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Mar 6, 2023, 1:02:50 AM3/6/23

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The "apparent" force exists in the accelerated frame but does not exist

in the inertial frame.

For example, in the case of the slingshot, the (apparent) centrifugal

force exists in the rotating frame.

Does this mean that (in the rotating reference) there really is a

centrifugal force acting on the stone or do we imagine that there is

but, in reality, it isn't there at all?

Obviously, in the second case, no one would ever think of asking to

which fundamental force it belongs but, in the first case, if a force

really acts on the stone, we should be able to establish what kind of

force it is.

Well, is the apparent centrifugal force that really acts on the stone

during the rotation and in the rotating reference part of one of the 4

fundamental forces?

[[Mod. note -- No. The apparent centrifugal force is an artifact of

working in non-inertial (in this case rotating) coordinates. -- jt]]

in the inertial frame.

For example, in the case of the slingshot, the (apparent) centrifugal

force exists in the rotating frame.

Does this mean that (in the rotating reference) there really is a

centrifugal force acting on the stone or do we imagine that there is

but, in reality, it isn't there at all?

Obviously, in the second case, no one would ever think of asking to

which fundamental force it belongs but, in the first case, if a force

really acts on the stone, we should be able to establish what kind of

force it is.

Well, is the apparent centrifugal force that really acts on the stone

during the rotation and in the rotating reference part of one of the 4

fundamental forces?

[[Mod. note -- No. The apparent centrifugal force is an artifact of

working in non-inertial (in this case rotating) coordinates. -- jt]]

Mar 7, 2023, 12:12:20 PM3/7/23

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On Monday, March 6, 2023 at 12:02:50=E2=80=AFAM UTC-6, Luigi Fortunati wrote:

> The "apparent" force exists in the accelerated frame but does not exist

> in the inertial frame.

>

...
> The "apparent" force exists in the accelerated frame but does not exist

> in the inertial frame.

>

> Does this mean that (in the rotating reference) there really is a

> centrifugal force acting on the stone or do we imagine that there is

> but, in reality, it isn't there at all?

>

...
> centrifugal force acting on the stone or do we imagine that there is

> but, in reality, it isn't there at all?

>

It is easy to tell the difference between a real force and fictitious forces

due to an accelerating reference frame. Ask yourself if an observer riding

on the object would feel the force. If not it is a fictitious force.

Alternatively, use an inertial reference frame to describe the motion of

the object. If it is still accelerating in that frame it is a real force,

otherwise it is fictitious.

Rich L.

Mar 7, 2023, 12:19:30 PM3/7/23

to

that, and instead seeks to explain the behaviour of freely moving

objects as if the observer were in an inertial frame, the observer then

has to invent the centrifugal force.

So the force is not real. It arises from a mistaken world-view.

Sylvia.

Mar 9, 2023, 3:35:51 PM3/9/23

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Richard Livingston il 07/03/2023 02:12:15 ha scritto:

>> The "apparent" force exists in the accelerated frame but does not exist

>> in the inertial frame.

>>

> ...

>> Does this mean that (in the rotating reference) there really is a

>> centrifugal force acting on the stone or do we imagine that there is

>> but, in reality, it isn't there at all?

>>

> ...

>

> It is easy to tell the difference between a real force and fictitious forces

> due to an accelerating reference frame. Ask yourself if an observer riding

> on the object would feel the force. If not it is a fictitious force.

>

> Rich L.
>> The "apparent" force exists in the accelerated frame but does not exist

>> in the inertial frame.

>>

> ...

>> Does this mean that (in the rotating reference) there really is a

>> centrifugal force acting on the stone or do we imagine that there is

>> but, in reality, it isn't there at all?

>>

> ...

>

> It is easy to tell the difference between a real force and fictitious forces

> due to an accelerating reference frame. Ask yourself if an observer riding

> on the object would feel the force. If not it is a fictitious force.

>

Right.

An observer riding any particle of the string or sling stone feels the

centripetal force of the innermost adjacent particle and the

centrifugal force of the outermost adjacent particle.

If so (and it seems to me that it is) both of these forces are real and

there are no fictitious forces anywhere (of the spinning slingshot).

