hand, cord and ball where there is the action

> of a single force without the reaction of the opposite one.

Sure, third law always holds.

> > So in the end, you are stretching the rope.

>

The forces on the ball are not balanced,

hence it accelerates all the time.

At the attachement point the forces do balance.

(force of rope on ball equals force of ball on rope)

It is an error to call that reaction force 'a centrifugal force'.

THERE ARE NO CENTRIFUGAL FORCES.

(high school teacher, again)

Jan

meaningfully said in this thread. Thus, any future posts must present

something truly new rather than just a repeat (rephrased or not) of

previous exchanges. -P.H.]

Luigi Fortunati <fortuna...@gmail.com> wrote:

> Torn Rumero DeBrak alle ore 15:59:04 di giovedÃ 21/07/2022 ha scritto:

> >> But the case with my animation

> >> https://www.geogebra.org/m/mrthyefq

> >> it is completely different because the elastic cord that stretches is

> >>there.

> >>

> >> So how could the elastic cord stretch (in all references) if (in this

> >> specific case) there is no centrifugal force?

> >

> > It is simply (for your understanding level) your force, that

> > accellerates your ball on the cord keeping it on a circular path.

>

> A single force is not enough to stretch an elastic cord, you need two of

> opposite sign.

YES.
> opposite sign.

> But let's see how the forces work in this case.

>

> The centripetal force of my hand acts on one end of the bungee cor

> and the centrifugal force of the end of the bungee cord acts on my hand.

> They are two opposing forces and not just one.

> The centripetal force of the other end of the cord acts on the ball and

> the centrifugal force of the ball acts on the other end of the string.

> Again, the forces act in pairs and not alone.

there could be no momentum conservation.

> There is no point on the hand, cord and ball where there is the action

> of a single force without the reaction of the opposite one.

> > So in the end, you are stretching the rope.

>

> Yes, I am stretching the rope (pulling it to one side) but on the other

> side there is the ball is pulling it from the opposite side!

YES.
> side there is the ball is pulling it from the opposite side!

The forces on the ball are not balanced,

hence it accelerates all the time.

At the attachement point the forces do balance.

(force of rope on ball equals force of ball on rope)

It is an error to call that reaction force 'a centrifugal force'.

THERE ARE NO CENTRIFUGAL FORCES.

(high school teacher, again)

Jan

Jul 22, 2022, 1:39:58 PM7/22/22

to

On Friday, 22 July 2022 at 13:04:58 UTC+2, J. J. Lodder wrote:

> [Moderator's note: It seems that all has been said which can be

> meaningfully said in this thread. Thus, any future posts must present

> something truly new rather than just a repeat (rephrased or not) of

> previous exchanges. -P.H.]

But no agreement has been reached even among those posting
> [Moderator's note: It seems that all has been said which can be

> meaningfully said in this thread. Thus, any future posts must present

> something truly new rather than just a repeat (rephrased or not) of

> previous exchanges. -P.H.]

answers, indeed here I feel compelled to try and again object/

question:

> Luigi Fortunati <fortuna...@gmail.com> wrote:

>> Torn Rumero DeBrak alle ore 15:59:04 di giovedÃ 21/07/2022 ha scritto:

> It is an error to call that reaction force 'a centrifugal force'.

> THERE ARE NO CENTRIFUGAL FORCES.

I'd say that is indeed an error, but not for that reason:
> THERE ARE NO CENTRIFUGAL FORCES.

If you hop on a merry go round and don't hold yourself...

and then I won't repeat what I have said upthread, but if we

actually *measure* the acceleration locally, we do find that

there is in a force, a very concrete one: so, to say that

centrifugal forces plain "do not exist" is simply wrong and

eventually misleading.

In fact, here is my own summary of the scenarios here:

In the inertial frame in which -say- a ball is attached to and

rotating around a central pivot at rest, there is a *centripetal*

force from the ball directed to the center (at every instant),

and the reaction is the contrary force pulling the central

pivot towards the ball.

OTOH, in the rotating frame in which the ball is at rest, there

is a *centrifugal* force pulling the ball *away from* the central

pivot, and the reaction is again the contrary force at the central

pivot, i.e. here *away from* the ball. So, indeed the analogous

to the inertial description, but in opposite directions: outwards

instead of inwards.

