Can centripetal and centrigugal force act together?

In summary: 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.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.
  • #1
monty37
225
1
can centripetal and centrigugal force act together?
 
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  • #2
monty37 said:
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.

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.)
 
  • #3
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.
 
  • #4
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.)

¿do you mean that in the rotating frame you have both accelerations?
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.
I don't agree with that , a stationary observer in a rotating frame doesn't feel a centripetal acceleration, he is at rest .

If the angular speed of the non inertial reference frame remains constant , these are the forces (in this non inertial frame):

-gravity

-normal, equal to the gravity force, so they cancel

- the friction force pushing inward at your feet

-the centrifugal pushing outward to your feet.

So the friction force is equal to the centrifugal (if you are at rest), and that means:

Friction force= mrw2 =centrifugal force

The net result is a=0, and as a consequence there is not centripetal acceleration.

¿do you agree?

---------------It is very useful to make these problem:

A mass is at rest respect to the laboratory frame, while a frictionless turntable rotates beneath it . In the turntable frame ¿what are the forces?. And verify F=ma.
 
  • #5
jonjacson said:
¿do you mean that in the rotating frame you have both accelerations?
No. In the rotating frame the two forces (centripetal and centrifugal) cancel and the acceleration is zero.

I don't agree with that , a stationary observer in a rotating frame doesn't feel a centripetal acceleration, he is at rest .
Don't confuse centripetal acceleration with centripetal force. The centripetal force still exists in the rotating frame.
 
  • #6
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.

I don't understand that, ¿can you show me the mathematical expression of the centripetal force in the non inercial frame?.
 
  • #7
jonjacson said:
I don't understand that, ¿can you show me the mathematical expression of the centripetal force in the non inercial 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.
 
  • #8
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.
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?

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?.

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.

¿everything is ok?
 
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  • #9
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?
Sounds good.

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?.
That's correct--ω is the rotational speed of the 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.
Right! In the turntable problem all forces are fictitious, since the centripetal acceleration itself is just an artifact of using a rotating frame.
 
  • #10
monty37 said:
can centripetal and centrigugal force act together?

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?"
 
  • #11
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.
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?:confused:.

Please say centrifugal, this already seems a tricky game with words :biggrin:
 
  • #12
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?:confused:.

Please say centrifugal, this already seems a tricky game with words :biggrin:
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.)
 
  • #13
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?"
Fictitious inertial forces--such as coriolis and centrifugal--are extremely useful when analyzing motion from a rotating frame.
 
  • #14
my book has given the "stone tied to a string " case as an example of centrifugal force.
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?
 
  • #15
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?
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.
 
  • #16
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.)

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".

Because I think that you use the concept of centripetal acceleration in the inertial frames too, so it is not something artificial developed to understand rotational frames (like coriolis or centrifugal force).

Perhaps I didn't express it sufficiently clearly.
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.
 
  • #17
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".
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.
 
  • #18
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.
can you apply the same balancing principle to planet rotation around the sun?
 
  • #19
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.
What book are you using? What force do they say balances the centrifugal force?
can you apply the same balancing principle to planet rotation around the sun?
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.)
 
  • #20
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.
Surely you meant a non-inertial frame, not an inertial frame. There is of course zero centrifugal force in an inertial frame.
 
  • #21
D H said:
Surely you meant a non-inertial frame, not an inertial frame.
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.
There is of course zero centrifugal force in an inertial frame.
That's certainly true.
 
  • #22
actually it is a theory of machines book ,where the topic is about the balancing of
rotary masses,so these are inertial masses and in order to balance this,another
mass in the same plane is being attached to the shaft,in the opposite direction.
 
  • #23
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.
 
  • #24
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.
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.

If there is a specific statement or problem in your book that has you confused, perhaps you can scan those pages in so we can see it. (But I must admit, some books are confusing.)
 
  • #25
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. It can be regarded as a reaction against the (somehow acceptable) centripetal force but, 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.
It's only the same type of phenomenon as the force which is pushing up un your bum as you read this.
 