Luigi.

Mar 11, 2023, 2:49:24 AM3/11/23

to

the behavior of a moving object by an observer who is in an accelerated

reference) is good for the case of the lighter on the dashboard of the

car when cornering, where the lighter moves.

On the other hand, the rope and stone of the sling do not move (in the

rotating reference).

So, your definition is not good for the slingshot, because (again in the

rotating reference) there are neither centrifugal nor centripetal

accelerations.

The string and the stone rotate only in the inertial reference but not

in the accelerated one.

Luigi.

Mar 12, 2023, 7:23:42 AM3/12/23

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On 3/9/23 2:35 PM, Luigi Fortunati wrote:

> An observer riding any particle of the string or sling stone feels

> the centripetal force of the innermost adjacent particle

Yes.
> An observer riding any particle of the string or sling stone feels

> the centripetal force of the innermost adjacent particle

> and the centrifugal force of the outermost adjacent particle.

In the inertial frame of the center their path is a circle with an

acceleration 3-vector pointing to the center of rotation. It is clear

there is no acceleration away from the center, and therefore no force on

the observer in that direction.

In coordinates rotating with the observer, the observer is motionless,

with zero acceleration. In these coordinates a FICTITIOUS "centrifugal

force" arises that cancels the real centripetal force.

"Centrifugal force" IS fictitious. It is due PURELY to choice of

coordinates, and thus cannot model any real, natural phenomenon.

> (again in the

> rotating reference) there are neither centrifugal nor centripetal

> accelerations.

rotating coordinates cancels the real centripetal force, giving a net

force of zero IN THESE COORDINATES, and ONLY in these coordinates.

[I repeat: a quantity that depends on coordinates, like

"centrifugal force", cannot possibly model a real,

natural phenomenon, because nature uses no coordinates.

Ditto for the other fictitious forces...]

Tom Roberts

Mar 12, 2023, 4:55:34 PM3/12/23

to

Tom Roberts il 12/03/2023 12:23:34 ha scritto:

>> An observer riding any particle of the string or sling stone feels

>> the centripetal force of the innermost adjacent particle

>

> Yes.

>

>> and the centrifugal force of the outermost adjacent particle.

>

> Nope.

Nope? If you put yourself in the shoes of any particle of the string or
>> An observer riding any particle of the string or sling stone feels

>> the centripetal force of the innermost adjacent particle

>

> Yes.

>

>> and the centrifugal force of the outermost adjacent particle.

>

> Nope.

stone, do you feel that the innermost adjacent particle pulls you

towards the center and don't you feel that the outermost adjacent

particle pulls you to the opposite side?

> ...

> "Centrifugal force" IS FICTITIOUS and appears in the rotating coordinates...

"Appears" in what sense?

Does it appear in the sense that we "see" the force appear?

And how is this apparition manifested?

Can it be observed or measured?

Or is it just supposed to be there?

I would like to know not in the abstract but in the concrete and real

case of the spinning slingshot.

>

> Tom Roberts

Luigi.

Mar 13, 2023, 2:34:22 PM3/13/23

to

On 3/12/23 3:55 PM, Luigi Fortunati wrote:

> Tom Roberts il 12/03/2023 12:23:34 ha scritto:

>>> An observer riding any particle of the string or sling stone

>>> feels the centripetal force of the innermost adjacent particle

>> Yes.

>>> and the centrifugal force of the outermost adjacent particle.

>> Nope.

>

> Nope? If you put yourself in the shoes of any particle of the string

> or stone, do you feel that the innermost adjacent particle pulls you

> towards the center and don't you feel that the outermost adjacent

> particle pulls you to the opposite side?

The adjacent particle on the outside exerts the centripetal force that
> Tom Roberts il 12/03/2023 12:23:34 ha scritto:

>>> An observer riding any particle of the string or sling stone

>>> feels the centripetal force of the innermost adjacent particle

>> Yes.

>>> and the centrifugal force of the outermost adjacent particle.

>> Nope.

>

> Nope? If you put yourself in the shoes of any particle of the string

> or stone, do you feel that the innermost adjacent particle pulls you

> towards the center and don't you feel that the outermost adjacent

> particle pulls you to the opposite side?

constrains the observer to move in a circle. There is no force exerted

by the adjacent particle on the inside, because the string is in tension

and is incapable of exerting an outward force.

