[rmolnav]

LAST CHAPTER  of the "story" ...
Then I got following answer:
"> Everybody knows the adjective "centrifugal" implies some kind of "fleeing from a center" (quoted from my previous mail)
Sure, and that's how the word is used in physics, to denote the fact that we notice things trying to fly outwards when we are in a rotating frame.  Newton's laws don't hold in non-inertial frames, so when something accelerates in a rotating frame, we are not obliged to say that this acceleration was produced by a force.  But if we insist on doing so, then we call the force centrifugal.
The force that a hammer exerts on the thrower is something we speak of in the thrower's frame, which is being taken as inertial for the sake of argument here.  It's a real force, unlike the one that we invent in the rotating frame if we want to apply Newton's laws there.  If we called the force produced by the hammer a centrifugal force, there would be a confusion about which frame we were discussing: inertial or non-inertial.  We just reserve the word "centrifugal" for rotating frames.
The force of the hammer on the thrower in the inertial frame is like the force of the ground on you or me; it's typically called a reaction force, but it doesn't get a more special name than that because it doesn't need one.  It's just the force that Newton's Third Law says is there.  You want to apply the word "centrifugal" to this scenario too, but like any word, if the meaning we give to it is too broadly applicable, it loses its value in any quantitative discussion.
Reserving that word for rotating frames only makes it more useful.
There's a big, big difference between a real force of tension and a force that we invoke in a rotating frame to make Newton's laws work outside their normal region of validity.  We signal that big difference by giving the latter invoked force the special name of "centrifugal".  Sure, it's a question of how such words are defined, but as I said, giving a word a wide meaning reduces its usefulness in quantitative discussions".
AND THEN I sent him what I posted here a week ago (#175).

[rmolnav]

Some of you, if has followed my posts with certain interest, surely has seen I´m not too keen on that  inertial and non-inertial system of reference mathematical "trick", when used to state that centrifugal force is not a real force, that is ficticious ...
Recently, when posting here several pieces of email I interchanged with the people from:
http://math.ucr.edu/home/baez/physics/General/Centrifugal/centri.html,
after seeing the bizarre explanations given by them, I showed my stand and said (#178):
"I INSIST: all that artificial "tricky" tool of inertial and non inertial frames of reference, and wherever things such as "Centrifugal force is, by definition, a force that appears in a rotating frame. That's its definition" are said, should not be considered as kind of a bible. Perhaps there should be a clear "User´s Manual", were it possible, to try an avoid confusion, or at least diminish its likelihood … "
I had previously refuted those type of ideas, e.g. on #159:
"In that post you referred to accelerations ("...objects which are moving with no acceleration in the inertial frame will accelerate in your frame"). That is quite clear... It´s similar to what happens, e.g., when pilots of two sailing ships, each with its own course and speed, look at each other ... We have what is called "relative course" ... What you see is the result of deducting your own vector velocity from the other´s ...
But I consider that to extend that idea of moving reference frames to forces is rather erroneous, or at least it can lead to confusion.   
I´m going to bring up another example, not mentioned before as far as I can remember.
Imagine a fun ground whip, where we all enjoyed when children. Let us put a weight between two springs, already a little stretched, forming a horizontal radial line with one extreme fixed to the closed handle bar, and the other to the seat back.
When the cart (?) at most bent part of its trajectory, inner spring will be clearly stretched much more ... If it were a spring scale it would show the increase of forces acting on its hooks. The weight would be exerting an outward force on outer scale hook, that is a REAL centrifugal force (scales don´t gauge only "apparent" or "fictitious" forces !!)
And that would be a fact, whatever it were observed by somebody standing still outside the installation, or by somebody inside the cart and experiencing same accelerations as the weight ..."
Yesterday, ruminating rather unwillingly on the issue, I had a kind of EUREKA moment, and I think I saw the actual root cause of existing confusion caused by that "tricky" mathematical tool.
I´ll draft another post carefully, in order to do my best trying to unveil the idea as clearly as possible ... It´s a kind of philosophical reasoning, rather than only physical, and I didn´t wish it could be to no avail (if I were not able to convey my idea), as so many times has happened !!

[rmolnav]

I´ll draft another post carefully, in order to ...

