# Centrifugal vs. Centripetal
**URL:** https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657
**Category:** Factual Questions
**Created:** [May 2, 2003, 4:39pm UTC](https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657 "2003-05-02T16:39:20Z")
**Posts on this page:** 15
**Page:** 1
### Author:  [@KnK](https://boards.straightdope.com/u/KnK)
#### Post date: [May 2, 2003, 4:39pm UTC](https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657/1 "2003-05-02T16:39:20Z")
Hey there, first time poster here so be gentle đ
At a recent family gathering, the subject of centripetal force came up. One family member insisted that centrifugal force does not exist, and that centripetal force is the only force (in this context). This member insisted that when people generally refer to centripetal force, they are in fact wrong and that they should really be saying centripetal force instead. Two other opposing family members insisted that centrifugal force is the force imposed AWAY from the center, and that it opposes centripetal force that imposes a force TOWARD the center. Kind of a âyou cant have one without the otherâ type of thing.
Who is correct?
KnK
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### Author:  [@Schnitte](https://boards.straightdope.com/u/Schnitte)
#### Post date: [May 2, 2003, 5:02pm UTC](https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657/2 "2003-05-02T17:02:40Z")
The centripetal force is there every time something is rotating, simply because it wouldnât be rotating without it - the centripetal force is directed at the center of the circle, keeping the rotating object on its orbit instead of flying away. The centrifugal force is directed outwards, away from the circle; it results from the objectâs inertia, because it resists the change of its movement. At any point on the circle, the object is trying to fly on straight forward, but itâs getting kept on its orbit by the centripetal force. As a result, inertia is pulling the object outward, tangentially to the orbit of the object. If itâs a continous stable rotation, centripetal and centrifugal forces are of equal value, but the first is pulling inwards, the other one outwards.
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### Author:  [@Schnitte](https://boards.straightdope.com/u/Schnitte)
#### Post date: [May 2, 2003, 5:25pm UTC](https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657/3 "2003-05-02T17:25:43Z")
The direction of the centrifugal force is not tangentially outward, sorry. The object is trying to fly on straight forward, tangentially to the orbit; the centrifugal force, a result of inertia, is directed just straight out of the circle (at a 90° angle to the direction the object is moving to at any given moment).
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### Author:  [@Gumbercules](https://boards.straightdope.com/u/Gumbercules)
#### Post date: [May 2, 2003, 5:26pm UTC](https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657/4 "2003-05-02T17:26:17Z")
There is some debate about whether it is real or not
[http://www.straightdope.com/classics/a881216.htmlCecil](http://www.straightdope.com/classics/a881216.htmlCecil) says itâs not real
Also look [http://scienceworld.wolfram.com/physics/CentrifugalForce.htmlhere](http://scienceworld.wolfram.com/physics/CentrifugalForce.htmlhere)
[http://observe.arc.nasa.gov/nasa/space/centrifugal/centrifugal5.htmlNASA](http://observe.arc.nasa.gov/nasa/space/centrifugal/centrifugal5.htmlNASA) doesnât agree
I guess you will have to decide for yourself.
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### Author:  [@Gumbercules](https://boards.straightdope.com/u/Gumbercules)
#### Post date: [May 2, 2003, 5:29pm UTC](https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657/5 "2003-05-02T17:29:06Z")
Damn, forget my spaces
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### Author:  [@Cheesesteak](https://boards.straightdope.com/u/Cheesesteak)
#### Post date: [May 2, 2003, 5:30pm UTC](https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657/6 "2003-05-02T17:30:37Z")
Schnitte is incorrect, if the centripetal and centrifugal forces were equal, the object would not accelerate towards the center, and would go straight instead.
Centripetal force is the only ârealâ force acting on an orbiting object. Imagine you are sitting in the passenger seat of a car, and it turns sharply left. You will be âflungâ towards the door to your right, and pressed against it during the turn. People say that is centrifugal force, but there is NO force pushing you right, NO force pressing you against the door, you are trying to go straight, and the car door moves into your path, pushing you left instead.
