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What is Centrifugal force?

  1. Mar 22, 2008 #1
    I've heard recently that centrifugal "force" doesn't exist. If this is true what is the actual force that creates the centrifugal effect?

    Also, do centrifugal and centripetal effects/forces exist in outer space, i.e. on space shuttle.

    Thanks
     
  2. jcsd
  3. Mar 22, 2008 #2
    It's the tendency of a mass to continue moving in a straight line, while its environment, it's "frame of reference", is turning.

    Sometimes you're a passenger in a car when the car turns suddenly, and your body presses hard against the door. If you visualize the car as being at rest, if your x,y,z axes are drawn on the car, it would seem to you that some force pushed you against the door. Actually all your mass was doing was continue going in a straight line, and the turning of the car made the door hit you.

    Yes, it happens in space also.
     
  4. Mar 22, 2008 #3

    rcgldr

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    Centripital force is the force that accelerates an object inwards, centrifugal force is the equal and opposite reaction force due to the objects inertial resistance to the inwards acceleration from the centrpetal force.
     
  5. Mar 22, 2008 #4

    Doc Al

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    A "real" force has an actor, something that exerts the force. Centrifugal force is not a real force as there is no actor creating a force; it's just an artifact of describing things from a non-inertial (rotating) frame of reference. (Often it is extremely useful to describe things from within a non-inertial frame.)

    As mikelepore explained, the source of the effect is just inertia (Newton's 1st law).

    On the other hand, centripetal force is a real force in every sense of the word. When something moves in a circle, there is a real force (with an actor) pushing it towards the center.

    Examples: (a) Tie a string around a ball and whirl it in a circle. The string (the actor) exerts a real centripetal force on the ball. (b) Drive your car around a circular track. The road (the actor) exerts the centripetal force on the car.

    Regarding Newton's 3rd law and "action-reaction" pairs: It's true that any real force is part of an equal and opposite 3rd law pair. For example (a), the 3rd law pair of forces is: String pulls ball inward & ball pulls string outward. But that outward force on the string is a real force, not the fictitious centrifugal force. (Centrifugal force would be an outward "force" on the ball.)
     
  6. Mar 22, 2008 #5
    I'm curious as to what you consider to a criteria for existance? It may seem odd but this question goes to the heart of the matter when it comes to things such as the inertial force, of which centrifugal force is one of them.

    Definition - Inertial force: When the motion of the reference system generates a force (defined as the time rate of change of momentum, i.e. F = dp/dt), as measured in that system, we call that force an inertial force.

    Albert Einstein had the following to say on this topic. That the relation of gravity to inertia was the motivation for general relativity is expressed in an article Einstein wrote which appeared in the February 17, 1921 issue of Nature
    There is a similar comment in Introducing Einstein's Relativity, by Ray D'Inverno, Oxord/Clarendon Press, (1992) page 122
    Another opinion on this subject comes from The Variational Principles of Mechanics, by Cornelius Lanczos - The subject of inertial force is also addressed in - 4th Ed., Cornelius Lanczos, Dover Pub., page 98.
    To top this off I'll reference one more view on the concept of inertial force. A.P. French - Inertial force is defined as the force on a body that results solely from observing the motion of the body from a non-inertial frame of reference. This in addressed in Newtonian Mechanics, A.P. French, The M.I.T. Introductory Physics Series, W.W. Norton Pub. , (1971) , page 499. After describing the inertial force as seen from an accelerating frame of reference French writes
    Pete
     
    Last edited: Mar 22, 2008
  7. Mar 22, 2008 #6
    The presence of a centrifugal force does not require the existance of a reaction force. Centrifugal forces are the inertial force on a particle which is only measured by observers in rotating frames of reference and is directed in the outward direction from the center of rotation.

    Pete
     
  8. Mar 22, 2008 #7

    dst

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    So in other words, a "fictitious force" in a particular reference frame is one which has no 3rd law pair for that reference frame?
     
  9. Mar 22, 2008 #8
    No.

    Pete
     
  10. Mar 22, 2008 #9

    Doc Al

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    I'd say yes. (In Newtonian physics, at least.)
     
  11. Mar 22, 2008 #10

    dst

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    So how would you define it? I would have thought that was the case since it works for centrifugal force + coriolis force.

    Edit: Oh ok...
     
  12. Mar 22, 2008 #11

    Dale

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    (emphasis added by DaleSpam)

    NO! The centrifugal force is not a reaction force. It is a ficticious force and as such it does not obey Newton's 3rd law.
     
  13. Mar 22, 2008 #12

    Doc Al

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    Exactly.
     
