[rmolnav]

#38 and 39
You say:
"...  if we include the centrifugal force to the equation of the system, total force would be zero which mean no acceleration, contrary to the observation" I insist: to reason that way, whoever does it, is erroneous. It is a kind of blunder, in physics and /or maths ...
I thought that in my posts the different objects and acting forces were not "mixed", that I had clearly distinguished which forces are acting on each object.
If you read them carefully, you would understand what I mean.
For instance, in the case of weight and string, basic Physics tell us we must not even consider the addition referred to in what above quoted. The UNIQUE force acting on the weight is the centripetal force (considering the weight as a unit: see#37). An inward pull from the string.
The centripetal force (at that place) is an outward pull, exerted by the weight but FROM THE STRING. That is why, if there were a dynamometer between weight and outer end of string, that centrifugal force would act on dynamometer´s outer side, and forces could be gauged.
 
 

[rmolnav]

#40 (continuation)
I had some doubts about which preposition should be used in each case, and I must have put it wrong once at least.
I checked that to pull does´n requires any, but with "a pull" I put "from" in opposite senses. I´ll express now what I meant only with the verb.
- The string pulls inwards the weight (centripetal force).
- As a reaction to that action, the weight pulls outwards the string (centrifugal force).
 To say what quoted " ... include the centrifugal force to the equation of the system , total force would be zero which mean no acceleration...", is erroneous, kind of blunder as I said. At second Newtons law (F=ma), F must be the sum of all forces acting on an object, exerted BY OTHERS ... We can´t add any other force, f.e. one exerted by the considered object on others.
Otherwise we could deduce something utterly absurd: from 2nd and 3rd Newton´s laws it can be deduced that NO object can make (by itself) any other accelerate, because being action and reaction forces equal and opposite, to add both gives zero force ...

[rmolnav]

#27:
"The force is MV^2/R where the rotational velocity is around 1000 miles per hour and the distance to the center of the Earth is around 4000 miles. So I guess this is small".
"Small", and "big", are rather meaningless terms, if not specified what compared to.
That could be considered small compared to normal gravitational force, the weight of objects.
But its result is enormous ... Due to that centrifugal force, the equator diameter is app. 43 km bigger than the distance between poles.
That is a kind of permanent high tide all around the equator, with low tides at poles, in the order of 10,000 times highest lunar/solar tides.
And, compared to something also very big, between 2 and 3 times mont Everest (!!)

[rmolnav]

#42 (continuation)
Sorry, "only" in the order of  1,000 times, instead of 10,000 !!

[rmolnav]

Over last week I tried to convey the main ideas of my stand on the subject. Modestly, I must say I feel pretty sure about this matter, since in last century sixties I studied Solid and Fluid Mechanics for my degree. And many other phenomena related to the subject I later learnt confirmed what initially studied. And I feel I must pass that information to anybody interested.
But I´m not sure about the success of my effort. Very few replies, and rather negative ...
To whoever is really interested in the subject: please read carefully my key posts: 28, 36 and 37.
Any doubt, or opposite view, please kindly tell me.
I still have to complete the application of those ideas to the real and most frequent case of a rotation due to gravity, without any string or similar tool.
But I´ll do it another day/days, most probably in more than one step.
Now, as a kind of divertimento, I´ll bring up a "funny" case, an experiment that anybody can do at home.
Take a cylindrical bucket, or something similar (with vertical inner side). Fill it with water (app. 2/3 of its hight), and put it on the center of a revolving chair. Make it revolve.
Initially flat and horizontal water level will change to a lower level at the center, and the more distant from the center, the higher the water level.  Kind of low tide at center and high tide near the bucket inner surface. Why?
Any comment would be welcome.

[alancalverd]

Take a cylindrical bucket, or something similar (with vertical inner side). Fill it with water (app. 2/3 of its hight), and put it on the center of a revolving chair. Make it revolve.
Initially flat and horizontal water level will change to a lower level at the center, and the more distant from the center, the higher the water level.  Kind of low tide at center and high tide near the bucket inner surface. Why?

Centrifugal force.