Since youâre being pushed against the door, you âthinkâ there must be something pushing you that way. See, even I said âpushed against the doorâ when there is nothing actually pushing you that way, you were already going that way.
One very important thing to remember about physics, if the forces zero out, no acceleration takes place. If there is no acceleration, things either sit still or move in a straight line however fast they were moving initially. This never, ever, changes, if you have a situation that seems to violate this, youâve messed something up.
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### Author:  [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)
#### Post date: [May 2, 2003, 5:31pm UTC](https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657/7 "2003-05-02T17:31:55Z")
Hereâs a column from a few years back with an excellent âin a nutshellâ explanation by **Chronos**.
[http://boards.straightdope.com/sdmb/showthread.php?threadid=41078](http://boards.straightdope.com/sdmb/showthread.php?threadid=41078)
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### Author:  [@El\_Marko](https://boards.straightdope.com/u/El_Marko)
#### Post date: [May 2, 2003, 6:04pm UTC](https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657/8 "2003-05-02T18:04:37Z")
Iâd have to concur with **Cheesesteak** on this. Iâm not sure where the idea of a centrifugal force first came from, but I offer as one point of data the fact that my college physics text (engineering-type physics) there is no mention of centrifugal force in the index, only centripetal force. It does, however, offer the following:
ââŚconsider a car traveling along a highway at a high speed and approaching a curved exit ramp. As the car takes the sharp left turn onto the ramp, a person sitting in the passenger seat slides to the right across the seat and hits the door. At that point, the force of the door keeps him from being ejected from the car. What causes the passenger to move toward the door? A popular, but improper, explanation is that some mysterious force pushes him outward. This is often called the âcentrifugalâ force. The passenger invents this ficticious force in order to explain what is going on in his accelerated frame of reference.â
So I guess you could say that centrifugal force is related to inertia as **Schnitte** said, but itâs really an artificial construct to explain the effects of inertia from the perspective of someone who is a part of the inertial system they are observing (i.e. the car passenger described above)
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### Author:  [@KnK](https://boards.straightdope.com/u/KnK)
#### Post date: [May 2, 2003, 6:38pm UTC](https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657/9 "2003-05-02T18:38:58Z")
I agree El Marko. My college physics text doesnt mention centrifugal eitherâŚonly centripetal.
However, in the âreal-lifeâ world, if you had a chainsaw that was malfunctioning⌠So you go to the parts store and tell them you need a CENTRIPITAL clutch??? Society has adopted centrifugal as the norm. But why? And why is this wrong?
Secondly, to quote [Webster.com](http://Webster.com),
CENTRIFUGE :
Main Entry: 1cen¡tri¡fuge
Pronunciation: 'sen-tr&-"fyĂźj
Function: noun
Etymology: French, from centrifuge centrifugal, from New Latin centrifugus
Date: 1887
: a machine using centrifugal force for separating substances of different densities, for removing moisture, or for simulating gravitational effects
Why does centripetal have no place in this definition or why isnt this machine called some derivitive of centripetal?
Thanks
KnK
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### Author:  [@CurtC](https://boards.straightdope.com/u/CurtC)
#### Post date: [May 2, 2003, 8:45pm UTC](https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657/10 "2003-05-02T20:45:35Z")
> [@](#):
>
> El Marko wrote:\*
> âŚitâs really an artificial construct to explain the effects of inertia from the perspective of someone who is a part of the inertial system they are observing
This is the essence of the situation. I think if a physics text says that centrifugal force is âimproperâ then itâs not an advanced enough textbook. Itâs proper to use it as long as youâre clear that youâre dealing in a rotating reference frame and can handle that fact.
What about the coriolis force? Does the textbook mention that? Itâs also one of those âfictitiousâ forces that only exists in a rotating reference frame. My college texts talked about it.
