[rmolnav]

In several threads, about object dynamics: gravity, tides, centrifugal forces, Newton´s Laws …, I´ve long been showing my stand.
I got some replies, but almost none agreeing with me clearly … I don´t actually know the stand of most readers about “my" ideas ...
There is a detail it would be very interesting for me to receive more comments about, before going any further with other consequences of the issue I have in mind and I´m planing to send. In order to have a cleaner ground to play on …
It´s the question of how we (and somehow objects) feel gravity.
If we (or an object) are pulled by a massive object, and nothing else prevent us to accelerate what required by Newton´s 2nd Law, internal stresses are almost negligible: as if with no gravity, either still or with constant velocity vector.
But if a 3rd object don´t let us move with required 2nd Law acceleration, exerting on us another force, Newton´s 3rd Law “turns up” and we feel internal stresses. THOSE stresses are what actually “tell” us we are in a gravity field (whatever the deep explanation of gravity), NOT GRAVITY itself.
If we are standing, mainly feet and legs compression. If sitting, our bottom compression (especially if a hard seat !!). If in bed, smaller compression along our body. If hanging from our hands and arms, we feel strong tensile stresses there …
And if only part of the theoretically required “g” is allowed to happen, above mentioned stresses will be smaller, proportionally to “not allowed” acceleration … I mean, if e.g. a skyscraper lift fell with g/3 acceleration, and we were standing on its floor, at our feet and legs (and rest of our body) we would feel 2/3 of the compression stresses we were feeling when no movement (or with constant speed movement)

[hamdani yusuf]

It´s the question of how we (and somehow objects) feel gravity.If we (or an object) are pulled by a massive object, and nothing else prevent us to accelerate what required by Newton´s 2nd Law, internal stresses are almost negligible: as if with no gravity, either still or with constant velocity vector.But if a 3rd object don´t let us move with required 2nd Law acceleration, exerting on us another force, Newton´s 3rd Law “turns up” and we feel internal stresses. THOSE stresses are what actually “tell” us we are in a gravity field (whatever the deep explanation of gravity), NOT GRAVITY itself.

We can see astronauts in ISS don’t feel gravity even though the gravitational fields there are still high (almost 90% of gravity on earth surface). That’s because the station is in free fall.
The internal stresses are caused by difference of force vectors working on different body parts. Let’s compare the feelings of ISS astronauts with divers, sky divers, or someone doing indoor skydiving, or levitated frog in a strong magnetic field.
I think we can still find out that we are in a gravity field even when we don’t feel the stresses, by calculating the apparent movement of distant stars.

[rmolnav]

We can see astronauts in ISS don’t feel gravity even though the gravitational fields there are still high (almost 90% of gravity on earth surface). That’s because the station is in free fall.

Thank you.
I would rather say "That’s because THEY are in free fall" ... I know you also mean that, but I kind of flee from expressions which seem to refer to the reference frames, as unique reason of what happens ...
Apart from that, I insist: they don´t actually feel gravity, because gravity itself can´t be felt. We feel internal stresses, that appears when not all parts of our body are allowed to fall "100% free",

The internal stresses are caused by difference of force vectors working on different body parts.

Yes. But those vectors are not only Earth´s gravity pull vectors on different body parts ...
I´ve already commented this in many different cases: as our body is "obliged" to move as a whole, and its parts are not equally far away from Earth´s C.G., on the one hand gravity pull per unit of mass varies from part to part, especially if we are in a "radial" position. The farther, the smaller. And on the other hand, required centripetal acceleration for actual rotation (ω²r), varies too. The farther the bigger.
That´s why our farther parts, not being sufficiently attracted by Earth to rotate at ω angular speed, have to be additionally pulled by inner contiguous parts ... That also causes pulls from outer parts on inner contiguos ones, in the opposite sense (Newton´s 3rd Law) (by the way, those last pulls are real centrifugal forces ...).
All those internal stresses, apart from stretching (in this case) our bodies, are the FACTS that we actually feel (whatever the frame of reference), NOT gravity itself. And not only in this particular case.
That "idea" is what I do want to convey, both in my last post and in this one.   