And now, just to be clear, back to errors and reasons: it is not

that centrifugal/centripetal are related by Newton's third law

(the error): rather, they exist in distinct frames of reference,

and each sees a corresponding reaction as per the third law

(the reason).

No?

Julio

Jul 23, 2022, 5:49:19 AM7/23/22

to

> [Moderator's note: It seems that all has been said which can be

> meaningfully said in this thread. Thus, any future posts must present

> something truly new rather than just a repeat (rephrased or not) of

> previous exchanges. -P.H.]

A piece of advice from a physics teacher, translated and somewhat
> meaningfully said in this thread. Thus, any future posts must present

> something truly new rather than just a repeat (rephrased or not) of

> previous exchanges. -P.H.]

paraphrased for politeness:

"Shut up and do the math".

The transformation equations to a Newtonian rotating frame are

well known. Just apply them to the problem at hand and study

the results.

Jul 23, 2022, 5:49:19 AM7/23/22

to

On 7/22/22 12:39 PM, Julio Di Egidio wrote:

> centrifugal/centripetal [forces] exist in distinct frames of reference,

It is quite clear that nature uses no frames of reference or

coordinates, so every natural phenomenon MUST be independent of frame or

coordinates -- they are purely human constructs we use to DESCRIBE what

happens. So "centrifugal force" cannot possibly be real (a natural

phenomenon).

> If you hop on a merry go round and don't hold yourself...

is undefined). This is only "misleading" to people who ignore the

context and use incomplete descriptions: Yes, if you measure the

acceleration or force in the rotating coordinates you find a non-zero

"centrifugal force". If you measure it in the inertial frame in which

the center is at rest, you find zero force. Now see my previous paragraph.

IOW: saying "centrifugal forces do not exist" is just using the usual

meaning that existence applies to the world, and figments of human

imaginations simply do not exist.

(I agree with the moderator that that all has been said which can be

meaningfully said in this thread.)

Tom Roberts

> centrifugal/centripetal [forces] exist in distinct frames of reference,

It is quite clear that nature uses no frames of reference or

coordinates, so every natural phenomenon MUST be independent of frame or

coordinates -- they are purely human constructs we use to DESCRIBE what

happens. So "centrifugal force" cannot possibly be real (a natural

phenomenon).

> If you hop on a merry go round and don't hold yourself...

> if we

> actually *measure* the acceleration locally, we do find that

> there is in a force, a very concrete one: so, to say that

> centrifugal forces plain "do not exist" is simply wrong and

> eventually misleading.

You didn't completely describe what you are discussing (your "locally"
> actually *measure* the acceleration locally, we do find that

> there is in a force, a very concrete one: so, to say that

> centrifugal forces plain "do not exist" is simply wrong and

> eventually misleading.

is undefined). This is only "misleading" to people who ignore the

context and use incomplete descriptions: Yes, if you measure the

acceleration or force in the rotating coordinates you find a non-zero

"centrifugal force". If you measure it in the inertial frame in which

the center is at rest, you find zero force. Now see my previous paragraph.

IOW: saying "centrifugal forces do not exist" is just using the usual

meaning that existence applies to the world, and figments of human

imaginations simply do not exist.

(I agree with the moderator that that all has been said which can be

meaningfully said in this thread.)

Tom Roberts

Jul 25, 2022, 1:44:54 AM7/25/22

to

On Saturday, 23 July 2022 at 11:49:19 UTC+2, Tom Roberts wrote:

> On 7/22/22 12:39 PM, Julio Di Egidio wrote:

> > centrifugal/centripetal [forces] exist in distinct frames of reference,

>

> It is quite clear that nature uses no frames of reference or

> coordinates, so every natural phenomenon MUST be

Utter nonsense: the physics is the same, the description is not.
> On 7/22/22 12:39 PM, Julio Di Egidio wrote:

> > centrifugal/centripetal [forces] exist in distinct frames of reference,

>

> It is quite clear that nature uses no frames of reference or

> coordinates, so every natural phenomenon MUST be

> independent of frame or

> coordinates -- they are purely human constructs we use to DESCRIBE what

> happens. So "centrifugal force" cannot possibly be real (a natural

> phenomenon).

> > If you hop on a merry go round and don't hold yourself...

> > if we

> > actually *measure* the acceleration locally, we do find that

> > there is in a force, a very concrete one: so, to say that

> > centrifugal forces plain "do not exist" is simply wrong and

> > eventually misleading.