Note also that "centrifugal force" is fictitious and appears only in the

rotating coordinates. It cannot ever be "felt" by an observer, because

it is not real (in any sensible sense of the word).

Remember that no coordinate-dependent quantity can model any real

phenomenon in the world we inhabit, because nature uses no coordinates.

(Coordinate dependence would mean that multiple calculated values would

all have to be equal to the single value of nature.) We humans use

coordinates to describe and model the world -- they are an arbitrary

construct of humans; coordinates are imaginary, though clocks and rulers

used to implement them are real.

>> "Centrifugal force" IS FICTITIOUS and appears in the rotating

>> coordinates...

>

> "Appears" in what sense?

those rotating coordinates.

> Can it be observed or measured?

placing a spring scale in the appropriate place. Such a scale cannot

measure the "centrifugal force" on an object because there is no place

to put its other end.

> Or is it just supposed to be there?

the "centrifugal force" -- otherwise Newton's laws to not describe what

happens.

Tom Roberts

Mar 17, 2023, 12:04:15 AM3/17/23

to

Op maandag 6 maart 2023 om 03:02:50 UTC-3 schreef Luigi Fortunati:

Don't we forget Newton's law here (action=reaction)?

In both positions, the anchor point and the rotating mass, two forces

hold each other in equilibrium.

In the anchor point, the centrifugal force transmitted via the rope is

compensated for at each moment by the ground reaction.

In the rotating mass, it is equally compensated by the ground reaction,

transmitted along the rope.

Even in the rotating system, the "rotating observer" will be aware of

their "free movement" being hindered by a reaction in the rope; and

conclude that this is accounted for by their not belonging to an inertial

system.

Consider being in a rotor: https://en.wikipedia.org/wiki/Rotor_(ride)

Do you imply that people are feeling fictitious forces, when they don't

see "proper" movement?

--

guido wugi

In both positions, the anchor point and the rotating mass, two forces

hold each other in equilibrium.

In the anchor point, the centrifugal force transmitted via the rope is

compensated for at each moment by the ground reaction.

In the rotating mass, it is equally compensated by the ground reaction,

transmitted along the rope.

Even in the rotating system, the "rotating observer" will be aware of

their "free movement" being hindered by a reaction in the rope; and

conclude that this is accounted for by their not belonging to an inertial

system.

Consider being in a rotor: https://en.wikipedia.org/wiki/Rotor_(ride)

Do you imply that people are feeling fictitious forces, when they don't

see "proper" movement?

--

guido wugi

Mar 17, 2023, 9:13:27 AM3/17/23

to

Tom Roberts il 13/03/2023 05:34:17 ha scritto:

>>>> An observer riding any particle of the string or sling stone

>>>> feels the centripetal force of the innermost adjacent particle Yes.

>>>> and the centrifugal force of the outermost adjacent particle.

>>> Nope.

>>

>> Nope? If you put yourself in the shoes of any particle of the string

>> or stone, do you feel that the innermost adjacent particle pulls you

>> towards the center and don't you feel that the outermost adjacent

>> particle pulls you to the opposite side?

>

> The adjacent particle on the outside exerts the centripetal force that

> constrains the observer to move in a circle...
>>>> An observer riding any particle of the string or sling stone

>>>> feels the centripetal force of the innermost adjacent particle Yes.

>>>> and the centrifugal force of the outermost adjacent particle.

>>> Nope.

>>

>> Nope? If you put yourself in the shoes of any particle of the string

>> or stone, do you feel that the innermost adjacent particle pulls you

>> towards the center and don't you feel that the outermost adjacent

>> particle pulls you to the opposite side?

>

> The adjacent particle on the outside exerts the centripetal force that

What you wrote is absurd!

The force that the innermost particle exerts on the outermost one and

that that the outermost particle exerts on the innermost one cannot

both be centripetal!

If one is centripetal, the other must be centrifugal, and vice versa.

Luigi.