  14. Mar 22, 2008 #13
    Thanks everyone for your replies and references.

    Einstein's idea that gravity and inertia could be identical is similar to what I was assuming. However, I was thinking more along the line that either Centrifugal and Centripetal is actually an affect of gravity, and possibly a form of counter gravity. And inertia is a kinetic (stored) energy, which isn't directly related to the inward pulling and outward pulling on an object due to centripetal or centrifugal force.

    For example, isn't the centripetal force that keeps an object in orbit, in space (planets, satellite) due to gravity? Why would centripetal force be any different on earth?

    With the string and ball analogy. Why does a mass rotating around an axis, rise up against gravity the faster it spins. Could a gravitational force be coming from within the string?
     
  15. Mar 22, 2008 #14

    Dale

    Staff: Mentor

    You are probably aware that there are many descriptions that you can use to accurately describe the same thing, each description capturing a different aspect of the thing.

    When you say "gravitational force" you are talking about the mechanism of the force or how the force is exerted. You can also say "electrostatic force" or "tensile force" or "friction force" all refering to the mechanism.

    When you say "centripetal force" you are talking about the function of the force, or what it is doing. You can also say "reaction force" or "restoring force" all refering to the function and not the mechanism.
     
  16. Mar 22, 2008 #15
    You can always use an infinite amount of words, and equations, to describe or complicate a simple concept.
     
  17. Mar 23, 2008 #16
    I don't understand, it clearly seems to be just like any other reaction force. Any acceleration on an object, regardless of direction, creates a reactionary force. Why change the rules for the one case where the force just happens to be perpendicular to the objects direction of travel? Why should the direction of the force matter at all?
     
  18. Mar 23, 2008 #17

    Doc Al

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    No "rules" are being changed. To accelerate something in a circular path requires a centripetal force; that force, like any other, will be paired with a "reaction" force per Newton's 3rd law. But that "reaction force" is not the centrifugal force. The reaction to a centripetal force is an equal and opposite force on whatever is creating the centripetal force. See my examples in post #3.

    Centrifugal force is not a "real" force, but an artifact of describing motion from a noninertial, rotating frame. Newton's 3rd law does not apply.
     
  19. Mar 23, 2008 #18

    Dale

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    NO! How can you possibly say it seems to be just like any other reaction force? An action-reaction pair act on two different bodies. The centripetal and centrifugal force act on the same body. They cannot possibly form an action-reaction pair.

    Let's consider a standard example of a car making a left turn around an unbanked turn of constant radius, and for simplicity let's consider only in the horizontal plane (i.e. ignore gravity and normal forces since they cancel in this problem). We will consider the horizontal forces on the driver and on a cup on the frictionless dashboard and we will analyze their motion in both the frame of the road (inertial) and the frame of the car (rotating).

    In the road frame the cup is not accelerating, it moves in a straight line with no horizontal forces acting on it.

    In the road frame the driver is accelerating to the left. He experiences a static friction force from the seat which is the centripetal force accelerating him.

    In the car frame the cup is accelerating to the right. There are no real forces acting on the cup, so how do we explain its acceleration? We posit a fictional force we call the centrifugal force pointing to the right which explains the acceleration. Since the dashboard is frictionless there is no balancing centripetal force and the cup accelerates in the car's frame.

    In the car frame the driver is stationary. There is a real frictional (centripetal) force from the seat, so how do we explain the lack of acceleration? We posit the same centrifugal force pointing to the right as above. This centrifugal force balances the frictional centripetal force and the driver remains stationary.

    Now, let's go back and look for action-reaction pairs.

    In the road frame the only force is the friction force to the left from the seat acting on the driver. The reaction to that force is a friction force acting to the right on the seat. Note that, as always with action-reaction pairs, they are of the same kind (friction) and act on differen bodies (driver and seat).

    In the car frame the friction forces on the driver and on the seat still exist and still form an action-reaction pair of the same kind acting on different bodies. However, the centrifugal forces all act to the right so they are not opposite to each other. They violate Newton's 3rd law.
     
    Last edited: Mar 23, 2008
  20. Mar 23, 2008 #19
    I disagree. The centrifugal force does obey Newton's third law.

    Pete
     
  21. Mar 23, 2008 #20
    Einstein was motivated by this idea, i.e. that what Newtonians called "fictitious" forces were really, in Einstein's opinion anyway, "real" forces because they behaved like the "real" force of gravity. However others have interpreted this to mean that since inertial forces are identical in nature to "fictitous" forces that it meant that gravity was also to be considered a "fictitous force." This was never Einstein's view though, although it appears to be the view of some physicists today.
    Inertia and kinetic energy are very different things. First off it is incorrect to think of kinetic energy as being stored anywhere and second it is incorrect to think that because kinetic energy and inertial mass are related that they are the same thing. That would be like saying that velocity and kinetic energy are the same thing and that's obviously wrong.

    Gotta go. More later.

    Pete
     
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