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Gabriele Pinna
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Can someone explain me what is the connetion between centrifugal force and weight force thanks ? In which situation should I use the centrifugal force (that is a fake force) ?
The force in the radial direction is call centripetal. Centrifugal "force" is the inertia which is trying to keep the object going in a straight line.jamie.j1989 said:Centrifugal force isn't a fake force, tie a mass to the end of a string and spin around, the string gets taught and stops the mass from flying off, the string applies a force in the radial direction towards you, this is centrifugal force. Weight is a gravitational force.
What I believe jamie.j1989 is referring to is the reactive centrifugal force. https://en.wikipedia.org/wiki/Reactive_centrifugal_force. What Gabriele Pinna is talking about is the inertial centrifugal force. https://en.wikipedia.org/wiki/Centrifugal_force.mathman said:The force in the radial direction is call centripetal. Centrifugal "force" is the inertia which is trying to keep the object going in a straight line.
add centrifugal force when in rotating frameGabriele Pinna said:In which situation should I use the centrifugal force (that is a fake force) ?
jbriggs444 said:What I believe jamie.j1989 is referring to is the reactive centrifugal force. https://en.wikipedia.org/wiki/Reactive_centrifugal_force. What Gabriele Pinna is talking about is the inertial centrifugal force. https://en.wikipedia.org/wiki/Centrifugal_force.
vanhees71 said:I'd rather call the centrifugal force an inertial force rather than a fictitious force, because then it's immediately clear that it occurs exclusively in non-inertial (rotating) reference frames.
Now, this sounds very unclear and confusing to me. In my view, it is better not to obsess so much about the "centrifugal"/"centripetal" terminology, or always provide a precise mathematical definition of those terms (which you didn't).Andrew Mason said:In my view, it is clearer and much less confusing to say that in any rotating system, all net forces are centripetal and all forces (which includes "reaction" forces) are either purely centripetal forces and, in cases where there are mechanical connections, a combination of centripetal net forces and bi or multi-directional tensions.
No one is "obsessing" about anything. It is a matter of being clear on the physics. If one must talk about centrifugal "force", it is the apparent force that appears in a non-inertial (rotating) frame of reference. To suggest that the reaction force to a centripetal force should be called "the centrifugal reaction force" is just wrong in many cases and confusing where one is referring to tensions that necessarily always operate in at least two directions.A.T. said:Now, this sounds very unclear and confusing to me. In my view, it is better not to obsess so much about the "centrifugal"/"centripetal" terminology, or always provide a precise mathematical definition of those terms (which you didn't).
Just to follow up on my last couple of posts, this is a perfect example of why it is so confusing to speak of the "reactive centrifugal force". The string applies a force to both the rotating mass and to "you". The real forces acting on the rotating mass and "you" are centripetal, not centrifugal. In the non-inertial frame of "you", there seems to be some force pulling the mass away from you so it seems centrifugal. That is not a real (Newtonian) force. "You" (the person who is holding the other end) are actually rotating (kind of wobbling) about an inertial point somewhere in between you and the rotating mass. The rope is supplying a centripetal force to both (the rotating) you and the (rotating) mass. So the string or rope is supplying centripetal forces at both ends.jamie.j1989 said:Centrifugal force isn't a fake force, tie a mass to the end of a string and spin around, the string gets taught and stops the mass from flying off, the string applies a force in the radial direction towards you, this is centrifugal force. Weight is a gravitational force.
What about the real forces on the string?Andrew Mason said:The real forces acting on the rotating mass and "you" are centripetal, not centrifugal.
What about them?A.T. said:What about the real forces on the string?
That is the net force on a string element. I'm asking about the force exerted by the mass on the string.Andrew Mason said:The real force on any string element dm is the sum of the tensions acting on it: →dF=∑Ti=−dmω2r^rdF→=∑Ti=−dmω2rr^\vec{dF} = \sum T_i = -dm\omega^2 r \hat r. It is always toward an inertial point, which is the centre of rotation. r is the distance from the mass element to that centre of rotation.
Andrew Mason said:"You" (the person who is holding the other end) are actually rotating (kind of wobbling) about an inertial point somewhere in between you and the rotating mass. The rope is supplying a centripetal force to both (the rotating) you and the (rotating) mass. So the string or rope is supplying centripetal forces at both ends.
It is equal and opposite to the force of the string on the mass.A.T. said:That is the net force on a string element. I'm asking about the force exerted by the mass on the string.