Preparing that, i´ve entered Wikipedia, initially looking for frames of reference, and there is a link to centrifugal force, where I´ve rarely entered, and I decided to have a look at.
They use the so common definition:
"In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) directed away from the axis of rotation that appears to act on all objects when viewed in a rotating frame of reference".
BUT further down one can read:
"Within this view of physics (?), any other phenomenon that is usually attributed to centrifugal force can be used to identify absolute rotation. For example, the oblateness of a sphere of freely flowing material is often explained in terms of centrifugal force. The oblate spheroidshape reflects, following Clairaut's theorem, the balance between containment by gravitational attraction and dispersal by centrifugal force. That the Earth is itself an oblate spheroid, bulging at the equator where the radial distance and hence the centrifugal force is larger, is taken as one of the evidences for its absolute rotation".
Could that "dispersal by centrifugal force", if this c.f. depended on the type of frame of reference, and were "ficticious" if in relation to a non-inertial frame of reference, have caused the oblateness of our planet, being the so called equatorial bulge higher than 40 km ??
We have to keep in mind that Earth oblateness is not only as far as oceans are concerned, it also affect solid Earth.
Curiously, Earth´s mountain peaks farthest from its C.G. are not in the Himalayas (e.g. the Everest), but in Peruvian Andes, precisely due to billions of year action of centrifugal forces (logically, quite REAL).

[PmbPhy]

The centrifugal force is an inertial force. For details on such forces see
http://www.newenglandphysics.org/physics_world/gr/inertial_force.htm

In particular, the centrifugal force is an inertial force which is observed in rotating (non-inertial) frames.

I has nothing to do with things on a string in an inertial frame. Inertial forces were important to Einstein when he was creating general relativity. In the February 17, 1921 issue of Nature Einstein wrote

Can gravitation and inertia be identical? This question leads directly to the General Theory of Relativity. Is it not possible for me to regard the earth as free from rotation, if I conceive of the centrifugal force, which acts on all bodies at rest relatively to the earth, as being a "real" gravitational field of gravitation, or part of such a field? If this idea can be carried out, then we shall have proved in very truth the identity of gravitation and inertia. For the same property which is regarded as inertia from the point of view of a system not taking part of the rotation can be interpreted as gravitation when considered with respect to a system that shares this rotation. According to Newton, this interpretation is impossible, because in Newton's theory there is no "real" field of the "Coriolis-field" type. But perhaps Newton's law of field could be replaced by another that fits in with the field which holds with respect to a "rotating" system of co-ordinates? My conviction of the identity of inertial and gravitational mass aroused within me the feeling of absolute confidence in the correctness of this interpretation.

[rmolnav]

The centrifugal force is an inertial force. For details on such forces see
http://www.newenglandphysics.org/physics_world/gr/inertial_force.htm
In particular, the centrifugal force is an inertial force which is observed in rotating (non-inertial) frames.

I´m afraid that is rather confusing ... What do you mean with "... which is observed in ..."?
Does it mean it is a real force "observed" by us? Or, in that "particular" case, is it rather a kind of tricky tool WE apply to bring a "fictitious" situation (an accelerating frame of reference) back to reality (with all REAL effects which have disappeared in that frame of reference?
Apart from that, linked site states (according to Einstein):
"In general relativity inertial forces are identical in nature to gravitational forces" and defines:
"Inertial force: When the motion of the reference system generates a force (defined as the time rate of change of momentum, i.e. f º dp/dt, which has to be, as measured in that system, we call that force an inertial force".
Therefore we could state:
"Centrifugal forces (as above defined) = inertial forces ("generated" ? by the reference system) = gravitational forces (acc. to Einstein)"
and, as far as I can understand, either both centrifugal and gravitational forces are REAL, or both are fictitious ...
Do you mean both are fictitious? ... Then, the question I posted yesterday (by the way, have you read it?):
"Could that "dispersal by centrifugal force", if this c.f. depended on the type of frame of reference, and were "ficticious" if in relation to a non-inertial frame of reference, have caused the oblateness of our planet, being the so called equatorial bulge higher than 40 km ??
would have an easy answer: that lifting of huge mountains is not the result of two opposite forces, but due to the distortion of space-time ...
But, whatever the deep nature of gravity and inertia, the phenomena considered by us can be explained within Newton´s Mechanics !!

[PmbPhy]

I´m afraid that is rather confusing ... What do you mean with "... which is observed in ..."?