[rmolnav]

#45 Alancalverd
Certainly ... And thank you. But I do expect comments from people who, both here and on:
http://www.thenakedscientists.com/forum/index.php?topic=49715.50,
have said bizarre things  considering that "centrifugal force" should be a kind of forbidden expression, that it isn´t actually a real force ...
There is a big confusion about this subject out there. Even between some scientists (!!).
I´m seeing you are very active on many subjects, but not on either of the two mentioned.
Perhaps you have not seen what I´m referring to. Otherwise you would have sent several refuting replies ...

[rmolnav]

#46 (continuation)
After talking about "bizarre things" that some people say, I want to express my greatest respect on everybody, especially on laymen who heard erroneous things said by presumably well educated people.
Before I sent my first post to linked site (#20), I had read some posts which I found utterly wrong. And I started:
“1) Centrifugal” force is NOT a forbidden word whatsoever: it is just a poorly understood and poorly explained force..."
And I carried on unfolding my stand on the subject. Not only on that post, but later on many others.
Many weird things can be seen there, due to the "huge" effort some people had to make trying to explain sea tides (especially the high tide opposite to Moon) without taking into account any centrifugal force, not even mentioning the "forbidden" word.
As far as I can remember, the "leader" was a rather extensive mathematical "contraption" somebody linked:
http://www.newenglandphysics.org/Science_Literature/Journal_Articles/schutz_tides.pdf
Perhaps I should be more humble, and think I could be the wrong one ... After all, I can´t actually fully understand the maths there ...
But I must say I feel pretty sure tides are due to all acting forces, mainly water own weight, lunar (and solar) attraction, and centrifugal forces. Also local conditions, due to the fact that water is not completely free to respond to mentioned forces.

[rmolnav]

I must also say I understand how careful some people are when dealing with our topic. In the extreme opposite to what mentioned on #47, there are educated people which consider centrifugal force as if it were a kind of "free for all" force, something we could even take energetic advantage from. That the problem would only be technological, and a lot of money is being spent trying to develop the idea.
And, watch out! Being centrifugal force a reaction f., it exists as long as the centripetal one does. This last one is necessary just to keep the object rotating, and there is nowhere we could take energy from whatsoever. And then, some people go to the #47 extreme !!
F. e., we can throw a stone with a sling. And, with no friction from air and far from Earth (no gravity), instead of throwing it, the stone could even carry on rotating for ever ... But it would be utterly absurd to think of using either of those two opposite forces for something else. The stone would change its trajectory, and the system would collapse.
You can google "space elevator" ...
As I said yesterday, perhaps I should be more humble and accept the possibility of being wrong. There are even several american universities trying to develop the idea. But, with all my respect for the people who are working hard on that idea, sorry, I found it absurd from its initial physical base.

[hamdani yusuf]

#40 (continuation)
I had some doubts about which preposition should be used in each case, and I must have put it wrong once at least.
I checked that to pull does´n requires any, but with "a pull" I put "from" in opposite senses. I´ll express now what I meant only with the verb.
- The string pulls inwards the weight (centripetal force).
- As a reaction to that action, the weight pulls outwards the string (centrifugal force).
 To say what quoted " ... include the centrifugal force to the equation of the system , total force would be zero which mean no acceleration...", is erroneous, kind of blunder as I said. At second Newtons law (F=ma), F must be the sum of all forces acting on an object, exerted BY OTHERS ... We can´t add any other force, f.e. one exerted by the considered object on others.
Otherwise we could deduce something utterly absurd: from 2nd and 3rd Newton´s laws it can be deduced that NO object can make (by itself) any other accelerate, because being action and reaction forces equal and opposite, to add both gives zero force ...

How to determine wether a force would produce acceleration?


Where is the centrifugal force in the first and second picture above?

[nilak]

I've made a computer simulation program in which if you select 2 particles with a certain mass and tie them together with a spring, they can rotate around each other without adding the centrifugal force into the motion equations. I only use F=ma; F=kx; dv=a*dt; dx=v*dt; The same happens if you substitute the spring with a gravitational force F=Gm1m2/r^2. So the centrifugal force is the effect of inertia when changing direction. When motion is restricted on a circle, the force of inertia is produced at every little change in the initial direction. This force can be calculated. If your motion is restricted by a different shape (elipse) it will have different formula ( you need to include an angle). Centrifugal force is not a fundamental force, it is only a formula to use for motion restricted on a circle.