Only if you think the hub centre is the inertial centre of rotation. To determine the inertial centre of rotation you have to take into account the mass of the earth. The hub/earth actually rotates about that point. So the chain supplies tension to the hub, which distributes that tension throughout the apparatus and the earth, causing the hub/earth to rotate or wobble (albeit imperceptibly) around that inertial point and accelerate centripetally toward that point.jbriggs444 said:To make this less obvious, consider a hub, as on a carnival ride with chains leading to the seats swinging around in a circle. The force exerted on the hub by the chains is in the centrifugal direction.
As long as the center of rotation is anywhere in the interior of the hub, the force of the chains on the hub will be at least primarily in the centrifugal (aka outward) direction.Andrew Mason said:Only if you think the hub centre is the inertial centre of rotation. To determine the inertial centre of rotation you have to take into account the mass of the earth.
So it's opposite to the "centripetal" force, and has the same point of application (just acts on a different object).Andrew Mason said:It is equal and opposite to the force of the string on the mass.
It's the same center that the centripetal force point towards. Since the two forces have the same point of application, but opposite directions, they can hardly both point toward the center of rotation.Andrew Mason said:Whether that force is toward or away from the centre of rotation depends on where the centre of rotation is and what you mean by the string.
So how useful is it to always refer to the reaction force to a centripetal force as centrifugal if it is sometimes not centrifugal (even when the force is tensile) and, in the case of force acting at a distance, never centrifugal? And, regardless of how the tension may be directed, that tension can never accelerate a mass away from the centre.jbriggs444 said:As long as the center of rotation is anywhere in the interior of the hub, the force of the chains on the hub will be at least primarily in the centrifugal (aka outward) direction.
Yes, the unbalanced component of seats and riders on the carnival ride and the ride and Earth are mutually orbitting a common barycenter and are all undergoing centripetal acceleration around that hypothetical point. But that's hardly a useful or even a particularly accurate description of the situation.
I dislike the term "reactive centrifugal force" and try not to use it. However, in the case I've described we have a force acting at a distance which is most certainly centrifugal in direction.Andrew Mason said:So how useful is it to always refer to the reaction force to a centripetal force as centrifugal if it is sometimes not centrifugal (even when the force is tensile) and, in the case of force acting at a distance, never centrifugal?
Nobody does that always. Some people do it only when it actually points away from the center.Andrew Mason said:So how useful is it to always refer to the reaction force to a centripetal force as centrifugal
Yes. But the direction of a force is defined as the direction which the force, acting alone on the body, would accelerate the centre of mass of the body, which I believe fits the Newtonian definition of force. The force and its "reaction" force act on different bodies, and therefore on different points. So it is not necessarily the case that the reaction force to a force in the centripetal direction will be in a non-centripetal direction.A.T. said:So it's opposite to the "centripetal" force, and has the same point of application (just acts on a different object).
Why not? Since they act on different bodies at different points, they could both be, and often are, both centripetal forces. The point of application does not determine the direction of the force unless you are only dealing with the mass elements whose centres of mass coincide with the point of application.A.T. said:It's the same center that the centripetal force point towards. Since the two forces have the same point of application, but opposite directions, they can hardly both point toward the center of rotation.
! In the example you gave, the centripetal forces were mechanical, not "acting at a distance". Where the forces between two bodies are acting at a distance (eg. gravity) both are always centripetal.jbriggs444 said:I dislike the term "reactive centrifugal force" and try not to use it. However, in the case I've described we have a force acting at a distance which is most certainly centrifugal in direction.
The chain can be replaced with a force at a distance without changing the situation in any material way.Andrew Mason said:! In the example you gave, the centripetal forces were mechanical, not "acting at a distance". Where the forces between two bodies are acting at a distance (eg. gravity) both are always centripetal.
This sense of "direction" alone tells you nothing about pointing towards or away from the center. You have to also consider the point of application.Andrew Mason said:But the direction of a force is defined as the direction which the force, acting alone on the body, would accelerate the centre of mass of the body, which I believe fits the Newtonian definition of force.
Does not follow. They act at the same geometrical point (connection between string and mass) and are opposite in direction. So they cannot both point towards the center.Andrew Mason said:act on different bodies, and therefore on different points.
How would that work?jbriggs444 said:The chain can be replaced with a force at a distance without changing the situation in any material way.
