# Is the centrifugal force fictitious or just incidental?

Is the "centrifugal force" fictitious or just incidental?

Is the "centrifugal force" fictitious or just incidental?

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NateTG
Homework Helper
The cetrifugal force is part of accounting for an accelerated frame of reference. It corresponds to being in a reference frame that is accelerated.

I'm not sure what you mean by ficticious or incidental.e

http://www.hcc.hawaii.edu/~rickb/SciColumns/FictForce.04Feb96.html

When pressed against the door on a curve you are experiencing a similar pair of forces as the door pushes inward on you through the curve. From the general reference frame outside the car it is clear that there is a pair of equal and opposite forces acting between the passenger and the car. From the specific reference frame inside the car, centrifugal force exists as a fictitious force which appears to try to eject you from the car as the door pushes on you to counteract your inertial tendency to keep moving in a straight line at a constant speed. When our state of motion changes our senses confuse the relationship between forces and inertia. As a result we feel forces acting on us in the opposite direction from the actual forces that are causing us to accelerate. This is because our perception of forces and motion evolved in a nonaccelerating reference frame and because there are always at least two objects involved which exert equal and opposite forces on each other.
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Originally posted by NateTG
The cetrifugal force is part of accounting for an accelerated frame of reference. It corresponds to being in a reference frame that is accelerated.

I'm not sure what you mean by fictitious or incidental.e
Centrifugal forces and coriolis forces are often referred to as a 'fictitious' because it is a result of measuring coordinates in a rotating frame as opposed to an actual force which is trying to accelerate you outward. I've even heard gravity, within the context of general relativity, referred to as a 'fictitious' force.

I never cared for that term because it causes a lot of confusion.

Arcon

Originally posted by Loren Booda
Is the "centrifugal force" fictitious or just incidental?
Don't think of it as "fictitious" since its just as 'real' as any other force. Refering to it as an "inertial force" is much better.
Jimmy wrote
I've even heard gravity, within the context of general relativity, referred to as a 'fictitious' force.
In GR the gravitational force is considered to be equivalent (i.e. of the same nature) to an inertial force so in that sense you're correct. Howevever regarding this "real/fictitious" thing - Newton may have considered the Coriolis force to be 'fictitious' but Einstein considered it "real," i.e. Einstein wrote
Can 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 "real" 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 Newton's theory there is no "real" field of the "Coriolis-field" type. But perhaps Newton's law of field could be replaced by another that fits in with the field which holds with respect to a "rotating" system of co-ordiantes? My conviction of the identity of inertial and gravitational mass aroused within me the feeling of absolute confidence in the correctness of this interpretation. Can 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 "real" 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 Newton's theory there is no "real" field of the "Coriolis-field" type. But perhaps Newton's law of field could be replaced by another that fits in with the field which holds with respect to a "rotating" system of co-ordiantes? My conviction of the identity of inertial and gravitational mass aroused within me the feeling of absolute confidence in the correctness of this interpretation.
Both A.P. French and Cornelius Lanczos consider inertial forces to be real as well.

In fact in Lagrangian dynamics there is nothing which requires an inertial frame. Lagrangian dynamics is based on the Hamilton's Principle (Principle of extremal action) which does not depend on the frame of referance chosen.

Mk
I remember looking at a study done by students at the University of Michigan that said either centrifical or centrifigal force doesn't exist. Can someone clear that up?

NateTG
Homework Helper
Originally posted by Mk
I remember looking at a study done by students at the University of Michigan that said either centrifical or centrifigal force doesn't exist. Can someone clear that up?
Are you sure that the distinction wasn't between centripetal and centrifugal. The centripetal acceleration is the acceleration that keeps an object traveling in circles in an inertial reference frame.

Arcon: In GR the gravitational force is considered to be equivalent (i.e. of the same nature) to an inertial force so in that sense you're correct. Howevever regarding this "real/fictitious" thing - Newton may have considered the Coriolis force to be 'fictitious' but Einstein considered it "real," i.e. Einstein wrote...
Thanks for posting that Arcon. So gravity can be thought of as our inertial tendency to follow geodesics through space-time and the force I feel as weight on the surface of the earth is an inertial force; the surface of the Earth is preventing me from moving along in a straight line (geodesic) through it's own space-time curvature.

I would say that the centrifugal force, as well as the centripital force, are clearly incidental. That is, these are forces that cannot be created independent of one another, therefore they are incidental unto each other.