> You didn't completely describe what you are discussing (your "locally"

> is undefined). This is only "misleading" to people who ignore the

> context and use incomplete descriptions: Yes, if you measure the

> acceleration or force in the rotating coordinates you find a non-zero

> "centrifugal force". If you measure it in the inertial frame in which

> the center is at rest, you find zero force.

exist applied to the ball as the ball is NOT in uniform motion!!

And my "locally" is perfectly defined, the one who is at least confused

and utterly misleading is still you (and co).

> IOW: saying "centrifugal forces do not exist" is

Enough said.

Julio

[Moderator's note: As Julio notes, and as did I in a recent moderator's

note, it does seem that all has been said in this thread; it is making

no more progress, so let's end it. -P.H.]

Jul 25, 2022, 3:01:22 AM7/25/22

to

In this discussion the following question remained unanswered: "In my

animation, and in the rotating reference in which the centrifugal force

exists, who is exercising it, who is undergoing it and what is its point

of application?".

animation, and in the rotating reference in which the centrifugal force

exists, who is exercising it, who is undergoing it and what is its point

of application?".

Jul 25, 2022, 5:06:55 AM7/25/22

to

Luigi Fortunati <fortuna...@gmail.com> wrote:

[one last time]

[Moderator's note: Yes, definitely! -P.H.]

> In this discussion the following question remained unanswered: "In my

> animation, and in the rotating reference in which the centrifugal force

> exists, who is exercising it,

No one, it is a universal force.

> who is undergoing it

Every mass point.

> and what is its point of application?".

There is no point of application.

The centrifugal force is a -force field- that acts everywhere,

on every mass element.

And one last time:

You really should find out what the 'centrifugal force' is

according to everybody else, before trying to invent your own.

Jan

[one last time]

[Moderator's note: Yes, definitely! -P.H.]

> In this discussion the following question remained unanswered: "In my

> animation, and in the rotating reference in which the centrifugal force

> exists, who is exercising it,

> who is undergoing it

Every mass point.

> and what is its point of application?".

The centrifugal force is a -force field- that acts everywhere,

on every mass element.

And one last time:

You really should find out what the 'centrifugal force' is

according to everybody else, before trying to invent your own.

Jan

Sep 22, 2022, 4:18:21 AM9/22/22

to

Hi together

"So, how is the centrifugal acceleration of water justified EVEN in the

inertial reference where the centrifugal force is not there? "

Simple (first, from the beginning) Question might have no answer.

I thought about it too. (Who not!)

So I had a private answer at least by the aid of Einstein I hope.

1. In reality no (100%) inertial frame of reference does exist.

But this does not answer the question. But helps to understand Mach?

2. If you are a part of Newtons rotating bucket, let us say a water

molecule! You (molecule) do not know: Is there a new gravitational

force acting or are all molecules simply rotating (accelerating by

Newton himself).

3. If you are an observer outside the rotating bucket, standing still on

earth (which does not stand still). You know there is not a new G-Force

acting on molecules, there is a centrifugal force only, which is not

affecting on you like a G-Force could or would but there is acceleration

on the molecules simple to see for you but not for

molecules. Unfortunately you can not tell them what happens. So they

have two unique explanations. One is wrong. They never can find out. Man

can. (Woman of course too)

Best wishes

M.

By the way. Newtons definition of force from the Momentum Equation adopted

F=dP/dt=f1+f2+f3+f4+f5

leads to 5 force contributions from pure math applied. (We do not find all in textbooks!)

Now the "physics(nature)" must give them reality (by inventing,

Einsteins wording, f1 up to f5) which nature might follow or not tells

us the experiment. f3 is Coriolis and f5 I don't know. f2 is

Mass*Acceleration. We do not have it in textbooks. But every student can

find f1 to f5 by simple product rule applied! If you like see "mano4848

Sommerfeld Fine Structure Constant" (private investigation on YouTube)

https://www.youtube.com/results?search_query=manfred+sommerfeld+FSK

"So, how is the centrifugal acceleration of water justified EVEN in the

inertial reference where the centrifugal force is not there? "

Simple (first, from the beginning) Question might have no answer.

I thought about it too. (Who not!)

So I had a private answer at least by the aid of Einstein I hope.

1. In reality no (100%) inertial frame of reference does exist.

But this does not answer the question. But helps to understand Mach?

