# Difference between pseudo force and centrifugal force

1. Jul 9, 2010

### utkarsh009

what is pseudo force and centrifugal force? can anyone differentiate between them? is one a special case of the other?

2. Jul 9, 2010

### mikelepore

Centrifugal force is an example of a pseudo-force, that is, an apparent force to someone whose frame of reference isn't at rest or moving with a constant velocity. In the case of centrifugal force, the frame of reference is rotating.

3. Jul 22, 2010

### utkarsh009

if so then what are the different types of pseudo forces?

4. Jul 22, 2010

### Cleonis

Well, I think it's better to speak of different names, that are descriptive of the particular circumstances. In all situations the underlying phenomenon is Inertia.

In the case of circular motion, sustained by a centripetal force: the tendency to move away from the axis of rotation is referred to as 'centrifugal force'

In the case of the Coriolis effect the tendency to deflect is referred to as 'Coriolis force'.

Note that you can choose to invent names for yet other circumstances. Example: as seen from the Earth the planet Mars goes into retrograde motion from time to time. You can choose to invent a 'retrograde pushing force', to account for the retrograde motion. What we see from the Earth is the sum of two motions: the actual Mars motion, and an apparent Mars motion due to the fact that the Earth itself is orbiting the Sun. It's called 'apparent motion' when it's not a motioncomponent of the observed object, but in fact a motion of the observer.

Anyway, 'centrifugal force' and 'Coriolis force' are the only names that are used often.

5. Jul 22, 2010

### utkarsh009

thank you very much cleonis and mikelepore!!!!!!!!!!!!

6. Aug 12, 2011

### thebiggerbang

I know that the OP will not be around, but I'd like to share this way of thinking about centrifugal force. It is a pseudo force, and in brief centrifugal force is nothing but the absence of centripetal force!

7. Aug 12, 2011

### A.T.

This is wrong. Both forces are acting on an object at rest in a rotating frame. The centrifugal force is always there in a rotating frame, regardless if there is a centripetal force to counter it.

8. Aug 12, 2011

### thebiggerbang

I quoted this http://www.regentsprep.org/Regents/physics/phys06/bcentrif/centrif.htm" and I suppose you mean it is always acting beacuse that is the pseudo force rising up due to the non-inertial frame! Isn't it?

Last edited by a moderator: Apr 26, 2017
9. Aug 12, 2011

### rcgldr

That page claims centrifugal force isn't real, but that ignores the real outwards force the tires of a turning car exert onto the surface of the earth. This is more of an argument over termonolgy than actual forces. In the case of the car turning, you have equal and opposing forces at the contact patch of the tires, the tires exert an outwards force onto the surface of the earth, coexistant with the earth surface exerting an inwards force onto the tires. The inwards force results in inwards acceleration of the car, while the outwards force results in a tiny amount of rotational and lateral acceleration of the earth (tiny because the earth is so much more massive).

In the case of an observer in a rotating frame, an object at rest or moving linearly with respect to the center of the frame, will appear to have fictitious centripetal (inwards), centrifugal (outwards), and/or coriolis (tangental) forces acting on it. If the object is interacting with the rotating frame, actual inwards or tangental forces will produce what appear to be ficticious forces from the object. As an example of fictitious centripetal force, imagine someone at the north pole of the earth looiking straight up at the stars; they'll all appear to be orbiting.

Getting back to the OP, you can have linear pseudo forces, such as the apparent sensation of a gravity like force when inside an linearly accelerating vehicle or space craft. There could be a combination of linear, rotational, and angular acceleration (rate of rotation changing), to produce complex fictitious forces from an observer in that accelerating frame of reference.

In an inertial (non-accelerating) frame of reference the term "reactive centrifugal force" may be used to describe the outwards reactive force that coexists with centripetal force and acceleration. As long as the qualifier "reactive" is used, it should prevent any confusion with the physics usage of the term centrifugal force to mean the fictitious force from a rotating frame of reference.

http://en.wikipedia.org/wiki/Reactive_centrifugal_force

http://en.wikipedia.org/wiki/Centrifugal_force

http://en.wikipedia.org/wiki/Coriolis_effect

Last edited: Aug 12, 2011
10. Aug 12, 2011

### A.T.

Yes, in a rotating frame there is always a centrifugal force. But only sometimes a centripetal force to counter it. If there is no centripetal force then there is centrifugal acceleration in the rotating frame (which is just inertial motion in the inertial frame). If there is a centripetal force that counters the centrifugal force, then there is no acceleration in the rotating frame (but there is centripetal acceleration in the inertial frame).

The sentence from the website:
is wrong. Centrifugal Force doesn't imply Lack-of-Centripetal Force. For objects at rest in the rotating frame both forces must be there and balance each other. You could say: Centrifugal Acceleration in the rotating frame implies Lack-of-Centripetal Force.

Last edited by a moderator: Apr 26, 2017
11. Aug 12, 2011

### rcgldr

Only if the object is somehow affected by the rotating frame. The path observed from a rotating frame of an object not affected by the rotating frame doesn't always appear to be affected by a centrifugal force. For example, if an observer (tlme lapse camera) is placed at the north or south pole of the earth looking straight up, all the stars appear to be orbiting, as if some fictitious centripetal force was causing them to follow a circular path.

12. Aug 13, 2011

### A.T.

Nope, in a rotating frame there is always a centrifugal force. And I have no idea what you mean by "affected by the rotating frame".

The coordinate acceleration in the rotating frame is a result of the net force, not just centrifugal force.

And that fictitious centripetal force is called Coriolis force. In this case it is opposed to the centrifugal force, and has twice the magnitude. So half of it cancels the centrifugal force and the other half acts as a centripetal force, keeping the stars on a circular path.

13. Aug 13, 2011

### rcgldr

One where there is a force between an object and a rotating frame. The previous example I used was a ball rolling or sliding inside a cylinder that is rotating on an axis perpendicular to the primary axis of the cylinder. The ball will accelerate outwards due to the tangental force from the walls of the rotating cylinder. I was trying to show an example where there was no real centripetal force acting on an accelerating object.

My mistake. I was thinking of a real "centrifugal" force (reaction to centripetal acceleration) observed from a rotating frame as opposed to the centrifugal force used to correct observation from a rotating frame. What about the case of an observed object on the axis of rotation? What if the object were moving along the axis of rotation?

Last edited: Aug 13, 2011
14. Aug 13, 2011

### A.T.

A reference frame is not a physical object, that can exert forces on masses. There is no such thing as a "force between an object and a reference frame".

The inertial forces depend on the position or velocity, and can be zero in some cases. But there are no such rules like "lack of centriperal force" or "force between object and reference frame" that say when to apply them. In general you always have to assume those inertial forces in rotating frames.

Last edited: Aug 13, 2011
15. Aug 13, 2011

### rcgldr

In this case I meant a physical frame that was rotating. Again I was mistakenly thinking about the inertial reaction to a centripetal force from an inertial frame of reference, instead of the fictitious forces from a rotating frame of reference.

Getting back to the ealier posts and my post #9, in the case of a car turning on a road, I'm not sure how you describe the force exerted by the tires onto the earth surface, and the force exerted by the earth surface onto the tires, from the car's perspective as part of a rotating frame of reference.

I think part of the OP was answered by explaining that from an linearly accelerating frame of reference, you get a fictitious linear force that appears similar to gravity, which is an example of pseudo force that is not a fictitious centrifugal force.

Last edited: Aug 13, 2011
16. Aug 13, 2011

### A.T.

I guess you mean a physical object that is at rest in the rotating frame.

The are both "real" forces and present in every frame, also the rotating rest frame of the car.