  • #26
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.
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.
It can be regarded as a reaction against the (somehow acceptable) centripetal force but,
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.
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.
Real forces don't "disappear" when you change frames.
It's only the same type of phenomenon as the force which is pushing up un your bum as you read this.
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.
 
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  • #27
Ok if that's a recognised definition. But the upwards force on your bum is surely no more real; it stops when you take the floor away. And how about the equivalence principle? Gravity and acceleration are equivalent are they not?
I'm sure you are in league with Mr. Scales, from 1962.
;-)
 
  • #28
when we stir tea,the dregs tend to get collected at the centre,but
it should settle at the corners,right,in accordance with centrifugal force?
 
  • #29
There's fluid dynamics at work here so I don't think that argument is my cup of tea. Does it even hold water? It 'leaves' out some facts and it may be an (Earl) Grey area.
Sorry - I'll get my coat.
 
  • #30
sophiecentaur said:
But the upwards force on your bum is surely no more real; it stops when you take the floor away.
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.

And how about the equivalence principle? Gravity and acceleration are equivalent are they not?
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.

Suppose you are in an enclosed spaceship (elevator car in Einstein's formulation). You feel a force from the floor of the vehicle pushing up on you. The vehicle might be out in space firing its engines or it might be standing still on the surface of a planet. Ignoring that the planet's gravitational field is not uniform, there is no way that you can tell the difference between these two scenarios. Now suppose instead that the vehicle is not firing its thrusters and that you are floating around inside the vehicle. Is this the vehicle is in deep space, far from any gravitational source, or free-falling (e.g., in orbit) about some massive body? Once again, you cannot distinguish between these scenarios using only experiments conducted within the vehicle.

I'm sure you are in league with Mr. Scales, from 1962.
;-)
Ad hominem attacks, and particularly those of a non sequitur nature, do not make for a good argument.
 
  • #31
Which 'homo' am I supposed to be attacking? Mr Scales was my hero! He taught Physics like no other could. And I wouldn't attack YOU - you might be bigger than me. ;) - and you are telling me to take a running jump off a tall building, too.

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. BUT, there is a force, which can be felt and measured and it IS in the direction opposite to the centripetal force. Furthermore, the direction of motion, when the central force is removed, asymptotes towards the 'away' direction even if it starts off tangential. The force's realness or otherwise is at another level of sophistication and it just seems over precise to make too much of a thing of it. I just wonder how many of the teachers who have shouted and screamed about it have given it as much thought as this thread has.
 
  • #32
sophiecentaur said:
WMr Scales was my hero! He taught Physics like no other could.
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.

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.
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.

For another thing, orbits in general are not circular. They are elliptical.

Here are nine elliptical orbits with the same semi-major axis, eccentricity = 0.1 to 0.9 in steps of 0.1, as viewed from the perspective of an inertial frame:

aadff4.png


With this inertial perspective, the shapes are simple and well-known (ellipses that share a common focus, and in this case, share a common semi-major axis). The descriptions of the orbits are once again simple and well-known (Kepler's laws). The equations of motion are yet again simple and well-known (Newton's second law + Newton's law of gravitation).


Here are the same orbits, but this time viewed from the perspective of a frame rotating with the mean motion of these orbits:

2zfk2fl.png


With this rotating perspective, the shapes are complex (this family of curves may have a name, but I don't know what it is). The descriptions of the orbits are complex, as are the equations of motion (Newton's second law + Newton's law of gravitation + centrifugal force + coriolis force).

BUT, there is a force, which can be felt and measured and it IS in the direction opposite to the centripetal force.
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.
 
  • #33
Did I ever say that orbits are explained using centrifugal force?
'My' centrifugal force is the one you feel when you are on a fairground ride. It's there - I have felt it.
'My' Mr Scales was also notable to all his students; Google doesn't know everything.
 
  • #34
sophiecentaur said:
'My' centrifugal force is the one you feel when you are on a fairground ride. It's there - I have felt it.
That's not a centrifugal force. You feeling a centripetal force on those fairground rides.