[rmolnav]

#49 hamdani yusuf
That question has already been dealt with by me (mainly on #28, 36 and 37).
In the first place, we should keep in mind that “centrifugal forces” show up in DIFFERENT fashions ...
Those pictures are only mathematical abstractions of the real thing. The central object is supposed to be much more massive than the other, and, for the sake of simplicity, it is supposed to keep stand still. But, unless it mass were infinite, it is imposible for an object to pull another without moving itself at all.
In both cases we have a primary force: the central object pulls the other (centripetal force).
According to 3rd Newton´s principle, outer object also exerts an equal but opposite force on central one: centrifugal force.
In those cases where there is no physical connection (string, rope, chain ...), both forces are gravitational. By the way, we already know that gravity happens in both senses. It couldn´t be otherwise.
And both objects are considered with their masses concentrated on their gravity center. That is also a simplification. Every particle of each object “feels” independently the gravity pull from the other, not necessarily with same “intensity”, as long as distances are actually different. Those different centripetal forces on each particle originate internal stresses between contiguos particles, that (also due to 3rd principle) go in pairs, being half of them centrifugal.
But I have to come back again to that another day, with clarifying examples.
I already did it last year, under the previously mentioned head subject : “Why are there two high tides a day?”.
In cases where there is a physical connection, the centripetal force is also acting on each particle of both objects, and on each section of the “linking” device, from central to outer object. And on all those places the correspondent centrifugal reaction happens too.   


 

[rmolnav]

#50 Nilak
You say:
"So the centrifugal force is the effect of inertia when changing direction".
Not bad idea. But, have you realized that, after all, it is a way of expressing (in a particular case) the three Newton´s Principles?
Whatever happens with linear speed of an object, any change of its direction is a velocity vector change, an acceleration. That results in a curve trajectory.
That acceleration requires an acting force (2nd principle), towards the concave side of the curve (by the way, not necessarily a circumference): centripetal acceleration. If that is exerted (whatever the way) by object  A on B, B by INERTIA tries to maintain its own speed (1st principle), and exerts an equal but opposite force on A: (3rd principle).
But, apparently, you mean centrifugal force is "the effect of inertia" on the object that changes direction ... That seems to establish the idea that centrifugal force in a kind of fictitious force, just an "effect" of inertia.
Please kindly read my #50. There you can see I talk about the "infinite" pairs of centripetal/centrifugal forces acting on the particles of any rotating  object (or just with a curve trajectory). That can be considered an "effect" of inertia,  but it is a REAL force, and it doesn´t mean centrifugal force is not  fundamental. 

[nilak]

#50 Nilak
You say:
"So the centrifugal force is the effect of inertia when changing direction".
Not bad idea. But, have you realized that, after all, it is a way of expressing (in a particular case) the three Newton´s Principles?
Whatever happens with linear speed of an object, any change of its direction is a velocity vector change, an acceleration. That results in a curve trajectory.
That acceleration requires an acting force (2nd principle), towards the concave side of the curve (by the way, not necessarily a circumference): centripetal acceleration. If that is exerted (whatever the way) by object  A on B, B by INERTIA tries to maintain its own speed (1st principle), and exerts an equal but opposite force on A: (3rd principle).
But, apparently, you mean centrifugal force is "the effect of inertia" on the object that changes direction ... That seems to establish the idea that centrifugal force in a kind of fictitious force, just an "effect" of inertia.
Please kindly read my #50. There you can see I talk about the "infinite" pairs of centripetal/centrifugal forces acting on the particles of any rotating  object (or just with a curve trajectory). That can be considered an "effect" of inertia,  but it is a REAL force, and it doesn´t mean centrifugal force is not  fundamental.

I've just checked the Wikipedia to see what it says about centrifugal force and I understand the same thing I've tried to explain.
Sorry, reply no 50 is mine.

[hamdani yusuf]

...
According to 3rd Newton´s principle, outer object also exerts an equal but opposite force on central one: centrifugal force.
...

If both objects have equal mass, e.g. binary stars, where is the centrifugal force?

[nilak]

...
According to 3rd Newton´s principle, outer object also exerts an equal but opposite force on central one: centrifugal force.
...

If both objects have equal mass, e.g. binary stars, where is the centrifugal force?

There are two forces here, for each star, pointing away from the centre of rotation. Also two,
gravity forces, equal to each other and to the centrifugal forces, pointing to the centre of rotation. Instead of gravity you can say the spacetime curvature is causing the attraction if you want to be more precise.