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### Author:  [@Ring](https://boards.straightdope.com/u/Ring)
#### Post date: [May 2, 2003, 9:11pm UTC](https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657/11 "2003-05-02T21:11:10Z")
> [@](#):
>
> âCan gravitation and inertia be identical? This question leads directly to the General Theory of Relativity. Is it not possible for me to regard the earth as free from rotation, if I conceive of the centrifugal force, which acts on all bodies at
> rest relatively to the earth, as being a ârealâ gravitational field of gravitation, or part of such a field? If this idea can be carried out, then we shall have proved in very truth the identity of gravitation and inertia. For the same property which is regarded as inertia from the point of view of a system not taking part of the rotation can be interpreted as gravitation when considered with respect to a system that shares this rotation. According to Newton, this interpretation is impossible, because in Newtonâs theory there is no ârealâ field of the âCoriolis-fieldâ type. But perhaps Newtonâs law of field could be replaced by another that fits in with the field which holds with respect to a ârotatingâ system of co ordinates? My conviction of the identity of inertial and gravitational mass aroused within the feeling of absolute confidence in the correctness of this interpretation.â
>
> Albert Einstein
Thus in one respect, as **CurtC** says, the _centrifugal_ force is a real force but since it still doesnât have a source it remains to some extent a pseudo force.
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### Author:  [@bugg](https://boards.straightdope.com/u/bugg)
#### Post date: [May 2, 2003, 9:30pm UTC](https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657/12 "2003-05-02T21:30:08Z")
This is easy. The centrifugal force is fictional in an intertial (newtonian) reference frame, it is real in a rotational reference frame.
We describe forces in order to explain something that is going on, if we are consistent in doing so then all types of reference frames are equally valid and in some contexts you could legitimatelly refer to centrifugal force.
Everyone here should complain about their professors; my _high school_ professor made this issue fairly clear (my college professor didnât really, but hey)
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### Author:  [@robby](https://boards.straightdope.com/u/robby)
#### Post date: [May 4, 2003, 1:45am UTC](https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657/13 "2003-05-04T01:45:27Z")
There was some discussion about this last week (in a hijack of another thread) [here](http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=179054&pagenumber=2).
My $0.02:
First of all, centripetal force is the **only** proper rotational force to consider if you are looking at a problem from an inertial (non-accelerating frame). **In an inertial (Newtonian) reference frame, centrifugal force does NOT exist.**
Centrifugal force is only added if you are looking at a problem from a noninertial (accelerating) reference frame.
Centrifugal force is equal in magnitude and opposite in direction to centripetal force. Since centripetal force points inward, centrifugal force points directly outward.
**Cheesesteak** , centrifugal force and centripetal force are indeed equal in magnitude. In a rotating reference frame, they do indeed cancel outâthat is the whole point. Remember, centrifugal force can only be considered when you are in a noninertial (accelerating) reference frame, such as a rotating reference frame.
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### Author:  [@sailor](https://boards.straightdope.com/u/sailor)
#### Post date: [May 4, 2003, 11:33am UTC](https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657/14 "2003-05-04T11:33:56Z")
I canât see what the big fuss is about. It is fairly simple and it seems to me the discussion here is more about semantics than about physics.
Take any mass and apply a force to it and it will accelerate with an acceleration a = F/m. You can say that a mass being accelerated creates an opposite force of magnitude F = a \* m which is pretty much saying the same thing. In the case of rotary movement the centripetal force is the active (ârealâ if you want) and the centrifugal force is the reactive force.
Friction is similar in that it creates forces which do not exist on their own but exist as a consequences of other things. Move something to the right and you feel a force to the left. And yet, move it to the left and you feel the force pulling to the right. Leave the object in place and there is no force. Centrifugal force is similar to the forces created by friction. They exist as a consequence of other things and cannot exist on their own but they are definitely real.
