Centrifugal Force

1. Apr 6, 2012

Lengalicious

I don't understand how its a fictitious force and why you need to be in the rotating frame of reference to observe it? In the example of the moon and the earth and its tides. You have the centripetal force causing the tides to increase at the point where the earth is facing the moon and the centrifugal force to form tides where the earth is opposing the moon. Now this can be observed when your at some arbitrary point in space in an inertial reference frame cant it? If im floating in space above the earth at a point where i can observe both points of the earth in mention simultaneously then i would observe both tides would i not? If the centrifugal force was fake, then i would not observe the tide in the opposing side of the earth and if im standing on the earth in the non inertial reference frame i still observe the tide? So i dont understand, i must have something terribly misconstrueded here.

2. Apr 7, 2012

aroc91

The tides are the result of the moon's gravity, not centrifugal force. Your propositions are severely flawed and I suggest you read up on tides.

http://en.wikipedia.org/wiki/Tide
http://en.wikipedia.org/wiki/Tidal_force

That said, centrifugal force is merely the result of centripetal force combined with tangential movement. If something is in a centrifuge, it's not literally being pushed straight out from the center, it's being restricted when trying to travel on a tangent by centripetal force.

3. Apr 7, 2012

Emilyjoint

In our class we were taught that centripetal and centrifugal forces were an example of newtons 3rd law.
Centripetal force is the force towards the circle centre which acts on the object going in a circle
Centrifugal force is the force acting on the object at the centre of the circle and this is an outdated force.
In circular motion questions it always seems to be about the object going in a circle so centrifugal force never really comes up.
If you imagine a hammer thrower whirling round ready to throw the hammer this explanation fit

4. Apr 7, 2012

haruspex

It takes a force to cause an object to do something other than travel at constant speed in a straight line. In the case of an object being spun in a circle, by rope or gravitational attraction, the force that makes it do so is called centripetal force.
Centrifugal force is the opposing force - e.g. it's the force experienced by the rope. It's a real force, but it's wrong to think of it as a force acting on the spun object.

The moon and earth are in orbit around a common point inside the earth, somewhere between the earth's mass centre and the moon's. The water on the moonward side, being nearer fo the moon than is the centre of the earth, is attracted more strongly than the earth is, so bulges up towards the moon. The oceans on the opposite side are attracted to the moon less strongly than the earth is, so bulge the other way.

5. Apr 7, 2012

Emilyjoint

Centripetal force is the resultant force acting on the object in circular motion.
In the simplest cases such as an electron in orbit around a nucleus, a stone on the end of a string in a horizontal circle and the moon/ earth in orbit then the resultant force is the only force acting in each case.
For a stone in a vertical circle the resultant (centripetal) force is a combination of tension and gravity.

6. Apr 7, 2012

Staff: Mentor

Yikes!

That's not common usage. In modern usage, 'centrifugal force' is an inertial force that only appears when analyzing things from a non-inertial frame of reference. It's not the reaction force to centripetal force.

(For that reaction force, some use the term reactive centrifugal force.)

7. Apr 7, 2012

DrStupid

The 3rd law says: ''To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions''.

What are the two bodies in the case of centripetal and centrifugal forces?

8. Apr 7, 2012

genericusrnme

Centrifugal force is what happens because there's an r in the moment of inertia for a body rotating about the origin!

In all seriousness, the 'fictitious' in fictitious force shouldn't be taken so literally.

9. Apr 7, 2012

haruspex

For a rock tied to the end of a string and whirled around, the rock feels centripetal force from the string, while the string feels centrifugal force from the rock.
The whole kerfuffle over centrifugal force is from the popular misunderstnding that centrifugal force is a force experienced by the object going in circles.

10. Apr 7, 2012

Emilyjoint

What are the two bodies in the case of centripetal and centrifugal forces?.....
In the case of an electron in orbit about a nucleus...electron and nucleus
In the case of the moon/satellite in orbit around the earth....moon/satellite and earth
In the case of a stone on the end of a string in a horizontal circle... stone and person at centre.
I think all of these cases illustrate newtons 3rd law.
Where it does not work is in eg: stone in vertical circle, plane turning by banking, car turning on banked track
Because centripetal force is a resultant force.... not a force in its own right

11. Apr 7, 2012

Staff: Mentor

Well, the force that the string feels is outward and thus in a dictionary sense is 'centrifugal'. But that's not what physicists usually mean by the term centrifugal force. (Some use the term reactive centrifugal force to mean that outward force on the string.)
Centrifugal force is a (fictitious) force acting on the object going in circles. The popular misconception is that it is a 'real' force that somehow explains circular motion. It only appears as an artifact of describing things from a rotating reference frame.

12. Apr 7, 2012

Emilyjoint

none of my physics text books make any reference to centrifugal force.
There must be a good reason for that.
They all refer to Centripetal force and cover it very clearly

13. Apr 7, 2012

Staff: Mentor

Are you speaking of the modern, standard meaning of centrifugal force? Or are you using the old-fashioned meaning, better described as reactive centrifugal force (if you insist on giving it a label)?

Using the standard meaning of centrifugal force, then both the centripetal and centrifugal forces act on the same body and are thus not 3rd law pairs. (And the centrifugal force is a fictitious force--as opposed to an interaction force between bodies.)

Wherever you have a real interaction force, you'll have a 3rd law pair.
Again, real forces have 3rd law pairs: The string pulls the stone and the stone pulls the string; the air pushes the plane and the plane pushes the air; the road pushes the car and the car pushes the road.
Good! Think in terms of real forces and you won't go wrong. Instead of centripetal force, think of centripetal acceleration and Newton's 2nd law.

14. Apr 7, 2012

Staff: Mentor

Generally, the concept of centrifugal force is only introduced when learning about non-inertial reference frames. And that's usually done in a second course in mechanics, not the first year. (Mileage may vary, of course.) Any classical mechanics text will go into all sorts of inertial forces, such as centrifugal and Coriolis. Introducing it too early only creates confusion (IMO).

15. Apr 7, 2012

Emilyjoint

Don't say that things get 'thrown out' in circular motion.
Just identify real forces (centripetal and centrifugal are not real forces) find the resultant then use F =ma

16. Apr 7, 2012

DrStupid

I am speaking about centrifugal force. I never heard of "reactive centrifugal force". What action would a centrifugal force react to?

Last edited: Apr 7, 2012
17. Apr 7, 2012

DrStupid

Do you really think that the centripetal force is acting on one part of these pairs and the centrifugal force on the other?

18. Apr 7, 2012

Staff: Mentor

The reactive centrifugal force is a reaction to a centripetal force. It's not a term I would ever use. (But some do: Reactive centrifugal force)

19. Apr 7, 2012

Staff: Mentor

I wouldn't put centripetal and centrifugal forces in the same boat as being not real. While I agree that 'centripetal force' is just a term signifying the net force that produces centripetal acceleration (and not a new kind of force), the forces that make up the centripetal force in any given situation are usually quite real (string tension, gravity, etc.) and have 3rd law pairs (like any interaction force). But centrifugal force (using the standard terminology) is never 'real'--it's not an interaction with any other body.