[rmolnav]

#54 hamdani yusuf
As I said on #51, if you are considering those stars as if all their constituent material were concentrated on their center of gravities, you are right. All action and reaction forces would go towards the center of the twin system.
But that simplification is not what actually happens. Star A material closer to star B, as well as the material of B closer to A, will be pulled by the other star with stronger force than if we consider outer material. Those (NOT EQUAL to each other) centripetal forces are supposed to produce the centripetal acceleration that the rotation requires. But all parts of the stars, due to own gravity, are obliged to rotate at SAME angular speed. Force excess and/or deficit can only be compensated by internal stresses, changes in pressure when gases.
Gases "try" to concentrate either at closest parts of the pair (centripetal force bigger than what necessary for required centripetal acceleration), or at outer parts (centripetal force smaller than what required). If you consider a traverse "slice" at those outer parts, pressure at outer side will be smaller than at inner side (comparing with pressures if they were not rotating). The net force acting on the slice would be CENTRIFUGAL (apart from own gravity force or weight)
By the way, that is also a good picture of the two high tides on Earth: one at Moon side, and the other at opposite side. Whatever the actual material, phenomena are very similar

 
 
 

[rmolnav]

Yesterday I happened to run into Einstein … and I think he “told" me that, at least on one of my key ideas, I´m right.
Do NOT worry! I am not going to talk about relativity theory. Our discussion about centrifugal force, and gravity, must continue within Newton´s Physics.
You can see on #37 the case of a bar hanging from one of its extremes, and the different stresses on each of its sections.
My plan was to pass from that rectilinear situation to a rotational one: the bar, in the space without gravity, somehow rotating around an imaginary hinge at one of its extremes. I haven´t done it yet because I preferred to comment other posts.
It would be very, very similar to the vertical hanging bar case. Each part of the bar would “feel” the centrifugal force in a way quite similar to the effect of the bar weight when hanging vertically. The centripetal acceleration implies an effect on the bar equivalent to a kind of outward, radial weight.
And, a real coincidence, yesterday afternoon tve2 (Spain) emitted “Inside Einstein´s Mind”.
There I learnt that when he was young, and “ruminating” about gravity (before any relativity idea), he imagined something almost exactly equal to what above exposed.
When announcing the video, they said it was from NOVA. From previous cases I know they have plenty of videos on their web site, and I found it:
http://www.pbs.org/wgbh/nova/physics/inside-einsteins-mind.html
Here in Spain is not possible to watch those videos, unless paying for them ...
Approximately at 1/3 of the video, there is a scene with a huge square box, with open front side, and a man “floating” inside (as without gravity). Then he falls onto the box floor, and a little later the point of observation goes back, and we can see Earth globe supporting the box. And afterwards, again at a no gravity place, they lift the box with “g” acceleration.
They say it was imagined by Einstein, who thought the man would feel “floating” or falling down, but, when standing on its feet, he couldn´t tell apart the situations when attracted by Earth´s gravity and when no gravity but the box was being accelerated upwards with “g”.
Its conclusion was that gravity and acceleration must be the “same" thing … And that helped him continue his ideas of space-time deformation, and rest of stuff.
Subsequently, I say, centripetal force/acceleration acts on the rotating object as Einstein´s box lifting force, and makes the object “feel” a centrifugal force, exactly as the man on the box floor feels either his weight (no acceleration) or the acceleration (no weight). And, in other cases (f.e. a child on a swing), both forces added up. 



 
 
 

[rmolnav]

By the way, as far as I can remember the word "inertia" doesn´t appear ... (?)
Another day I have to send a post about that example of the child on the swing. And another to say something more about the revolving bucket with water ...
And sorry for my rather poor English (after sending the post I´ve seen I say "its conclusion" referring to Einstein´s !!)

[rmolnav]

The case of a child on a swing can be simplified as a pendulum.
There is a difference, as long as in the pendulum case the weight is supposed to be hanging from a string knot (the string pulls the weight), but the swing seat is actually pushing upwards the child. That changes the actual field of reaction forces within child´s body, compared to if he were hanging from his hands.
Another day we can talk about the real case of the swing. Today I propose the simplified pendulum example:
Oscillating_pendulum.gif
Any comments)