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monty37
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can centripetal and centrigugal force act together?
Realize that in standard physics usage "centrifugal force" refers to a fictitious (or inertial) force that only appears as an artifact of viewing things from a rotating--and thus noninertial--reference frame.monty37 said:can centripetal and centrigugal force act together?
Doc Al said:Realize that in standard physics usage "centrifugal force" refers to a fictitious (or inertial) force that only appears as an artifact of viewing things from a rotating--and thus noninertial--reference frame.
To answer your question directly: Sure. Imagine a ball tied to a string being swung in a horizontal circle. There is of course a "centripetal" force on the ball being provided by the string tension. If viewed from a rotating frame in which the ball is at rest, then you'd also have a centrifugal force acting on the ball. (Note that nothing actually pushes the ball outward.)
I don't agree with that , a stationary observer in a rotating frame doesn't feel a centripetal acceleration, he is at rest .daniel_i_l said:Centrifugal force is an imaginary force that's felt by observers in a rotating frame.
The centripital force is the actual force that's causing the rotating movment. So a stationary observer in a rotating frame feels a centripital force towards the center of rotation and a centrifugal force in the opposite direction. The two cancel out which is why the observer is stationary in the rotating frame.
No. In the rotating frame the two forces (centripetal and centrifugal) cancel and the acceleration is zero.jonjacson said:¿do you mean that in the rotating frame you have both accelerations?
Don't confuse centripetal acceleration with centripetal force. The centripetal force still exists in the rotating frame.I don't agree with that , a stationary observer in a rotating frame doesn't feel a centripetal acceleration, he is at rest .
Doc Al said:No. In the rotating frame the two forces (centripetal and centrifugal) cancel and the acceleration is zero.
Don't confuse centripetal acceleration with centripetal force. The centripetal force still exists in the rotating frame.
In the example I gave in post #2, the centripetal force--which equals mω²r--is provided by the string tension. That string tension exists regardless of the frame you use.jonjacson said:I don't understand that, ¿can you show me the mathematical expression of the centripetal force in the non inercial frame?.
Okey, now everithing is clear(I think), I was talking of another system, without a string, simply a mass above a rotating disk, so in that case the friction is the centripetal force ( because points to the center of rotation), and in the case of the string the tension is the centripetal force, in both cases centripetal acceleration is cero because centrifugal force is of the same magnitude and opposite sense ¿do you agree?Doc Al said:In the example I gave in post #2, the centripetal force--which equals mω²r--is provided by the string tension. That string tension exists regardless of the frame you use.
Sounds good.jonjacson said:Okey, now everithing is clear(I think), I was talking of another system, without a string, simply a mass above a rotating disk, so in that case the friction is the centripetal force ( because points to the center of rotation), and in the case of the string the tension is the centripetal force, in both cases centripetal acceleration is cero because centrifugal force is of the same magnitude and opposite sense ¿do you agree?
That's correct--ω is the rotational speed of the frame.Other question when you use mw2r, that w is the angular speed of the rotation frame not of the object because is at rest, ¿am I wrong?.
Right! In the turntable problem all forces are fictitious, since the centripetal acceleration itself is just an artifact of using a rotating frame.Finally, in the turntable problem, you have that the body is not at rest, it has a speed v, so it appears a coriolis force pointing to the center of the rotating frame, so now that coriolis force is the centripetal force, we have the centrifugal force like always (if r is not parallel to the angular speed vector), and the net result of these two forces (coriolis pointing inward, and centrifugal pointing outward) is a centripetal acceleration mv2/r, the same as in the inertial frame.
monty37 said:can centripetal and centrigugal force act together?
When I have seen your reply, I was happy because I had started to think that I understand this, but about the third point ¿did you mean centripetal or centrifugal?.Doc Al said:Sounds good.That's correct--ω is the rotational speed of the frame.Right! In the turntable problem all forces are fictitious, since the centripetal acceleration itself is just an artifact of using a rotating frame.
I meant centripetal--towards the center. As seen in the rotating frame, the mass execute circular motion and thus is centripetally accelerated. (I'm a bit confused by your question since you identified the acceleration as centripetal yourself in post #8.)jonjacson said:When I have seen your reply, I was happy because I had started to think that I understand this, but about the third point ¿did you mean centripetal or centrifugal?.
Please say centrifugal, this already seems a tricky game with words
Fictitious inertial forces--such as coriolis and centrifugal--are extremely useful when analyzing motion from a rotating frame.ruko said:Centrifugal force is fictitious. It does not exist. A centrifuge should be called an inertiafuge. The question in my opinion should read, "Can centripetal force and inertia act together?"
The shaft pulls the mass in a circle; the inward force it exerts can be called the centripetal force. Viewed from a rotating frame, you would have a fictitious centrifugal force acting outward on the mass. The two "forces" balance each other.monty37 said:well,applying the same to engineering concepts ,a mass tied to a shaft undergoing rotary motion, the book says there is a centrifugal force acting ,but with respect to an observer outside there is also a centripetal force?so they need to balance each other out,right?