[rmolnav]

Let’s compare the feelings of ISS astronauts with divers, sky divers, or someone doing indoor skydiving, or levitated frog in a strong magnetic field.

Also in those cases what I say applies.
Internal stresses felt by astronauts are very, very small, and distributed across their bodies.
When on Earth much bigger additional forces, pushing or pulling, have to be exerted on our bodies by other external objects, to keep us somehow rotating at zero altitude. Newton´s 3rd Law implies we exert opposite forces on those objects ...
Each part of our body can have only the required centripetal acceleration at zero altitude (apart from sky divers case). As also Newton´s 2nd Law applies for each body part (due to same reasons as what said when in ISS) internal stresses occur, in each case depending on the way external forces are exerted on us ...
As in all those cases external forces are applied distributed over big areas of our bodies, in no body part internal stresses accumulate significantly, as if if we were "floating"  ... But they do occur.
As I previously said, in all cases what is felt is NOT GRAVITY itself, but those internal stresses. And they will be proportional to the fraction of Earth´s attraction which is not "allowed" to cause any actual acceleration, centripetal for the rotation, or linear if any (e.g. sky divers).

[rmolnav]

In an e-mail discussion with a scientist from NOAA, after some comments I sent him in relation to a publication from them I had seen on the internet (that resulted rather old), he told me literally:
"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"."
I would add that, if correctly considered applied NOT to the object following a curved path, but to the other object which produces the centripetal acceleration of the former, what he calls "this effect", not an "imaginary force" as he says, is quite a REAL force (certainly due to inertia, clearly expressed by Newton in his 3rd Law of Motion) ... The same I´ve always been saying here, since my #28 of 2/10/2016 !!
By the way, I suggest you to have a look at:
https://www.thenakedscientists.com/forum/index.php?topic=49715.new
Recently comments directly connected to centrifugal forces have been posted there ...
Allow me say I consider them very interesting, though I myself sent them ...

[alancalverd]

Back to the elementary "whirling rock on a string". There is nothing imaginary about the tension in the string. You can calculate it, and measure it, and the moment you cut or release the string so the rock is no longer following a curved path, it disappears. So centrifugal force is the force exerted by the rock on the string.It is the force that makes a rubber tyre expand radially, and shatters an overspeeding grindstone.

Imaginary forces do not break real objects.

[rmolnav]

Back to the elementary "whirling rock on a string". There is nothing imaginary about the tension in the string.

You are right, and I´ve posted tens of similar examples saying the same.
But it is not that simple when there is no physical link.
E.g.: Earth follows its elliptical orbit thanks to the pull from the Sun: centripetal force.
Newton´s 3rd Law: Earth exert an equal (but opposite) pull on the Sun ...
What real force could be called centrifugal force? ...

[rmolnav]

Some months ago I interchanged a few emails with a NOAA scientist, discussing some details I previously saw on a publication of them about tides which I was not completely in agreement.
Recently I´ve been talking about that in:
https://www.thenakedscientists.com/forum/index.php?topic=49715.new#lastPost
where tides, and CENTRIFUGAL FORCES as something directly related to tides, have been  long discussed by some of thenakedscientist members …
What follows is one of the first things mentioned NOAA scientist told me:
"The publication you are referring to is "Our Restless Tides", a 10-page pamphlet developed in the 1950's 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”.”
I added in my last post:
RESULT OF INERTIA ... I do agree: Newton´s Motion Laws come from inertia !!
"THIS EFFECT" ... Centrifugal forces, in the cases when they are REAL, are not just an "effect" or tendency due to inertia ... Affected objects "tend" to keep their velocity vector due to their inertia, and centrifugal FORCE is derived from inertia, but it is something more than that "effect" ...

