# Does centrigual force exist?

1. Feb 2, 2010

### xxpbdudexx

I have always thought centrifugal force existed. My physics teacher last year told me it doesn't. I did some research on centrifugal and centripetal force and am still convinced centrifugal force exists as the equal and opposite force to centripetal force. What do you guys think?

2. Feb 2, 2010

### DocZaius

My understand of centrifugal force is that it is a pseudoforce. If you stand on a rotating disc and the frame of reference is rotating with you, there will be a force in that frame that points away from the center of the disc. That is the centrifugal force. If you do not move from your spot on the disc in the rotating frame, the centrifugal force and centripetal force perfectly balance. In this rotating frame, you could therefore say you are not accelerating.

Someone observing the situation from an inertial frame outside the disc will note that your body merely tries to go in a direction tangential to the rotation and if it does not, friction (or something keeping you to the disc) is the centripetal force responsible. It points towards the center of the disc. This observer sees you constantly accelerating towards the center of the disc. This observer sees no centrifugal force.

The reason I believe someone could say that the centrifugal force does not exist, is that it is not necessary to describe the dynamics observed in an inertial frame of reference. It can be helpful in a rotating frame of reference though.

Last edited: Feb 2, 2010
3. Feb 2, 2010

### boit

I read an article that affirms centripetal force as a true force but treats centrfugal force as an observed phenomenon. I think it exist only due to the presence of the former as per newtons first law "an object continues moving straight untill compelled by a force (read centripetal) to act otherwise. I used to believe it was a force too.

4. Feb 2, 2010

### Vasara

Centrifugal force doesn't exist per se, rather it's a fictitious force that is needed in order to explain motion within a non-inertial reference frame; in an inertial reference frame it's not necessary. It's not considered a real force because it doesn't result from a physical interaction, but the reference frame itself accelerating.

5. Feb 2, 2010

### Staff: Mentor

This can be very confusing for a student. There are two completely different and unrelated meanings to the term "centrifugal force".

1) The less common meaning of "centrifugal force" is the Newton's 3rd law reaction force to the centripetal force. So for example, if you are spinning a bucket on the end of a string then the force that the string exerts on the bucket points inward and is called the centripetal force, by Newton's 3rd law the bucket exerts an equal force on the string which points outward and is called the centrifugal force.

2) The more common meaning of "centrifugal force" is a fictitious force that is present in the non-inertial rotating reference frame and, in that non-inertial frame, acts to pull objects outward. This force only exists in the rotating reference frame and not in other reference frames. It is not a real force because it does not obey Newton's 3rd law. However, in the rotating reference frame it can do work, cause, stress, accelerate objects, balance other forces, and basically do anything else that you would expect a force to do.

Hope that wasn't too confusing.

6. Feb 2, 2010

### A.T.

For more explanations here the corresponding wikis about both forces:
1) http://en.wikipedia.org/wiki/Reactive_centrifugal_force
2) http://en.wikipedia.org/wiki/Centrifugal_force_(rotating_reference_frame)

7. Feb 3, 2010

### Zula110100100

An explanation I came to without all that reference frame stuff is that the seeming centrifugal force is a result of the ABSENCE of centripetal force.

Any object attached(by a force) to the axis of rotation is getting centripetal acceleration from that force. An object that does not receive centripetal acceleration and has a velocity relative to the axis of rotation moves in a linear path while the centripetal accelerations of the objects attached to the center is angular, curving in front of the linear path making it seem like you were pushed out to it.

Imagine a hollow sphere rotating in space, if you were inside, floating a few inches away from the wall, you would feel no force. But if you were touching it, and friction were accelerating you perpendicular to the axis of rotation, it would continually be in your path and there would be a force of you pressing against it and it pressing against you. That Fapp(I guess) force is the so-called centrifugal force.

8. Feb 3, 2010

### rcgldr

My guess is that your teacher is confusing "fictitious" forces with non-existant forces. The reaction force to centripetal acceleration is real. As pointed out, in the case of a mass twirling around on a string, the string exerts a centripetal force onto the mass, and the mass exerts and equal and opposing reactive (centrifugal) force onto the string.

The main exception may occur when the force doesn't involve contact, such as gravity or electical forces. Say you have two objects connected by string twirling in space away from any external forces, and their mass is so low that gravity can be considered non-existant. At each end of the string, you have the same Newton 3rd law pair of forces, string exerting centripetal force on the object, the object exerting reactive centrifugal force on the string.

Now remove the string and instead have the two objects orbiting on oppposite sides of a circular path due to gravity. In this case, the only force present is the gravitational atrractive force, which is dependent on the mass and distance between objects but not affected by the speed of those objects (ignoring speed of gravity and relativistic effects). The path of the objects is affected by their speed, and is only circular when v^2/r = gravitational acceleration, but the force is independent of their speed. In this case, the Newton 3rd law pair of forces are the attractive forces of each object towards the other.

9. Feb 4, 2010

### Tao-Fu

10. Feb 4, 2010

### campo133

Its all about the inertial reference frames. Remember, Newton's first law, which says you can only apply his laws IFF you are in an inertial reference frame. Centrifugal force comes about if you are in a non-inertial reference frame (like a rotating disk), and is required to explain the motion of objects. If you are in an inertial frame, you do not need the centrifugal force to explain the motion of the object. Hence the name, fictitious or pseudo.

11. Feb 4, 2010

### rcgldr

Although the reactive centrifugal force is not needed to explain the motion of the object, it is needed to explain why there's an equal and opposing force on whatever is generating the centripetal force (Newton third law pair of forces, except for a non-contact attractive force like gravity or opposite charged objects).

In the real world, most interactions are reactive, where linear and angular momentum are conserved, unless you consider one of the objects, such as the earth, immovable. For example imagine a rocket in outer space, free from the effects of any external forces, following in a circular path due to rotating so it's engine always points "outwards", and the proper amount of thrust to maintain the circular path (adjusted to compensate for the decrease in mass over time). There's a compressive reaction at the engine that generates a pair of forces that accelerate fuel outwards and the rocket inwards. In this case, both forces are "reactive", and neither the centripetal or centrifugal force is more or less "fictitious" than the other in this case.

12. Feb 5, 2010

### Stonebridge

The confusion is one of nomenclature as well as physics.
The usual meaning of "centrifugal" force, when incorrectly used, is not the same as the Newton 3 reaction to centripetal force.
The commonly misconceived term, centrifugal force, usually refers to a non-existent force on the object moving in a circle, tending to make it want to move in a straight line. The Newton 3 reaction to centripetal force is perfectly legitimate, and you can call it what you want, but it is not a force on the object under consideration, it is a force on something else.
The myth that needs to be eliminated regarding this "force", is the one that says that when you are in a car that is going round a corner, there is a force on you tending to move you in a straight line. You can even feel it! This is false. This is the mythical centrifugal force.

13. Feb 5, 2010

### Fredrik

Staff Emeritus

14. Feb 5, 2010

### Newtons Clock

It can't be balanced because your velocity is changing, in order for it to be balanced you would have to be moving in a straight line path. Pretend you're in a centrifuge, there is absolutley something going on that appears to be acting radially outwards, but it's a phenominon that we don't truely understand, inertia. Your inertia does not want you to go on that curved path, it wants you to stay on that straight line path. If you drew a free body diagram from the reference frame from the cabin (A non-inertial reference frame, this is where issues arise), you would have a normal provided by the back of the cabin, which is reacting to your inertia. So why isn't this a "real" force? Because it can only be observed from a non-inertial reference frame. That's my take atleast.