[rmolnav]

Pendulum I
It seems simple, but there are small but important details that, if not being careful, one can misinterpret facts, or at least confuse others …
That´s why I am now going to consider only the scenario statically (and step by step), without any movement: fixed hanging point, string and weight (I´ll call it W), all in a straight vertical line.
- a) Primary acting force: Earth pulls downwards W.
- b) W does not move. According to 2nd Newton´s principle, the sum of all forces acting on W has to be null.
- c) The unique object that can exert another force on W is the string: it must somehow pull W, with equal but opposite force (watch out: those two opposite forces are not action/reaction forces (3rd principle); they are acting on a unique object, and 3rd principle is about two objects exerting a force on each other).
- d) If the string pulls upwards W, applying now 3rd principle to that pair of objects, we can deduce that W must be pulling downwards the string lower extreme.
- e) That force seems to be a centrifugal one (and centripetal the one mentioned on - c)), but we should keep in mind that if there is no rotatory movement at all, a proper “center” does not actually exist.
- f) And when with movement, we have also to be careful with the term “centripetal force”, because it usually refers to the the radial component of adding up all forces acting on W,  which divided by the mass would give us the “centripetal acceleration” that makes W not to follow a rectilinear trajectory. And in many cases some of those added forces may be in the sense of the “center”, but compensated by others and not producing any acceleration by themselves.
 

[hamdani yusuf]

Pendulum I
It seems simple, but there are small but important details that, if not being careful, one can misinterpret facts, or at least confuse others …
That´s why I am now going to consider only the scenario statically (and step by step), without any movement: fixed hanging point, string and weight (I´ll call it W), all in a straight vertical line.
- a) Primary acting force: Earth pulls downwards W.
- b) W does not move. According to 2nd Newton´s principle, the sum of all forces acting on W has to be null.
- c) The unique object that can exert another force on W is the string: it must somehow pull W, with equal but opposite force (watch out: those two opposite forces are not action/reaction forces (3rd principle); they are acting on a unique object, and 3rd principle is about two objects exerting a force on each other).
- d) If the string pulls upwards W, applying now 3rd principle to that pair of objects, we can deduce that W must be pulling downwards the string lower extreme.
- e) That force seems to be a centrifugal one (and centripetal the one mentioned on - c)), but we should keep in mind that if there is no rotatory movement at all, a proper “center” does not actually exist.
- f) And when with movement, we have also to be careful with the term “centripetal force”, because it usually refers to the the radial component of adding up all forces acting on W,  which divided by the mass would give us the “centripetal acceleration” that makes W not to follow a rectilinear trajectory. And in many cases some of those added forces may be in the sense of the “center”, but compensated by others and not producing any acceleration by themselves.

I think your examples above don't clear things up, but rather confusing.

[rmolnav]

#61 hamdani yusuf
"I think your examples above don't clear things up, but rather confusing".
There are not "examples", it is only a rather simple case, analyzed step by step.
Could you please specify which concrete step and/or point you find confusing?
My aim there was that, reading later posts of mine including pendulum oscillation, nobody would erroneously think again (f.e.) similarly to what reflected on your own #38:
"Because 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”,
what was later dealt with by me on #39 and 40. I expected it was already clear to you ...
I consider what said on #60, if each paragraph is (one by one) read carefully, should be understood with no difficulty.
I understand they could seem rather far fetched details.
But I´m convinced that, not to have them clearly understood by many people, is the main reason why there is such a big confusion about the subject out there.

[hamdani yusuf]

#61 hamdani yusuf
"I think your examples above don't clear things up, but rather confusing".
There are not "examples", it is only a rather simple case, analyzed step by step.
Could you please specify which concrete step and/or point you find confusing?
My aim there was that, reading later posts of mine including pendulum oscillation, nobody would erroneously think again (f.e.) similarly to what reflected on your own #38:
"Because 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”,
what was later dealt with by me on #39 and 40. I expected it was already clear to you ...
I consider what said on #60, if each paragraph is (one by one) read carefully, should be understood with no difficulty.
I understand they could seem rather far fetched details.
But I´m convinced that, not to have them clearly understood by many people, is the main reason why there is such a big confusion about the subject out there.

When trying to explain centripetal and centrifugal forces, we should focus more on the generation of force due to circular movement of an object. Don't complicate things with additional forces whose direction is not radial.
In the case of astronauts in ISS, do they experience centrifugal force? Does it have the same magnitude as the centripetal force by earth gravity which keeps them in orbit?