2. If you are a part of Newtons rotating bucket, let us say a water

molecule! You (molecule) do not know: Is there a new gravitational

force acting or are all molecules simply rotating (accelerating by

Newton himself).

3. If you are an observer outside the rotating bucket, standing still on

earth (which does not stand still). You know there is not a new G-Force

acting on molecules, there is a centrifugal force only, which is not

affecting on you like a G-Force could or would but there is acceleration

on the molecules simple to see for you but not for

molecules. Unfortunately you can not tell them what happens. So they

have two unique explanations. One is wrong. They never can find out. Man

can. (Woman of course too)

Best wishes

M.

By the way. Newtons definition of force from the Momentum Equation adopted

F=dP/dt=f1+f2+f3+f4+f5

leads to 5 force contributions from pure math applied. (We do not find all in textbooks!)

Now the "physics(nature)" must give them reality (by inventing,

Einsteins wording, f1 up to f5) which nature might follow or not tells

us the experiment. f3 is Coriolis and f5 I don't know. f2 is

Mass*Acceleration. We do not have it in textbooks. But every student can

find f1 to f5 by simple product rule applied! If you like see "mano4848

Sommerfeld Fine Structure Constant" (private investigation on YouTube)

https://www.youtube.com/results?search_query=manfred+sommerfeld+FSK

Nov 1, 2022, 3:44:18 AM11/1/22

to

[[Mod. note --

1. I apologise for the delay in processing this article, which arrived

in my moderation email on 2022-10-27, but was unfortunately

misclassified as spam by my email provider.

2. I have rewrapped overly-long lines.

-- jt]]

On Wednesday, July 13, 2022 at 5:50:42 AM UTC-4, Luigi Fortunati wrote:

> When Newton's bucket starts to rotate, the water slowly starts to

> rotate as well and accelerates outwards due to the centrifugal force.

>

> But the centrifugal force is ONLY in the rotating reference and not in

> the inertial one.

active forces but reactive forces. The active force on any given

point of a spinning object is the centripetal force imposed on it

by the mass-points on the same radius ..namely the ONES which are

closer to the rotational axis. The reaction force ,to the centripetal

force exerted in-and-on this given point......is the so called

centrifugal force. Objects.......exterior to the spinning body...will

appear rotating on circles around the same centre ...that is they

appear to be .....one would say..under the effect of some centripetal

forces. That said..... the mandatory force to exist in a rotational

frame is...the centripetal force ..and NOT the centrifugal force !

So...for parts of a spinning solid ,there is an action (centripetal

force)..and accordingly... there is a reaction (centrifugal force).

For matter which is outside of the spinning solid...there is ONLY...a

centripetal force, in the reference frame of that given solid.

Best regards, LL

1. I apologise for the delay in processing this article, which arrived

in my moderation email on 2022-10-27, but was unfortunately

misclassified as spam by my email provider.

2. I have rewrapped overly-long lines.

-- jt]]

On Wednesday, July 13, 2022 at 5:50:42 AM UTC-4, Luigi Fortunati wrote:

> When Newton's bucket starts to rotate, the water slowly starts to

> rotate as well and accelerates outwards due to the centrifugal force.

>

> But the centrifugal force is ONLY in the rotating reference and not in

> the inertial one.

>

> So, how is the centrifugal acceleration of water justified EVEN in the

> inertial reference where the centrifugal force is not there?

Centrifugal forces in rotational reference frames are not
> So, how is the centrifugal acceleration of water justified EVEN in the

> inertial reference where the centrifugal force is not there?

active forces but reactive forces. The active force on any given

point of a spinning object is the centripetal force imposed on it

by the mass-points on the same radius ..namely the ONES which are

closer to the rotational axis. The reaction force ,to the centripetal

force exerted in-and-on this given point......is the so called

centrifugal force. Objects.......exterior to the spinning body...will

appear rotating on circles around the same centre ...that is they

appear to be .....one would say..under the effect of some centripetal

forces. That said..... the mandatory force to exist in a rotational

frame is...the centripetal force ..and NOT the centrifugal force !

So...for parts of a spinning solid ,there is an action (centripetal

force)..and accordingly... there is a reaction (centrifugal force).

For matter which is outside of the spinning solid...there is ONLY...a

centripetal force, in the reference frame of that given solid.

Best regards, LL

Nov 2, 2022, 4:04:51 AM11/2/22