[alancalverd]

Centrifugal force was always an "imaginary force" (not a real / measurable force).

So what is providing the measured tension in the string? What is making the real blood measurably separate in what I now have to call the Imaginary Centrifuge in my Virtual Laboratory? What is pulling on my arms and legs when I make a 60-degree banked turn in the plane? What caused the radial expansion of the tyres on my racing car?

Inertia? No way. Inertia is proportional to mass, and there's no mechanism for converting circular motion into mass.

[PmbPhy]

What is making the real blood measurably separate in what I now have to call the Imaginary Centrifuge in my Virtual Laboratory? What is pulling on my arms and legs when I make a 60-degree banked turn in the plane? What caused the radial expansion of the tyres on my racing car?

Its known in GR circles as Inertial Induction. See Peacocks book Cosmological Physics page 6.
Einstein said it was a gravitational force which is produced when the field of reference changed from an inertial frame to a non-inertial frame. The centrifugal force is an inertial force since its directly proportional to inertial mass.

As far as what Einstein actually said. From my website at
http://www.newenglandphysics.org/physics_world/gr/inertial_force.htm

Albert Einstein - That the relation of gravity to inertia was the motivation for general relativity is expressed in an article Einstein wrote which appeared in the February 17, 1921 issue of Nature

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.

[quoter]
Inertia? No way. Inertia is proportional to mass, and there's no mechanism for converting circular motion into mass.
[/quote]
Prove it.

[alancalverd]

Inertia? No way. Inertia is proportional to mass, and there's no mechanism for converting circular motion into mass.

Prove it.

Wirth pleasure, my friend.

Circular motion is defined by angular velocity r. ds/dt  with dimensions [L]²[T]^-1. Mass either has dimension [M] or energy equivalent E/c² which unsurprisingly also has dimension [M].

The fact remains that you can't claim a force that anyone can measure, to be "imaginary and unmeasurable". And if it acts away from the center of revolution (why else would the tyre fall off the wheel?) then it's etymologically justifiable to call it centrifugal.

Start with an object moving in a straight line.If you want to make it move in an arc, you must apply a centripetal force perpendicular to its momentum vector (Newton number 1!). Therefore the object will apply an equal and opposite centrifugal force to whatever is applying the centripetal force (Newton number 3!)

[rmolnav]

So what is providing the measured tension in the string? What is making the real blood measurably separate in what I now have to call the Imaginary Centrifuge in my Virtual Laboratory? What is pulling on my arms and legs when I make a 60-degree banked turn in the plane? What caused the radial expansion of the tyres on my racing car?
Inertia? No way. Inertia is proportional to mass, and there's no mechanism for converting circular motion into mass.

I do regret you seem not to have read, or to have forgotten, the so many posts I started to send here as early as one and a half year ago, #28 ... I´ve given many examples in line with the first paragraph of what quoted.
And regarding second paragraph, I´m afraid you are not keeping in mind that, what you yourself said afterwards:

Start with an object moving in a straight line.If you want to make it move in an arc, you must apply a centripetal force perpendicular to its momentum vector (Newton number 1!). Therefore the object will apply an equal and opposite centrifugal force to whatever is applying the centripetal force (Newton number 3!)

,
something I´ve already said many, many times, wouldn´t be possible without the physical phenomenon of INERTIA ... Newton puts that phenomenon in physical terms (force, mass and acceleration).
And 3rd one shows objects are kind of "stubborn", as they "think":" I want to keep keep my velocity vector (INERTIA) ... If any other object tries to "force" me and change it, I´ll exert back an equal force on it, trying to continue with my speed ..."
Excuse me for the joke, but it reflects reality.
And please kindly note what quoted by you from my post:

Quote from: rmolnav on Today at 11:34:43
Centrifugal force was always an "imaginary force" (not a real / measurable force).

was not said by me, but by the NASA scientist I had mentioned.
And he says "IT WAS ...", showing the big confusion about the concept that was, though perhaps not "always", so common decades ago, and even today according to my recent year experience.
Well, I have to leave it now.
Tomorrow, more quietly, I´ll say something more, and reply to PmbPhy too.