[rmolnav]

#63 hamdani yusuf
« on: Today at 04:29:08 » Quote (selected):
"Don't complicate things with additional forces whose direction is not radial".
Certainly, we should not complicate things with any unnecessary stuff ... But please note that a pendulum oscilates because of the action of the very existence of a tangential component of its weight, which produces a tangential acceleration.
My concrete aim was nobody would mix them later when analyzing the oscillation, that is a kind of partial rotation, but with variable speed.
And you also ask:
"In the case of astronauts in ISS, do they experience centrifugal force? Does it have the same magnitude as the centripetal force by earth gravity which keeps them in orbit?"
In that scenario there are also subtleties one has to take into account carefully.
Considering the mass of an astronaut as a hole, 3rd Newton´s principle tell us that, if earth gravity pulls him down, he also pulls earth upwards.
But that pull is negligible to earth.
As a unit, the astronaut only experiences centripetal force, which makes him rotate.
But his body parts and members, if he were much, much taller, would differently experience earth gravity, especially when in  "vertical"  (radial) position.
Why? Because the distance from his head to earth would be greater than that distance from his feet ...
Besides that, the trajectories of those two body parts would be slightly different circumferences ... For a unique angular speed of rotation, that would mean his feet would require a smaller centripetal force than what corresponds to experienced gravity, and the opposite at his head.
Internal stresses/forces would happen to compensate those differences. And internal stress/forces go in pairs, due to 3rd Newton´s principle.
Half of them would be centripetal, and half centrifugal.
That seems bizarre, because distance differences are relatively negligible. But if ISS were much, much bigger in radial sense, other analogous effects would happen there, and be sure they should be taken into consideration. 
 
 

[hamdani yusuf]

...
"In the case of astronauts in ISS, do they experience centrifugal force? Does it have the same magnitude as the centripetal force by earth gravity which keeps them in orbit?"
In that scenario there are also subtleties one has to take into account carefully.
Considering the mass of an astronaut as a hole, 3rd Newton´s principle tell us that, if earth gravity pulls him down, he also pulls earth upwards.
...

The pulling of the earth is also centripetal, toward the barycenter of the system. Hence there is no apparent centrifugal force here.

[puppypower]

The centrifugal force appears when linear momentum, meets angular momentum, and two merge into angular momentum, while remaining separate and distinct. If we break the merger, the distinction will reappear.

[rmolnav]

#65 hamdani yusuf
Please note the expression “centrifugal force” does NOT appear in what quoted by you. Not even the term “reaction force” …
3rd principle affects to any object exerting a force on another, and viceversa. But not always is easy to say which is action and which reaction. I have seen it is better not to introduce that issue in my expositions.
Let me transform a little the astronaut case.
Let us imagine he goes out of ISS, to a position keeping the orbit but many meters after the station.
There would be what we can call a dynamic equilibrium: earth pulls the man, but he does not move downwards because there gravity acceleration is exactly equal to the square of his tangential speed divided by the radius of his orbit. Velocity vector only changes direction, and that lets him rotate, as when inside the station.
Surely he would have gone out of ISS with a safety “rope”, let us suppose with a knot around one of his wrists. If another astronaut inside the station tights the rope and strongly pulls him, the dynamic equilibrium ends: that pull increases his tangential velocity. And if we divide gravity attraction by his mass, we get an acceleration smaller than what required to keep him in that orbit (the square of the actual speed divided by the radius) …
The astronaut would move not only tangentially, but also radially, upwards. Then the rope pull would have a downward component. That “centripetal” component of the rope pull, according to 3rd principle would mean our man would also pull the rope, upwards: a CENTRIFUGAL force.
Besides, within the astronaut body, there would be stresses (wrist and forearm would be pulling rest of the body … 3rd principle would also apply there ...
Somebody could say: rather confusing example … Or: well, that is a particular case, with more than two objects (earth, ISS, rope and astronaut).
But in ALL real cases there are many, many more “objects” to be considered, because their different parts (whatever their size) experience gravity attraction from other massive objects independently …
Nature is much more complex than most mathematical simplifications ...