I don't think it is false to say that in a rotating rigid system, all net forces are centripetal. As I stated, there are tensions within rigid systems that can go in many directions. Exactly where a force acts on a rigid body can be a philosophical debate. What matters is how the body acts in response to the force. In the case of two rigid bodies in mutual gravitational orbit about their common barycentre, the centre of mass of each body accelerates in the direction of that barycentre. That does not depend on how the mass is distributed within the body.Your latter claim is simply false. It holds for the special cases of gravity on point masses and for gravity on spherically symmetric objects simplified by pretending that it acts at their centers of mass. It does not hold for general forces at a distance where the point of application of the force need not coincide with the target object's center of mass.
G. David Scott (1957). "Centrifugal Forces and Newton's Laws of Motion" 25. American Journal of Physics. p. 325. said:G. David Scott (1957). "Centrifugal Forces and Newton's Laws of Motion" 25. American Journal of Physics. p. 325.
A mass A is moving in a circle; a centripetal force must therefore be acting on A. What is the reaction to this force? The reaction in general will be the force which causes mass B to move in a circle. For example the reaction to the force which move the electron around the proton in the hydrogen atom is the force which moves the proton around their common centre of gravity. The same situation is revealed in the case of the moon and the earth, or the figure skater and her partner as they whirl about together. The reaction to a centripetal force is not a centrifugal force but another centripetal force. Though Newton's third law is used in statics, its real significance is in dynamics. When a force acts to accelerate one mass, then there is always a reaction which is accelerating another mass in the opposite direction.
Practical problems can always be solved using the strict Newtonian approach, but it is often more realistic and intuitively simpler to use a rotating reference frame and introduce the necessary centrifugal forces. The centrifugal forces must be recognized for what they are - non-Newtonian forces acting on masses at rest in a rotating frame.
Magnets, charged spheres, big blobs of mass, idealized cords. It does not matter. We have an object over here exerting a force-at-a-distance with a point of application over there.Andrew Mason said:How would that work?
That's not the false statement you made.I don't think it is false to say that in a rotating system, all net forces are centripetal.
Tensions act within bodies. But there are forces between the bodies, which can go in many directions, including away from the center.Andrew Mason said:As I stated, there are tensions within rigid systems that can go in many directions
How do you determine the point of application of a force? Does it the point of application really matter - or is this just a semantic debate? Where do the Earth and moon apply their forces to each other?A.T. said:This sense of "direction" alone tells you nothing about pointing towards or away from the center. You have to also consider the point of application.
Since the forces act on different bodies, why can they not be both centripetal if the bodies are on opposite sides of the inertial centre of rotation?Does not follow. They act at the same geometrical point (connection between string and mass) and are opposite in direction. So they cannot both point towards the center.
Ok. Put the ride in space and have the sufficiently negatively charged chair circling the sufficiently positively charged hub due to the electric forces between them. It would seem to me that both undergo centripetal acceleration about their common centre of mass.jbriggs444 said:Magnets, charged spheres, big blobs of mass, idealized cords. It does not matter. We have an object over here exerting a force-at-a-distance with a point of application over there.
So what is the false part of what I said?That's not the false statement you made.
The point of force application does matter in physics.Andrew Mason said:Does it the point of application really matter - or is this just a semantic debate?
Special case, that cannot be generalized.Andrew Mason said:if the bodies are on opposite sides of the inertial centre of rotation
If that was your only point, no one would object. But you make a lot of invalid generalizations and geometrically absurd statements.Andrew Mason said:My point is that using the term "centrifugal force"to denote a phenomenon other than the non-Newtonian pseudo-force that appears in non-inertial frames creates a great deal of confusion for anyone trying to learn about the physics of rotating bodies.
Please stop changing the setup to fit your pet scenario. It can be a charged blob on the hub, not a charged hub.Andrew Mason said:Ok. Put the ride in space and have the sufficiently negatively charged chair circling the sufficiently positively charged hub due to the electric forces between them. It would seem to me that both undergo centripetal acceleration about their common centre of mass.
So what is the false part of what I said?
You did not say "net", and did not mention a point of application.Andrew Mason said:Where the forces between two bodies are acting at a distance (eg. gravity) both are always centripetal.
A.T.'s response to this was entirely on point.My point is that using the term "centrifugal force"to denote a phenomenon other than the non-Newtonian pseudo-force that appears in non-inertial frames creates a great deal of confusion for anyone trying to learn about the physics of rotating bodies.
So tell us where my "invalid generalizations" differ from those of Prof. Scott in his article.A.T. said:If that was your only point, no one would object. But you make a lot of invalid generalizations and geometrically absurd statements.