Sorry. I had no idea that would be confusing, .To "observe" means. Whenever you see a term you don't understand find a dictionary and look it up. In this case:
https://en.oxforddictionaries.com/definition/observation

The action or process of closely observing or monitoring something or someone. etc

To observer "in" means that you must be at rest in the non-inertial frame of reference. E.g. In the movie Mission to Mars and The Martian the astronauts can go to part of the ship which is rotating with respect to an inertial frame. An object in this part of the spaceship which has no force on it as observed in the inertial frame is accelerating in this rotating frame. Newton attributed that to using the wrong frame. Einstein came along and said there are no wrong frames. This acceleration as determined by observers in the rotating frame is said to be caused by the coriolis force which Einstein's general relativity views as a gravitational force. Einstein said it was a real force. I did post a URL to my website which makes all of this crystal clear. If people don't read what I have to say then I don't see a reason for saying it.

[rmolnav]

Quote from: rmolnav
I´m afraid that is rather confusing ... What do you mean with "... which is observed in ..."?
Sorry. I had no idea that would be confusing, .To "observe" means. Whenever you see a term you don't understand find a dictionary and look it up

Sorry, but my question was about what YOU mean when saying "... force which is observed in...", rather than what said on a dictionary … Because, as on Oxford dictionary:
"Observe: Notice or perceive (something) and register it as being significant", and
"Force: An influence tending to change the motion of a body or produce motion or stress in a stationary body".
One can "notice or perceive" directly a body and its motion, but  NOT something which is "an INFLUENCE tending to change the motion of a body or …"
To say which possible "influence" is the one you "notice or perceive" requires the intervention of not only your senses, but also your mind …
And your ideas could be right or wrong. Therefore one can´t actually "notice or perceive" a certain "influence" is the one "tending to change the motion of a body or produce motion or stress in a stationary body".
By the way, I also said:
"Does it mean it is a real force "observed" by us? Or, in that "particular" case, is it rather a kind of tricky tool WE apply to bring a "fictitious" situation (an accelerating frame of reference) back to reality (with all REAL effects which have disappeared in that frame of reference?"
Have you read that and consider it at least as a possibility? ...
I´m pretty sure many people misinterpret frame of reference issue, and draw wrong consequences.
As Einstein said, "there are no wrong frames" ... but what above said can and does happen !!

[Vasyl]

Can anyone explain an attached example of centrifugal force?

[Vasyl]

I mean that a centrifugal force can lift conveyors up if conveyors accelerate body on one side of tape and decelerate it on the other one. How is it possible? I can not understand my mistake.

[Halc]

Can anyone explain an attached example of centrifugal force?

It labels F as centrifugal force, but the picture doesn't depict it.

The formula for centrifugal force (in a rotating frame) is mωr (where ω is angular velocity, labeled as L in the picture).  mv²/r is centripetal force, but in a rotating frame, v is zero, so that doesn't work.  In an inertial frame, there is no centrifugal force.
The F in the picture seems to depict a force being applied to the pulleys to apply tension in the belt.  It doesn't affect the speed of anything nor does it affect the forces being applied to m as it goes around.

Somehow the V in the picture is zero at the bottom and max at the top, which isn't what one would expect of a belt turning with constant linear speed at any point in the belt.

There is mention of acceleration, but it is unclear what is accelerating.

I mean that a centrifugal force can lift conveyors up if conveyors accelerate body on one side of tape and decelerate it on the other one. How is it possible? I can not understand my mistake.

The body seems to be moving at constant speed in the picture.  Why would the tape be slowing when the weight is on one side and speeding up when the weight is on the other?  That is just not depicted in the picture.

Yes, the belt will tend to bow out its sides at high speeds, and that inertial effect is a function of the tension on the belt, the distance between the pulleys, and of course the speed of the belt.  If the belt speed is changing, it needs to be specified.  The picture suggests that the belt is stopped at the bottom and moving at the top, which makes no sense.  The entire belt needs to move at the same speed or the belt breaks.


The center of gravity of the whole setup never moves, regardless of speed of any of the parts.

As for using a bigger wheel at one end and constant belt speed, that indeed makes no changes except that the weight isn't going to slow at one end or the other.  There will be no acceleration/deceleration since you specified constant belt speed.

[Vasyl]

The body seems to be moving at constant speed in the picture.  Why would the tape be slowing when the weight is on one side and speeding up when the weight is on the other?

A motor drives a tape and we can accelerate it or decelerate it. The topic is "is lifting force possible?"

[Vasyl]

The picture suggests that the belt is stopped at the bottom and moving at the top, which makes no sense.

There are arrows with lettering "acceleration", "deceleration". It means that we accelerate it from zero to max speed and decelerate it from max to zero.

[Vasyl]

The entire belt needs to move at the same speed or the belt breaks.