Think about it this way: You are going around in a circle on those rides. That means your acceleration vector is pointing toward the center of the circle. That in turn means some force directed toward the center of the circle is acting on you, and that is by definition a centripetal force.
 
  • #35
I know very well that the seat of the ride is pushing me inwards. The sums are quite clear. But I FEEL a force pushing my soft bits towards my back- 'outwards'. That force is away from the centre. Centrifugal, in fact. It is there just as much as the force of the ground on my feet. They are both reaction forces due to an acceleration. Why is this such a big deal? I just give a force a name and people get twitchy.
 
<h2>1. Can centripetal and centrifugal force exist at the same time?</h2><p>Yes, centripetal and centrifugal forces can exist simultaneously in a rotating system. Centripetal force is the inward force that keeps an object moving in a circular path, while centrifugal force is the outward force that pulls an object away from the center of rotation. These two forces work together to maintain the object's circular motion.</p><h2>2. How do centripetal and centrifugal forces differ?</h2><p>The main difference between centripetal and centrifugal forces is their direction. Centripetal force acts towards the center of rotation, while centrifugal force acts away from the center. Additionally, centripetal force is required to keep an object moving in a circular path, while centrifugal force is a result of the object's inertia trying to keep it moving in a straight line.</p><h2>3. Can centripetal and centrifugal forces cancel each other out?</h2><p>No, centripetal and centrifugal forces cannot cancel each other out. Centripetal force is necessary to maintain circular motion, so it cannot be eliminated. Centrifugal force, on the other hand, is a result of the object's inertia and cannot be eliminated either. These two forces work together to keep an object in a circular path.</p><h2>4. Do centripetal and centrifugal forces have the same magnitude?</h2><p>No, centripetal and centrifugal forces do not have the same magnitude. Centripetal force is always greater than centrifugal force because it is responsible for changing the direction of an object's motion and keeping it in a circular path. Centrifugal force is a reactive force and is dependent on the object's mass and velocity.</p><h2>5. Can centripetal and centrifugal forces be measured separately?</h2><p>Yes, centripetal and centrifugal forces can be measured separately. Centripetal force can be measured using the formula Fc = mv^2/r, where m is the mass of the object, v is its velocity, and r is the radius of the circular path. Centrifugal force can be calculated using the formula Fcf = mω^2r, where ω is the angular velocity of the object. These two forces can also be measured using force sensors and accelerometers.</p>

1. Can centripetal and centrifugal force exist at the same time?

Yes, centripetal and centrifugal forces can exist simultaneously in a rotating system. Centripetal force is the inward force that keeps an object moving in a circular path, while centrifugal force is the outward force that pulls an object away from the center of rotation. These two forces work together to maintain the object's circular motion.

2. How do centripetal and centrifugal forces differ?

The main difference between centripetal and centrifugal forces is their direction. Centripetal force acts towards the center of rotation, while centrifugal force acts away from the center. Additionally, centripetal force is required to keep an object moving in a circular path, while centrifugal force is a result of the object's inertia trying to keep it moving in a straight line.

3. Can centripetal and centrifugal forces cancel each other out?

No, centripetal and centrifugal forces cannot cancel each other out. Centripetal force is necessary to maintain circular motion, so it cannot be eliminated. Centrifugal force, on the other hand, is a result of the object's inertia and cannot be eliminated either. These two forces work together to keep an object in a circular path.

4. Do centripetal and centrifugal forces have the same magnitude?

No, centripetal and centrifugal forces do not have the same magnitude. Centripetal force is always greater than centrifugal force because it is responsible for changing the direction of an object's motion and keeping it in a circular path. Centrifugal force is a reactive force and is dependent on the object's mass and velocity.

5. Can centripetal and centrifugal forces be measured separately?

Yes, centripetal and centrifugal forces can be measured separately. Centripetal force can be measured using the formula Fc = mv^2/r, where m is the mass of the object, v is its velocity, and r is the radius of the circular path. Centrifugal force can be calculated using the formula Fcf = mω^2r, where ω is the angular velocity of the object. These two forces can also be measured using force sensors and accelerometers.

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