[hamdani yusuf]

#65 hamdani yusuf
Please note the expression “centrifugal force” does NOT appear in what quoted by you. Not even the term “reaction force” …
3rd principle affects to any object exerting a force on another, and viceversa. But not always is easy to say which is action and which reaction. I have seen it is better not to introduce that issue in my expositions.
Let me transform a little the astronaut case.
Let us imagine he goes out of ISS, to a position keeping the orbit but many meters after the station.
There would be what we can call a dynamic equilibrium: earth pulls the man, but he does not move downwards because there gravity acceleration is exactly equal to the square of his tangential speed divided by the radius of his orbit. Velocity vector only changes direction, and that lets him rotate, as when inside the station.
Surely he would have gone out of ISS with a safety “rope”, let us suppose with a knot around one of his wrists. If another astronaut inside the station tights the rope and strongly pulls him, the dynamic equilibrium ends: that pull increases his tangential velocity. And if we divide gravity attraction by his mass, we get an acceleration smaller than what required to keep him in that orbit (the square of the actual speed divided by the radius) …
The astronaut would move not only tangentially, but also radially, upwards. Then the rope pull would have a downward component. That “centripetal” component of the rope pull, according to 3rd principle would mean our man would also pull the rope, upwards: a CENTRIFUGAL force.
Besides, within the astronaut body, there would be stresses (wrist and forearm would be pulling rest of the body … 3rd principle would also apply there ...
Somebody could say: rather confusing example … Or: well, that is a particular case, with more than two objects (earth, ISS, rope and astronaut).
But in ALL real cases there are many, many more “objects” to be considered, because their different parts (whatever their size) experience gravity attraction from other massive objects independently …
Nature is much more complex than most mathematical simplifications ...

Why in ISS centrifugal force doesn't seem to be equal to centripetal force?

[rmolnav]

#68 hamdani yusuf
I think I´ve already answered similar questions.
If ISS were considered a single object, with all its mass concentrated in its center of gravity, NO centrifugal force would occur there: it only would be interacting with earth, pulling it back with a force equal to the centripetal force required for ISS´s actual speed of rotation at that distance.
But, as ISS has a size, particulary its radial hight, some of its parts are further from earth than others ... Gravity is not EXACTILY the same all through its hight. But all of them rotate at same speed. That produces an actual centripetal force "surplus" at its lower parts, and a "deficit" at upper parts.
That has to be compensated by mean of a field of internal stresses/forces, in pairs to satisfy 3rd Newton´s principle.
Half of them go downwards, and the other half upwards, which can be called CENTRIFUGAL.
Those forces would make ISS stretch in radial sense, the same that happens with ALL cosmic objects rotating around their barycenter.
Certainly, those effects at ISS must be almost negligible, but not completely. Haven´t you realized how  vertically small ISS is? Surely it is so to further minimize those effects.

[hamdani yusuf]

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 reference frame.

The term has historically sometimes also been used to refer to the reaction force to a centripetal force.

The centrifugal force is an outward force apparent in a rotating reference frame; it does not exist when measurements are made in an inertial frame of reference.

That's what wikipedia said.

[rmolnav]

#70
Wikipedia is not always completely right. Even it has errors.
Last year I already said:
“I know centrifugal force can be considered as an inertial, actually kind of ficticious force … But not only that way”.
(#28 of  http://www.thenakedscientists.com/forum/index.php?topic=49715.25 ).
Besides, what quoted by you is self-contradictory, because they also say:
"The term has historically sometimes also been used to refer to the reaction force to a centripetal force",
and all my examples are based on that fact. They say just "sometimes", but in all my examples that term has been used because pairs of centripetal forces and their reactions were present ... How do you think we should call a force opposite to a centripetal one?

 

 

[rmolnav]

#70 (continuation)
Rather than being self-contradictory, Wikipedia article author seems to consider that centrifugal f. actually is the ficticious f. he refers to, that to use the expression for the reaction to the centripetal f. is a kind of exceptional use …
I do consider it would rather be the opposite. His field must be Theoretical Physics or Maths, and he refers to a “trick” for when an inertial frame of reference: the use of a fictitious force, equal but opposite to centripetal f., but both forces applied on SAME unique object.
But the other case refers to a REAL force, which has been acting in the Universe since several billion years ago, since first pair of rotating stars appeared. Since then, hose forces and centripetal ones, together with own gravity of considered cosmic objects, have been deforming, and even broking apart, many of those objects:
"The limiting distance to which a satellite can approach without breaking up depends on the rigidity of the satellite. At one extreme, a completely rigid satellite will maintain its shape until tidal forces break it apart. At the other extreme, a highly fluid satellite gradually deforms leading to increased tidal forces, causing the satellite to elongate, further compounding the tidal forces and causing it to break apart more readily ..."
https://en.wikipedia.org/wiki/Roche_limit

[hamdani yusuf]

... How do you think we should call a force opposite to a centripetal one?