[chiralSPO]

Without making any claims myself, I will simply leave this here:


centrifugal_force.png (44.52 kB . 400x595 - viewed 1388 times)

https://xkcd.com/123/

[rmolnav]

Without making any claims myself, I will simply leave this here: ... (with three animated pictures)

Not bad way to show existing confusion... Even the author himself is confused: the introduction of a "rotating system" of reference, a mathematical "trick" not always justified, requires the concept of a centrifugal f. actually "fictitious", but applied to same considered rotating object ... Nothing to do with 3rd Newton´s Motion Law, because REAL centrifugal "reaction" force is exerted by the rotating object, on ANOTHER object: the one exerting the centripetal force required for the rotation of the former !!
THERE is where main cause of existing confusion resides, as I´ve been saying here (and in many, many other sites) since my very first post #28, 17 months ago !! Sadly, to no or little avail !!

[rmolnav]

Einstein said it was a gravitational force which is produced when the field of reference changed from an inertial frame to a non-inertial frame. The centrifugal force is an inertial force since its directly proportional to inertial mass.

I´m afraid there is too much confusion out there, even strictly within Newton´s Mechanics, to try an understanding beyond its limits, e.g. Einstein´s theories ...
Whatever the deep nature of inertia and gravity could be, Newton´s Mechanics is quite valid in most of usual cases.
Let me to suggest you to have a look to my post here six months ago, #159 (#160 too), replying #158 (yours)

[rmolnav]

... there is too much confusion out there ...

As a prove of that, please have a look to my last mail of several I sent to an "edu" well considered site (in 2016):
"Thank you VERY MUCH for your ideas. But you know, it is not that I want to apply "centrifugal" to a broader scenario. At least in Spanish, and in other latin origin languages, "centrifugal" does not need any definition: with its two parts "centri" and "fugal", you can tell its meaning. In English you also have "fugitive", from same origin, but perhaps it is not so self-evident. It´s fanny that in Spanish the "primary" force, centripetal, does have to be defined, because its part "petal", surely from latin too, does not tell us anything … I can´t remember any other similar common word! (though there must be some).
A more restrictive definition (back to centrifugal) could be useful to Physicists, but the term is something of everyday life, and it wouldn´t be sufficient to avoid confusion.
Many people, even from education institutions, seem to consider that term a kind of forbidden word, and they give erroneous explanations trying to specify the physical causes of phenomena which need a real centrifugal (or whatever we wanted to call it) force to be correctly understood.
Just to give you an example, what follows trying to answer the question "Would I weigh less at the Equator?":
"Earth has a bulge at the equator created by the planet’s rotation and a moving body’s tendency to continue in a straight line. (Sometimes this is misleadingly called "centrifugal force", but it's really just Newton’s laws of motion.) While standing at the equator you are further away from the bulk of Earth’s mass than at the poles, so the planet exerts less pull on you."
By the way, apart from de definition you gave me, I´ve seen others from important editors:
"Centrifugal force, a fictitious force, peculiar to a particle moving on a circular path, that has the same magnitude and dimensions as the force that keeps the particle on its circular path (the centripetal force) but points in the opposite direction" (which object does it affect? …)
"In physics, centrifugal force is the force that makes objects move outwards when they are spinning around something or travelling in a curve (??!!)
"Newton's third law states that for every acting force there is an equal and opposite reaction force.
Therefore there must be an equal and opposite reaction force to the Centripetal Force: the Centrifugal Force" (not mentioning the two objects of Newton´s law >>confusion) !!)
I MUST SAY I CAN´T UNDERSTAND WHY IT WOULD BE BETTER TO RESERVE A TERM FOR A FICTICIOUS FORCE RATHER THAN USING IT FOR THE REAL ONE, WHICH BESIDES, IT IS PRESENT IN MANY SCENARIOS ALL ACROSS THE UNIVERSE".

