# Appearence and reality -- centripetal and centrifugal forces...

For each action there is an equal and opposite reaction (third principle), and no one would ever think to say that the action is "real" and the reaction is "apparent".

Also the centripetal force is an "action" to which (as it happens for all the actions) it corresponds *always* an equal and opposite centrifugal "reaction".

A centripetal force could *never* exist without the corresponding centrifugal force!

So why in this case (and *only* in this case) the action (of centripetal force) would be "real" and the "apparent" (centrifugal) reaction?

PeroK
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The "action" and "reaction" pair of forces never apply to the same object. If something is subject to a real centripetal force (e.g. the Earth being pulled to the Sun) then the equal and opposite force is the force that object exerts on whatever is pulling it (e.g. the Earth pulling the Sun).

Centripetal and centrifugal forces on the object are not a third law pair.

• CWatters
The "action" and "reaction" pair of forces never apply to the same object. If something is subject to a real centripetal force (e.g. the Earth being pulled to the Sun) then the equal and opposite force is the force that object exerts on whatever is pulling it (e.g. the Earth pulling the Sun).

Centripetal and centrifugal forces on the object are not a third law pair.
The rope of the sling exerts its centripetal force (action) on the stone and the stone exerts its centrifugal force (reaction) on the rope.

PeroK
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The rope of the sling exerts its centripetal force (action) on the stone and the stone exerts its centrifugal force (reaction) on the rope.
Yes, exactly, and both are real forces. The fictitious centrifugal force on the stone in the stone's reference frame isn't relevant here.

If the force exerted by the stone on the rope is a "centrifugal" force and is also "real", why is it said that the centrifugal force is "apparent"?

It should be said that there exists the "apparent" centrifugal force (the one that is not there) and there exists *also* the "real" centrifugal force (which is there!).

PeroK
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If the force exerted by the stone on the rope is a "centrifugal" force and is also "real", why is it said that the centrifugal force is "apparent"?

It should be said that there exists the "apparent" centrifugal force (the one that is not there) and there exists *also* the "real" centrifugal force (which is there!).

Centrifugal means "away from the centre". Those forces can be as real as any others.

in the case of circular motion of a particle there is a real centripetal force that causes the circular motion, but no real centrifugal force on the particle.

However, if you are moving in a circle you have the impression that there is a centrifugal force pushing you out. This is a false impression: The real force is centripetal.

And, in your.reference frame you are at rest. You feel a real centripetal force in one direction, hence there is a fictitious centrifugal force to balance this.

In general you have fictitious forces in accelerating reference frames.

Dale
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If the force exerted by the stone on the rope is a "centrifugal" force and is also "real", why is it said that the centrifugal force is "apparent"?

It should be said that there exists the "apparent" centrifugal force (the one that is not there) and there exists *also* the "real" centrifugal force (which is there!).
There are two different types of centrifugal forces. One type of centrifugal force is the “reactive centrifugal force”. It is a real force which exists in all frames and which is the third law pair of a centripetal force. (Note, there is always a 3rd law pair to a centripetal force, but it is not always centrifugal)

The other type of centrifugal force is the “fictitious centrifugal force” or the “inertial centrifugal force”. It is a fictitious force which exists only in the rotating reference frame and it has no 3rd law pair.

They are different concepts, and which is implied should be clear from context. Obviously, if it says that it is “apparent” then it is talking about the latter one.

rcgldr
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TNote, there is always a 3rd law pair to a centripetal force, but it is not always centrifugal.
As an example of such a situation, consider two objects in space, both in a circular orbit about a common center of mass. Each experiences a centripetal force towards the common center of mass, and no centrifugal force is present. In this case the two centripetal forces towards the common center of mass are the 3rd law pair of forces.

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• Dale
As an example of such a situation, consider two objects in space, both in a circular orbit about a common center of mass. Each experiences a centripetal force towards the common center of mass, and no centrifugal force is present. In this case the two centripetal forces towards the common center of mass are the 3rd law pair of forces.
The Earth does not attract (centripetally) the Moon (Newton), the Earth attracts the surrounding space (Einstein) centrifugally (with respect to the center of common mass)!

There are two different types of centrifugal forces. One type of centrifugal force is the “reactive centrifugal force”. It is a real force which exists in all frames and which is the third law pair of a centripetal force. (Note, there is always a 3rd law pair to a centripetal force, but it is not always centrifugal)

The other type of centrifugal force is the “fictitious centrifugal force” or the “inertial centrifugal force”. It is a fictitious force which exists only in the rotating reference frame and it has no 3rd law pair.

They are different concepts, and which is implied should be clear from context. Obviously, if it says that it is “apparent” then it is talking about the latter one.
Nowhere is it said that there is a real centrifugal force and another apparent, everywhere it is written that the centrifugal force is "apparent".

Always.

Do you report only one case in which the pair of a centripetal force is not centrifugal?

PeroK
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The Earth does not attract (centripetally) the Moon (Newton), the Earth attracts the surrounding space (Einstein) centrifugally (with respect to the center of common mass)!

I'm sorry, I thought you asked these questions to learn. The problem is that in all your posts you simply argue with the answers you get. Here, you ask a question about classical mechanics, you get 3-4 answers all trying to explain it to you. In response, you argue that classical gravity was superseded by Einstein's Gravity.

So, what are you trying to learn? What's the point of asking a question only to argue against the answers you get?

• CWatters and Dale
Dale
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Nowhere is it said that there is a real centrifugal force and another apparent, everywhere it is written that the centrifugal force is "apparent".

Always
Then the sources you are reading are clearly always talking about the fictitious centrifugal force. It is indeed far more common for textbooks to talk about the fictitious centrifugal force than the reactive centrifugal force.

That doesn’t mean that there aren’t two distinct concepts, as I mentioned. It just means that one is discussed far more frequently than the other.

The Earth does not attract (centripetally) the Moon (Newton)
Sure it does. Newtonian gravity is perfectly adequate to describe the interaction of the earth and the moon.

Do you report only one case in which the pair of a centripetal force is not centrifugal?
Usually, for two-body uniform circular motion, contact forces will have a centrifugal reaction force and non-contact forces will have a pair of centripetal forces. For multi-body problems or non-uniform circular motion or extended objects there may be no third law pairs pointed in the radial direction at all.

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A.T.
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