Yes, It does. You can even move a body at the same speed but with bigger wheel radius at the bottom and you will have the same result.

[Vasyl]

The center of gravity of the whole setup never moves, regardless of speed of any of the parts.

Thanks a lot for your reply. I will try to understand it.

As for using a bigger wheel at one end and constant belt speed, that indeed makes no changes except that the weight isn't going to slow at one end or the other.  There will be no acceleration/deceleration since you specified constant belt speed.

I wanted to say that centrifugal force will be bigger on a top of conveyor and less at the bottom of it. After sum of two forces you have force in direction against gravity.

[Halc]

The topic is "is lifting force possible?"

The topic is "What is centrifugal force?".  There never was a mention of 'is lifting force possible", including on the diagram.  Of course lifting force is possible.  I lift things every day.
I think you're trying to describe a mechanism for reactionless thrust, which would violate physics.

As for using a bigger wheel at one end and constant belt speed, that indeed makes no changes except that the weight isn't going to slow at one end or the other.  There will be no acceleration/deceleration since you specified constant belt speed.

I wanted to say that centrifugal force will be bigger on a top of conveyor and less at the bottom of it. After sum of two forces you have force in direction against gravity.

The mass spends twice the time at the bottom going around the circumference of the larger wheel, so half the force for double the time makes for zero net difference.

The forces that are depicted in the picture point the wrong way.  The weight is going to pull on the belt, not the wheels.  Tension on the belt is what pulls the opposite wheel, so the weight going over the top will put tension on the belt that pulls up on the lower wheel.  The weight going slow at the bottom pulls down at a minimum on the upper wheel.  Same net effect, but the wheels are always being pulled together in such a setup and would need some sort of bar to keep them separated.

What is missing from the picture is the reaction thrust needed to accelerate the weight as it goes up and down.  That (in both directions) causes a downward force on the upper wheel that exactly cancels the upward force on the lower wheel.

[Vasyl]

I think you're trying to describe a mechanism for reactionless thrust, which would violate physics.

I have understood my mistake. I knew that reactionless thruster is fake but I didn't see it in mechanism above. No, I am not trying to violate physics' laws. Now, I see my mistake, thanks.

[Vasyl]

What is missing from the picture is the reaction thrust needed to accelerate the weight as it goes up and down.  That (in both directions) causes a downward force on the upper wheel that exactly cancels the upward force on the lower wheel.

By the way, Newton's third law is fulfilled. On the picture you see two conveyors which are rotated in different directions like in helicopter with two propellers.

[rmolnav]

The formula for centrifugal force (in a rotating frame) is mωr (where ω is angular velocity, labeled as L in the picture).  mv2/r is centripetal force, but in a rotating frame, v is zero, so that doesn't work.  In an inertial frame, there is no centrifugal force.