Arcon
Originally posted by Jimmy
Thanks for posting that Arcon. So gravity can be thought of as our inertial tendency to follow geodesics through space-time and the force I feel as weight on the surface of the earth is an inertial force; the surface of the Earth is preventing me from moving along in a straight line (geodesic) through it's own space-time curvature.
The force the you exert on the floor is an inertial force. The force the floor exerts on you is not.

For example: Suppose in your living room there is a uniform gravitational field. Suppose also that there is an electric field in your living room which is in the direction pointing from the floor to the ceiling. Place a ball which is charged such that it floats. I.e. the gravitational force is equal and opposite to the electric force. Then the force on the charge due to the gravitational field is an inertial force. The force on the charge due to the electric field is not an inertial force.

Are all non-inertial forces non-gravitational (i. e., exclusively electromagnetic, strong or weak)?

Originally posted by Arcon
Don't think of it as "fictitious" since its just as 'real' as any other force. Refering to it as an "inertial force" is much better.

But it's not just as real as any other force. It only exists in applying newton's laws of motion in an accelerated reference frame.

To go in a circle something must supply the centripetal force or you go in straight line. That centripetal force on the body is a real force and by Newton's 3rd law has a reaction force. If a string pulls on a weight to make it go in a circle then the weight pulls on the string. Real forces.

Now analyze the weight's motion in the rotating reference frame. The weight is not accelerating in this frame and so there is no centripetal acceleration. Yet the string still pulls on the weight. Why doesn't the weight move inward? To explain why, the centrifugal force is 'invented' to make the net force on the weight zero in the accelerated reference frame. But nothing is really pulling the weight inward. In that sense it is a 'fictious' force.

The 'proof' is that when you let go of the string the weight flies off tangentially, not radially.

All that said, I agree that 'fictious force' is a bad term that creates more confusion.

Originally posted by Arcon
The force the you exert on the floor is an inertial force. The force the floor exerts on you is not.

For example: Suppose in your living room there is a uniform gravitational field. Suppose also that there is an electric field in your living room which is in the direction pointing from the floor to the ceiling. Place a ball which is charged such that it floats. I.e. the gravitational force is equal and opposite to the electric force. Then the force on the charge due to the gravitational field is an inertial force. The force on the charge due to the electric field is not an inertial force.
I'm a little confused.

I understand the force of the electric field is not inertial. I also understand that the force I exert on the floor is inertial. What do you call the force that the floor exerts on me?

Let me put it this way: 2 spheres (A and B) of equal mass, say 10 kg, with no other forces affecting them except their own gravity, are separated by 1 m. Naturally, they are attracted to each other. The force that each sphere exerts on each other due to gravitation is inertial. At the instant they come in contact from the perspective of sphere A, sphere A exerts an inertial force against sphere B. Sphere B exerts an equal and opposite force on sphere A. What is that force that B exerts on A?

I promise I'm not trying to debate what you said. I really don't understand.

Doc Al
Mentor
Originally posted by Arcon
The force the you exert on the floor is an inertial force. The force the floor exerts on you is not.
I believe you can view (as you point out) the gravitational force as an inertial force, but the force you exert on the floor is not the gravitional force: it is a "real" electromagnetic force---your feet pushing against the floor.

Arcon
Originally posted by Jimmy
I'm a little confused.

I understand the force of the electric field is not inertial. I also understand that the force I exert on the floor is inertial. What do you call the force that the floor exerts on me?
There is no other name for such a force other than "non-inertial force."

There are contact forces between two bodies which are better explained through solid state physics and you've just described the contact forces. However I gave the electric force so as to simplify the description. Instead of thinking in terms of solid spheres think of charged bodies. Take two identical bodies, i.e. they have identical mass and identical charge, and place them at a distance from each other such that the electric force exactly balances the gravitational force. Then apply the explanation I gave above.

I promise I'm not trying to debate what you said. I really don't understand.
What's wrong with debate? That's part of what many physics discussions are.

Thanks Arcon, I understand what you are saying.

Originally posted by Arcon
There is no other name for such a force other than "non-inertial force."

What's wrong with debate? That's part of what many physics discussions are.
Nothing at all. I really wouldn't be much of a debater as my knowledge is fairly limited. Debate wasn't the proper word in this case. I should have said that I wasn't trying to start an argument based on some preconceived notion I have in order to forward my own ideas and try to catch you in a mistake. I wasn't trolling in other words. I always want to remain objective in order to learn new things and unlearn anything I may think is right but is in reality, wrong.

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