It could also be centripetal force. See binary stars.

Centrifugal force is considered pseudo/fictitious because it does not exist when measurements are made in an inertial frame of reference.

[rmolnav]

#73
Please kindly see #56, were your #54, with two identical binary stars case, was already dealt with.
AT BOTH OUTER PARTS of the pair, gas pressure decreases, and bulges appear, for same reason as our equator diameter is bigger than pole to pole distance: CENTRIFUGAL forces experienced by ANY molecule whose distance to the other star C.G is bigger than the distance between the two C.G.
You seem unable to distinguish the simplified case when considering all star masses concentrated on their respective C.G. (a rather theoretical trick), from REAL cases, in which each molecule actually "feels" gravity (from rest of massive objects) independently.
And 3rd Newton´s principle also applies to ALL those "infinite" tiny forces, and to internal stresses that happen throughout all objects. 

[rmolnav]

#72 (continuation)
"Jupiter – the most massive planet in the Solar System – causes the Sun to move at roughly 35km/h”
(both actually rotate around the barycenter axis of the pair, very near to Sun´s C. G.  - apart from other much smaller wobblings due to rest of planets)
"… When the star moves towards or away from you, the wavelength of its light shortens or lengthens, respectively".
http://www.bbc.com/earth/story/20161019-the-first-planet-around-another-star

[hamdani yusuf]

#73
Please kindly see #56, were your #54, with two identical binary stars case, was already dealt with.
AT BOTH OUTER PARTS of the pair, gas pressure decreases, and bulges appear, for same reason as our equator diameter is bigger than pole to pole distance: CENTRIFUGAL forces experienced by ANY molecule whose distance to the other star C.G is bigger than the distance between the two C.G.
You seem unable to distinguish the simplified case when considering all star masses concentrated on their respective C.G. (a rather theoretical trick), from REAL cases, in which each molecule actually "feels" gravity (from rest of massive objects) independently.
And 3rd Newton´s principle also applies to ALL those "infinite" tiny forces, and to internal stresses that happen throughout all objects.

There is alternative name for the cause of those bulges : inhomogeneity of centripetal forces among the molecules making up the stars.

[hamdani yusuf]

May I know your objection to wikipedia's statement that
Centrifugal force is considered pseudo/fictitious because it does not exist when measurements are made in an inertial frame of reference?

[rmolnav]

#77 hamdani yusuf
I have no objection to that statement whatsoever, if THAT is the case: "when measurements are made in an inertial frame of reference".
But in most real cases the frame of reference has NOT to be necessarily an inertial one.
Besides, forces do exist in nature, centrifugal ones included, long, long before anybody could make measurements ... 

[rmolnav]

REVOLVING WATER BUCKET
On #44 I brought up this case and asked for possible reasons of water surface change.
There was only an answer, from Alancalverd, “centrifugal force”.
I consider it could be very clarifying to analyze what happens in detail (aiming even laymen could understand) , especially because it is an example of in how different ways matter actually can respond to acting forces: gravitational, centripetal, centrifugal (or whatever we could call them,  “outward inertial forces”? ). As I´ve already said, they produce internal stresses and/or deformations, in different fashions depending on each case.
What causes the water rotate is inner bucket surface rotation, thanks to friction …  Outer water molecule inertia tries to keep constant (tangential) speed vectors. The bucket makes it impossible, kind of PUSH (not "pull" as in other cases we´ve previously seen) inwards the water (centripetal force), and water rotates.
That (forces and movement) is transmitted inwards up to the axis of rotation.
All those transmitted centripetal forces have their “mirror” forces (3rd Newton´s principle), centrifugal ones.
One could ask himself: ok, but if all rotations are on horizontal plans, and centripetal and centrifugal (and friction) forces are also horizontal, why outer water rises (the bigger the radius, the higher the water level)?
If instead of water there were solid stuff (f.e., concrete), only internal (mainly compression) forces would occur. But liquids respond with an increase of what is usually called hydraulic pressure, which acts in ALL directions.
In any point pressure must also be equal to the weight of a 1 cm2 section column of water, from the point up to water surface.
That makes water surface acquire its concave shape. It is a kind of dynamic equilibrium, result of ALL acting forces.