I don´t know if you have gone on reading the really long discussion David Cooper and myself have had on the thread "Why we do have two high tides a day?" ...
Most important question there have been precisely whether what discussed here, centrifugal force, intervenes or not in the formation of tides ...
Over last several months I´ve got a rather fresh approach to the issue, especially to its relation to the idea, as in what quoted from you, that " In an inertial frame, there is no centrifugal force".
What follows is my post #382 in mentioned thread, and it might be interesting for you, or other members, at least to have a look at.
"MY ULTIMATE GO ? (5th part)
Before trying and conveying my stand on non-inertial frames issue, let us go back to the drawing board, as if we didn´t have any education on dynamics …
Same image with Earth (E) on upper part, and vertically below the Moon (M).
Let us suppose we only know how they move, with kind of the mind of a “smart” child.
And then we learn that (as David Cooper said many times), if Earth and Moon tangential speeds somehow became null, they would begin to accelerate straightly towards each other, because of gravitational mutual attraction …
If just stopping their circular movements, they get “free” to accelerate towards each other … THOSE CIRCULAR MOVEMENTS ARE WHAT, WHEN EXISTING, WERE SOMEHOW COUNTERING THOSE STRAIGHT ACCELERATIONS NOW THEY HAVE GOT !!  No other logical possibility for a “clean” child mind …
How come? Let us see …
Curiously, with a rotating (so called "non-inertial”) frame of reference, same thing happens, but let us say “virtually” …
As the frame of reference rotates with E and M, E and M don´t "rotate" anymore  relatively to that frame of reference … Centripetal forces are not required whatsoever, and gravitational pull would make them fall onto each other … But that´s far from reality …
Then physicists go on: "How can we do “real” maths with that “artificial” (the adjective is mine ...) reference system ? … We have to apply a “fictitious” force, which we´ll call centrifugal force". That way WE GET BACK TO THE REAL SCENARIO … as far as dynamics is concerned.
Well, that force is certainly “fictitious”, because it has been introduced by us to work with that non-inertial frame of reference … BUT THAT MEANS THAT THE REAL CIRCULAR MOVEMENTS, NOT NEEDING THE ADDITION OF ANY “ARTIFICIAL” FORCE, SOMEHOW PRODUCE AN INERTIAL EFFECT THAT KEEPS E AND M WITHOUT FALLING STRAIGHTLY ONTO EACH OTHER !! TO ME (AND NOT ONLY ME), THAT IS A CENTRIFUGAL FORCE, QUITE REAL !!
In case B (1st part of “MY ULTIMATE GO ?"), as E and M were moving horizontally at same speed, Moon´s pull was able not only to bend E´s trajectory, but also to accelerate E straightly towards M with its full strength, quite a “free” acceleration. That “freedom” was made possible by the movement of the moon towards the left, with same horizontal speed as E.
The real case is dynamically quite the opposite: the movement of M in opposition to E´s speed, and with speeds which make them to revolve/rotate around the barycenter, makes impossible the decrease of E-M distance, and even any proper orbiting of E around M. It gives us a “rotating” scenario, where all forces have basically same direction as the line between centers of mass, and where the angular position doesn´t matter much (as far as Earth-Moon dynamics is concerned), as long as we accept centrifugal forces keep the separation constant …
After all, if Earth and Moon were the only celestial objects in the universe, to talk about angular position of the system would have no sense at all …
BUT, STILL, THE WOULD HAVE TO KEEP ROTATING. THAT WAY INHERENT INERTIAL FORCE, THE CENTRIFUGAL FORCE, WOULD KEEP THEM WITHOUT FOLLOWING STRAIGHTLY ONTO EACH OTHER !! 
Therefore, as I´ve said many times, I find quite correct what a NOAA scientist told me:
"... to provide a basic description of the forces which create the tides.  It's intended audience were the grade school children and adults of that time.  It used terminology of science and forces which were common in the 1950s.  Such as centrifugal force.  Centrifugal force was always an "imaginary force" (not a real / measurable force).  But that type of description made the concepts easier to understand and explain.  That  description and use of centrifugal force continued to be common practice until the 1970-80's.  At that point, the terminology shifted and the TEXTBOOKS USED IN GRADE SCHOOLS WERE CHANGED TO USE A MORE MODERN TERMINOLOGY AND DESCRIPTION OF THIS “EFFECT”BEING A RESULT OF INERTIA RATHER THAN AN “IMAGINARY FORCE”".
Initially I didn´t fully grasp his point ... But later I did.
The problem is that many books, dictionaries included, keep following models with ideas which are several decades out of date …"


   

 

[PmbPhy]

What is centrifugal force?

Its the force on an object resulting from observing the object in a rotating frame of reference.

In the previous discussion Alancalverd stated that there was no such thing as centrifugal force.

That's because he uses a definition of force which applies only in inertial frames.

I recommend reading the web page I created on this subject at:
http://www.newenglandphysics.org/physics_world/gr/inertial_force.htm

[alancalverd]

In the previous discussion Alancalverd stated that there was no such thing as centrifugal force.

I sincerely hope I didn't! Others, and the writers of trendy textbooks, deny its existence, but I have felt the tug on the string, and wondered why, if the only force on a rotating object is centripetal, the tyres fly off wheels at high speed and the denser parts of blood end up on the outside of the centripete (just because it says "centrifuge" on the outside of the machine doesn't mean that generations of scientists, engineers and aviators knew what they were talking about, children!)

"Ah, but if you cut the string, the stone doesn't fly away from the center!" Oh yes it bloody does. Tangentially, not radially, but still away from the center (assuming that you and I share the definition of "away").

Here's the simple truth

1. A body continues in a straight line unless acted upon by a force.

2. To make a body turn in a circle, you need to apply a force towards the center of the circle

3. Every action provokes an equal and opposite reaction

so the tension pulling the stone inwards is balanced by the force I feel pulling my hand outwards

But ...... and here's a paradox for those interested in such things..... if the inward and outward forces are identical, there is no net force inward, so the stone doesn't move in a circle after all! The inward force must exceed the outward force! Free energy, perpetual motion, collapse of Newtonian physics......