 

[alancalverd]

So would someone please tell me the preferred name of the magic that separates cream from milk, platelets from plasma, and heavy ions from light ones, in the various bits of everyday kit we use to do these useful things? If the force is imaginary, why does the g meter in my aeroplane (and the passengers' drinks) succumb to the same illusion as me and all my passengers?

[rmolnav]

So would someone please tell me the preferred name of the magic that separates cream from milk, platelets from plasma, and heavy ions from light ones, in the various bits of everyday kit we use to do these useful things? If the force is imaginary, why does the g meter in my aeroplane (and the passengers' drinks) succumb to the same illusion as me and all my passengers?

I can´t understand why you keep asking that, as if WE ALL were saying "the force is imaginary" ...
I´ve said quite a lot of times the opposite, BUT that idea is very, very spread ... Have you really read at least my last post, especially what at the end in capital letters???
Another example of the enormeous confusion (and my fight against it). What follows is an email I sent in november 2016:
"On your site:
https://global.britannica.com/science/centrifugal-force
(THE VERY BRITANNICA !!!)
it is said:
"Although it is not a real force according to Newton’s laws …"
Sorry, but I found that erroneous. REAL centrifugal forces frequently produce quite real effects …
E.g. when an athlete is rotating a hammer to be thrown, the wire is pulling the hammer (centripetal f. which produces the centripetal acceleration required for the rotation). And according to 3rd Newton´s Law, correctly applied, the hammer is also pulling outwards THE WIRE extreme.
That is quite REAL force, and CENTRIFUGAL. Should we have put a spring dynamometer between wire and hammer, we could have gauged that force …
The spring of the dynamometer requires two REAL forces applied to its opposite hooks to get stretched and show any figure”.
As you can see in linked site, they also say:
"A stone whirling in a horizontal plane on the end of a string ...
If the stone is moving at a constant speed and gravity is neglected, the inward-pointing string tension is the only force acting on the stone. If the string breaks, the stone, because of inertia, will keep on going in a straight line tangent to its previous circular path; it does not move in the outward direction as it would if the centrifugal force were real “ (!!!)
In a word: UNBELIVABLE …
And I could give tens of similar examples.
And, logically, if laymen read that, any effort to convey the actual physical reality is useless !!!

[rmolnav]

... there is too much confusion out there ...
As a prove of that, please have a look to my last mail of several I sent to an "edu" well considered site (in 2016)

For anybody who could be interested in having more "food for thought", what follows is my first mail to them, where there is also a link to mentioned site:
"On 
http://math.ucr.edu/home/baez/physics/General/Centrifugal/centri.html
it is said:
"Centrifugal force was invented to allow us to …"
I WOULD RATHER say it is NOT an invention, but something quite real. It is present in many real phenomena.
E.g.  when an athlete is rotating a hammer to be thrown, the wire is pulling the hammer (centripetal f. which produces the centripetal acceleration required for the rotation). And according to 3rd Newton´s Law, correctly applied, the hammer is also pulling outwards THE WIRE extreme.
That is quite REAL force, and CENTRIFUGAL. Should we have put a spring dynamometer between wire and hammer, we could have gauged that force …
The spring of the dynamometer requires two REAL forces applied to its opposite hooks to get stretched and show any figure"
THEN I WAS SENT WHAT FOLLOWS:
"Centrifugal force is, by definition, a force that appears in a rotating frame.  That's its definition.  The athlete you describe is in a non-rotating frame, if we neglect Earth's rotation.  He pulls on the hammer as he spins the hammer around him, and this pulling force manifests as a tension in his arm.  It produces the centripetal acceleration of the hammer around the athlete. 
The hammer certainly pulls on his arm with the same force, and this could certainly be
measured by a spring device.  But that force is not a centrifugal force, because it is not something that is being described in a rotating frame (, this is mine)
If you were a miniature creature riding on the hammer, you -would- be in a rotating frame, and then you would certainly say that a centrifugal force was present: this force would be trying to pull you away from the hammer--you'd have to hold on tight to the hammer to counteract this force that you felt.  But someone in an inertial frame says that the force you feel is not present in the inertial frame, and it is certainly not needed to apply Newton's laws to the situation in the inertial frame. Rather, it's due to the fact that the hammer is being pulled away from you by the athlete, which thus requires you to hold on to the hammer.  When you need to hold on, you interpret that as the result of a force that's pulling you away from the hammer.  But nothing physical is pulling you away from the hammer.  The hammer is being pulled away from you by the athlete--in the inertial frame".
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 ...