Doc Al said:I meant centripetal--towards the center. As seen in the rotating frame, the mass execute circular motion and thus is centripetally accelerated. (I'm a bit confused by your question since you identified the acceleration as centripetal yourself in post #8.)
Doc Al said:Sounds good.That's correct--ω is the rotational speed of the frame.Right! In the turntable problem all forces are fictitious, since the centripetal acceleration itself is just an artifact of using a rotating frame.
All I meant was that when viewed from an inertial frame, the mass is not accelerating at all and no net force acts on it.jonjacson said:Yes the acceleration is centripetal in the rotating frame , but i did not understand why you said at the end of your sentence "since the centripetal acceleration itself is just an artifact of using rotating frame".
What book are you using? What force do they say balances the centrifugal force?monty37 said:that is what i thought,they balance out each other,but there is no mention of centripetal force in the book,it is being balanced differently.
Sure, if you wanted to view it from a rotating frame in which the planet is at rest. (Not clear why you would want to do that, though.)can you apply the same balancing principle to planet rotation around the sun?
Surely you meant a non-inertial frame, not an inertial frame. There is of course zero centrifugal force in an inertial frame.Doc Al said:All I meant was that when viewed from an inertial frame, the mass is not accelerating at all and no net force acts on it.
No, I actually meant inertial frame. My comment referred to the "turntable" problem, in which the mass is stationary and suspended above a rotating turntable. Viewed from the rotating frame, the mass is centripetally accelerated, but from the inertial frame it is at rest.D H said:Surely you meant a non-inertial frame, not an inertial frame.
That's certainly true.There is of course zero centrifugal force in an inertial frame.
I'm not sure what you're looking for. If something is moving in a circle, then there must be a net radial force pulling it in. That net force is called the centripetal force. Centrifugal force is a "fictitious" force that is only used when analyzing motion from a rotating frame.monty37 said:so how do you conclude the nature of this force,as to where which acts,centripetal or centrifugal,i mean generally,as in books especially in engineering concepts,it is highly unclear.
Non-fictitious, a.k.a. "real", forces have agents. The centrifugal "force" has none, since it's just an artifact of analyzing things from a rotating frame. Just because you "feel" an outward force on you does not mean that there is one.sophiecentaur said:If you are on a turntable you will experience a non-fictitious force on you and if you hold up a pendulum it will lean 'out' along a radius. I really can't see why people get so up themselves when this force is called centrifugal.
No it can't, unless you are using "reaction" in some loose, non-Newton's 3rd law sense. The centripetal force is acceptable because it is "real"--there really is something exerting an inward force on something moving in a circle.It can be regarded as a reaction against the (somehow acceptable) centripetal force but,
Real forces don't "disappear" when you change frames.as it is there, and you can feel it, why do people get their knuckles rapped for naming it? It disappears as soon as you remove it's cause, of course.
The force pushing up on your butt is a real force between your chair and you. Nothing fictitious about it, and nothing to do with centrifugal force.It's only the same type of phenomenon as the force which is pushing up un your bum as you read this.
The floor and the force it exerts most certainly are real. Of course you can test the hypothesis that it isn't real by jumping off the top of a tall building.sophiecentaur said:But the upwards force on your bum is surely no more real; it stops when you take the floor away.
Yes and no. A lot of people misinterpret the equivalence principle. The equivalence principle says that no local experiment can distinguish between free-falling through empty space versus free-falling in a gravitational field.And how about the equivalence principle? Gravity and acceleration are equivalent are they not?
Ad hominem attacks, and particularly those of a non sequitur nature, do not make for a good argument.I'm sure you are in league with Mr. Scales, from 1962.
;-)
How are we to know who your Mr. Scales was? I thought you were referring to Junius Scales, the only Mr. Scales I could find who did something noteworthy in the early 60s.sophiecentaur said:WMr Scales was my hero! He taught Physics like no other could.
Teachers (and textbook authors) who centrifugal force to explain orbits are, in my opinion, doing an incredible disservice to their pupils. For one thing, that explanation typically goes side-by-side with a drawing of a planet/satellite moving in circular path around the Sun/Earth. That circular path implies that the teacher or author is looking at things from the perspective of an inertial frame of reference. There is *zero* centrifugal force in this frame. The only perspective from which a circular orbit has a centrifugal force that exactly opposes the gravitational force is a rotating frame of reference in which the planet/satellite is stationary.My point was that, at an early age, I was given all the arguments about things not 'flying off' due to centrifugal force - which I, of course, appreciate because it could mean a big misconception and a false prediction.
There is no measurable centrifugal force. The centrifugal force, if there is one, depends on the frame in which the picture is drawn. Whether you want to call it "real" is one thing. That it is measurable is quite another. It isn't.BUT, there is a force, which can be felt and measured and it IS in the direction opposite to the centripetal force.
That's not a centrifugal force. You feeling a centripetal force on those fairground rides.sophiecentaur said:'My' centrifugal force is the one you feel when you are on a fairground ride. It's there - I have felt it.