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#50 NilakYou 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.
... According to 3rd Newton´s principle, outer object also exerts an equal but opposite force on central one: centrifugal force....
Quote from: rmolnav on 13/10/2016 18:37:39... 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?
Pendulum IIt 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.
#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.
..."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....
#65 hamdani yusufPlease 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 ...
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.
... How do you think we should call a force opposite to a centripetal one?
#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.
The main problem with the idea of centrifugal force is that the vector tangent to the rotation is producing the outward 'force'. So that if a bucket on the end of a rope stops being confined, the rope is released, it does not move away in the radial direction. If the bucket were on a bungee rope then the experiment would be more interesting. As the speed of rotation increases how does the rope react?
#90 jerrygg38Einstein´s gravity theory is difficult to grasp to me (as to most people, I am afraid). So, on this subject I don´t feel sure enough to discuss. That´s why I have not commented your post last days, preferring to deal with others.But now it seems we are having a kind of rest, and I´ve decided to say a couple of things.On the one hand, what you say may be right, but I think last words should be: "and this is a centrifugal force”. I´ve already shown many cases where we also have centrifugal forces, quite different to your case.And on the other hand, the deep roots of the forces on my examples (gravity, inertial forces, centrifugal forces) may be, according to Einstein´s theory, "bending" of space-time, or related mass changes ... But I consider that, at least at speeds so much smaller than light speed, ALL cases where centrifugal force may appear can be explained within Newton´s Physics.