Mar 17, 2023, 9:13:49 AM3/17/23

to

Guido Wugi il 16/03/2023 15:04:10 ha scritto:

> Don't we forget Newton's law here (action=reaction)?

I won't forget it for sure because I wrote about these.
> Don't we forget Newton's law here (action=reaction)?

> In both positions, the anchor point and the rotating mass, two forces

> hold each other in equilibrium.

> In the anchor point, the centrifugal force transmitted via the rope is

> compensated for at each moment by the ground reaction.

> In the rotating mass, it is equally compensated by the ground reaction,

> transmitted along the rope.

And they are contact forces between adjacent particles (of the string

and of the stone) and not between the particles and the ground!

See my animation

https://www.geogebra.org/m/kx5kk285

Particle B communicates and exchanges action and reaction forces

(centripetal and centrifugal) with particles A and C, particle C with B

and D, and so on.

Only particle A interacts with the ground, not the others.

> Even in the rotating system, the "rotating observer" will be aware of

> their "free movement" being hindered by a reaction in the rope;

force on the rope!

> and conclude that this is accounted for by their not belonging to an

> inertial system.

> Consider being in a rotor: https://en.wikipedia.org/wiki/Rotor_(ride)

> Do you imply that people are feeling fictitious forces, when they don't

> see "proper" movement?

They feel the real centripetal force of the wall on them and feel that

they are exerting a real centrifugal force on the wall.

Mar 17, 2023, 7:17:58 PM3/17/23

to

On 3/17/23 8:13 AM, Luigi Fortunati wrote:

> Tom Roberts il 13/03/2023 05:34:17 ha scritto:

>> The adjacent particle on the outside exerts the centripetal force

>> that constrains the observer to move in a circle...

>

> What you wrote is absurd!

No. What I wrote is correct. You misread and added your own misconceptions.
> Tom Roberts il 13/03/2023 05:34:17 ha scritto:

>> The adjacent particle on the outside exerts the centripetal force

>> that constrains the observer to move in a circle...

>

> What you wrote is absurd!

> The force that the innermost particle exerts on the outermost one

> and that that the outermost particle exerts on the innermost one

> cannot both be centripetal!

between particles of the string are called tension. The string has a

tension that enables its particles to exert a centripetal force on the

observer (because the observer is connected to the string).

> If one is centripetal, the other must be centrifugal, and vice

> versa.

of the "fictitious forces" that arise in rotating coordinates to permit

one to apply Newton's laws in the rotating coordinates as if they were

inertial. In particular, we NEVER use that term for any other

outward-directed force. You violate this usage, and have confused yourself.

In this physical situation, the observer is tethered by a (massless)

string to a central mounting point, and moves in a uniform circular path

around it. The forces are:

a) the string exerts an outward-bound force of tension on the

central mounting point.

b) the string exerts an inward-bound force of tension on the

observer.

c) the central mounting point exerts a reaction force on the

string that is equal and opposite to (a).

d) the observer exerts a reaction force on the string that is

equal and opposite to (b).

These are the only forces in the problem; here they are all referenced

to the inertial frame of the central mounting point. We rarely discuss

(c) and (d) as they are trivial; the pairs (a,c) and (b,d) each satisfy

Newton's third law. Note that (b) is the only force on the observer, and

the acceleration corresponding to it makes the observer move in a

uniform circle around the central mounting point, while the observer and

string rotate around it; that is a basic application of Newton's second law.

If one wants to analyze this using the rotating coordinates in which the

observer and string are at rest, one must imagine an additional

"centrifugal force" equal and opposite to the tension force on the

observer (because in these coordinates the observer is at rest, so must

have zero total applied force) [#]. This is all well known, and the

"centrifugal force" is determined by the rotation of the coordinates and

by the radius and mass of the observer. Note that the "centrifugal

force" is proportional to radius, and thus is zero on the central

mounting point (think about it -- that point is not rotating).

[#] The other "fictitious forces" of rotating

coordinates, the "Coriolis force" and the "Euler

force", are both zero in this physical situation.

[This is getting overly repetitive, and I will not

participate further. Get a good book on Newtonian

mechanics and STUDY IT. Perhaps also read

https://en.wikipedia.org/wiki/Centrifugal_force]

Tom Roberts

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