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### Author:  [@robby](https://boards.straightdope.com/u/robby)
#### Post date: [May 4, 2003, 8:50pm UTC](https://boards.straightdope.com/t/centrifugal-vs-centripetal/172657/15 "2003-05-04T20:50:31Z")
> [@](#):
>
> \*Originally posted by sailor \*
> \*\*I canât see what the big fuss is about. It is fairly simple and it seems to me the discussion here is more about semantics than about physics.
>
> Take any mass and apply a force to it and it will accelerate with an acceleration a = F/m. You can say that a mass being accelerated creates an opposite force of magnitude F = a \* m which is pretty much saying the same thing. In the case of rotary movement the centripetal force is the active (ârealâ if you want) and the centrifugal force is the reactive force.\*\*
The discussion here is indeed partly about semantics. However, **Sailor** , your post also shows why the concept of centrifugal force is so (needlessly) confusing.
Reactive forces do exist in an inertial reference frame. They are as real as âactiveâ forces. Centrifugal force (acting on a body undergoing rotational motion) does NOT exist in an inertial reference frame. The problem is that some people refer to centripetal forceâs reactive force as âcentrifugal force.â The difference, however, is that reactive forces (in the sense of Newtonâs third law) do not act on the same body as the âactiveâ force.
For example if I swing a ball around in a horizontal circle\* at constant speed, there is a centripetal force on the ball that causes it to constantly accelerate with uniform circular motion. The reactive force in the case does not act on the ball. It acts on my hand. It is equal in magnitude and opposite in direction to the centripetal force acting on the ball. This reactive force is a real force that is present even in an inertial reference frame. While this force is equal in magnitude and in the same direction as the **centrifugal** force that may be considered to be acting on the ball, it is really a different force, since this reactive force acts on my hand, and centrifugal force acts on the ball (in a rotational reference frame).
> [@](#):
>
> \*Originally posted by sailor \*
> **Friction is similar in that it creates forces which do not exist on their own but exist as a consequences of other things. Move something to the right and you feel a force to the left. And yet, move it to the left and you feel the force pulling to the right. Leave the object in place and there is no force. Centrifugal force is similar to the forces created by friction. They exist as a consequence of other things and cannot exist on their own but they are definitely real.**
No force exists on its own. A consequence of Newtonâs Third Law is that all forces come in pairs: My hand exerts a force on the wall; the wall exerts a force back **on me**.
Centrifugal force is different. Both centripetal force and centrifugal force are considered to act on the **same body**.
[This website](http://www.xrefer.com/entry/486430) has a good explanation of the matter, using an example of satellite motion for this first paragraph quoted here:
> [@](#):
>
> âŚAnother way of looking at this situation, which was once popular, is to assume that the centripetal force is balanced by an equal and opposite force, acting away from the centre of the circle, called the centrifugal force. One could then say that the satellite stays in orbit when the centrifugal force balances the gravitational force. This is, however, a confusing argument because the centrifugal force is fictitious - it does not exist. The gravitational force is not balanced by the centrifugal force: it is the centripetal force.
>
> Another example is that of a car rounding a bend. To an observer in the car, a tennis ball lying on the back shelf will roll across the shelf as if it was acted on by an outward centrifugal force. However, to an observer outside the car it can be seen that the ball, because of its almost frictionless contact with the car, is continuing in its straight line motion, uninfluenced by the centripetal force. Occasionally the concept of a centrifugal force can be useful, as long as it is recognized as a fictitious force. **A true centrifugal force is exerted, as a reaction, by the rotating object on whatever is providing its centripetal force** [emphasis added].
This last sentence, as with **Sailor** âs post above, demonstrates how confused the term âcentrifugal forceâ is.
My take on the whole matter is to never use the term. đ
[sub]\*For simplicityâs sake, consider the swing to either take place on a horizontal frictionless surface, or in a weightless environment.[/sub]