[rmolnav]

THEN I WAS SENT WHAT FOLLOWS:

More "food for thought". After that really confusing "story" I was told, I replied:
"Hello Don,
You say:
"Centrifugal force is, by definition, a force that appears in a rotating frame.  That's its definition".
I consider that expression should not be defined such a narrow way. Everybody knows the adjective "centrifugal" implies some kind of  "fleeing from a center" …  According to Oxford Advanced Leaners Dictionary (7th Edition), "Moving or tending to move away from a center". And different types of forces can match that definition.
We could diminish confusion clearly specifying which force type are we talking about, rather than restricting its scope.
As it seems hammer throwing case interested you, please kindly have a look at what follows. Not long ago I sent it to a blog discussing the issue:

" Differences between the case of two celestial objects rotating around their barycenter, and the case when an object is being rotated by us by means of a cord o similar tool (as e.g. in hammer throwing), go beyond the “physical” difference that nothing connects the celestial objects to each other, apart from the “tele-connection" of gravity …
Somehow the celestial objects were initially given their speeds very long ago, and no energy is transferred now from/to each other.
When an athlete initiates hammer rotation, he has to transfer energy to the hammer, in order to increase its tangential speed the more the better.
For the sake of simplicity, i consider a rotation always around a vertical axis. The “tension” of the wire, I mean the inward force, is not strictly “centripetal”. It has three components:
One vertical, which has to support hammer weight (the wire can´t be completely horizontal).
Another horizontal and centripetal, which produces the centripetal acceleration necessary for the hammer to follow its circular path.
And yet another horizontal but tangential, the usually “forgotten” force. It is due to the fact that athlete hands are also rotating, but the straight line hands-wire-hammer is not a radius of hammer trajectory … The athlete tries and increases the angular speed of his hands, with the purpose of increasing angular and tangential speeds of the hammer, before releasing it. There is a sort of angular “delay” of the hammer relative to the situation of athlete hands. That´s the cause of the tangential component of wire tension.
So, the athlete increases more and more wire tension, and both its centripetal and tangential components increases too. Centrifugal force (opposite to centripetal one: the hammer pulling outwards wire end) increases as the centripetal one does… That outward, centrifugal force that increases more and more, could appear to cause the hammer to “fly” when the athlete releases it. But IT NOT DOESN´T.  The cause is actually mentioned tangential component of wire tension, that increases velocity revolution after revolution, with continuous transference of energy from the athlete to the hammer … As soon as the wire is released, hammer (+ wire) is “free” and go straight at its acquired tangential velocity (apart from weight and air friction effects).
After all, the centrifugal force was NOT being exerted on the hammer, and even if it had been, it would require time to increase any velocity, but it “dies” at same instant the hammer is released.
So, we can say centrifugal force rol is “important: the wire could not exert any centripetal force on the hammer not being tight, and it could not be tight acting only a force at one of its extremes !!
And don´t forget that, as I´ve been referring to during last weeks, centrifugal forces have other important roles in other cases. I have to decide which case may be worth to insist on."
Any further comments?
Yours ..."