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Centrifugal foce

  1. May 25, 2009 #1
    I read that the centrifugal force acts radially outwards on a stone being revolved by a person with a string. But I have just fallen into the confusion as what keeps the stone at a constant height when one is revolving it above one's head. Surely there must be a vertical force too, equal to the weight of the stone which prevents it from falling down.

    Now is it right to say that centrifugal force is radially outwards or I have some misconceptions

    Thank you
     
  2. jcsd
  3. May 25, 2009 #2
    It is right to say that centrifugal force is radially outward, but centrifugal force acts on the string, not on the stone. Centripetal force acts radially inward on the stone. This is what provides the vertical component of force against gravity and the horizontal component of force keeps the stone from traveling on it's "preferred" straight-line path. What is a little confusing here is that there is no outward force on the stone.
     
  4. May 25, 2009 #3
    Why the centrifugal force don't act on the stone. If you are right then the stone should not run away if the link between the stone and the rope breaks near the stone. Which certainly is not the case. Are should I interprate it as that, during motion centrifugal force acts only on string but it is instantaly transmitted to stone to go away when the link breaks?

    If the centripetal force acts radially inward, how it counter acts the action of gravity. Free body diagrams shold take the gravity into consideration as well, which I have never seen in any text book.

    It is still a puzzle to me as what is there to counteract the gravity...
     
  5. May 26, 2009 #4

    rcgldr

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    The stone rises because the misaligned linear forces creates a torque. The upper end of the string experiences an inwards force (from the persons hand), and the lower end of the string an outwards force (reaction force from stone).
     
  6. May 26, 2009 #5
    Hi there,

    elect_eng is right in his definition. This is also why the centrifugal force is called imaginary. It is not a real force acting on the stone. If you let the rope go, or if the rope is cut, you will not see the stone go away from the circular motion, but parallel to it.

    Cheers
     
  7. May 26, 2009 #6

    rcgldr

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    I left out the other part. The string is at an angle, so the tension in the string includes a downwards component on the upper end of the string and an upwards component on the lower end of the string.
     
  8. May 26, 2009 #7

    Doc Al

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    Realize that the string is at an angle with the horizontal. The vertical component of string tension balances the weight of the stone; the horizontal component of string tension provides the inward centripetal (not centrifugal) force.

    In standard usage, "centrifugal force" is a fictitious force that arises from analyzing the motion in a rotating frame of reference. Forget about it. There are only two "real" forces acting on the stone: The string tension and gravity.
     
  9. May 26, 2009 #8
    I agree with this. However, there is a non-standard usage "reaction centrifugal force" which is a real force in the non-rotating frame. This is simply defined as the reaction force to the centripetal force. The string exerts centripetal force on the stone, but the stone exerts a reaction force on the string. This is a real force necessary to have the string under tension. Another example of confusing terminology.
     
  10. May 26, 2009 #9

    Doc Al

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    Using the term "centrifugal" to describe the force that the stone exerts on the string is non-standard and confusing. So don't use it.
     
  11. May 26, 2009 #10
    I agree with that, but the non-standard usage still persists and seems relevant to the OPs confusion.

    Here is a link that describes the distinction between the real and fictitious forces.

    http://en.wikipedia.org/wiki/Centrifugal_force
     
  12. May 26, 2009 #11

    Doc Al

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    I am well aware of the distinction. (And this very issue has been discussed here ad nauseam.)

    If you look at the OP's first sentence:
    It's pretty clear that he's talking about forces on the stone and is making the usual error in viewing centrifugal force as a real force. Bringing up the non-standard usage of centrifugal force as a "reaction" force on the string is irrelevant and only adds to the confusion.
     
  13. May 26, 2009 #12

    tiny-tim

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    You mean with your hand (holding the string) below the stone?

    Not possible unless you're cheating. :biggrin:
     
  14. May 26, 2009 #13

    D H

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    That Wikipedia article is a prime example of why Wikipedia is not to be trusted. Wikipedia has more than its fair share of articles with a tinge of crackpot, fringe, or out-dated concepts. Unless one is already versant in the field, there is no way to know whether an article is legitimate or is a stinking pile of stuff.

    Let's look at what is happening to the stone from four different perspectives. To simplify things, I'll assume one end of the string is attached to a thin rotating vertical pole. The other end of the string is attached to the rock. The length of the string is l and the the angle between the string and the horizontal is θ. The pole is rotating with angular velocity ω. The rock revolves about the pole with this same angular velocity. The four different perspectives:
    1. Kinematic point of view, inertial observer
      The rock is undergoing uniform circular motion in a horizontal plane that intersects the pole at a point l sin θ below the attachment point on the pole. Call the intersection between the pole and the horizontal plane in which the rock is "central point". The radius of the circle traced by the rock is l cos θ. The rock's angular velocity about this central point is ω. Uniform circular motion means the rock is accelerating toward the central point. The magnitude of this acceleration is ω2r = ω2l cos θ. Multiplying by the rock's mass m yields the centripetal force. What does this centripetal force mean? Not much. Centripetal force is tautologically defined as the centripetal acceleration times mass. The exact same centripetal force results in many completely different circumstances.

      Note well: There is no centrifugal force in this point of view.

    2. Dynamics point of view, inertial observer
      The string is under some tensile force T. The forces on the rock are gravity Fg=mg directed downward and this tensile force, directed toward the attachment point. The vertical and horizontal component of this tensile force are T sin θ (directed upward) and T cos θ (directed inward). The vertical and horizontal components of the net force on the rock are T sin θ - mg and T cos θ. Reconciling this with the kinematic point of view, T sin θ - mg = 0 and T cos θ = mω2l cos θ. Thus T = mω2l and sin θ = g/T = g/ω2l. This point of view adds a lot to the kinematic point of view. Now we know the tension in the string and have derived the angle θ. In the kinematic point of view, the angle θ is a given. In the dynamics point of view, the angle is a consequence of the motion.

      Note well: There is no centrifugal force in this point of view.

    3. Kinematic point of view, rotating observer
      This point of view uses a rotating frame in which the rock is stationary. The origin of the frame is some point on the pole (which point doesn't matter). Since the frame is rotating and since rock is located some distance r=l cosθ from the rotation axis, there is a centrifugal force acting on the rock. This centrifugal force is directed away from the pole and has magnitude ω2r = ω2 l cosθ. The rock is stationary, so (a) there is no coriolis force and (b) the net apparent force is zero. In a rotating frame, the net apparent force is the sum of the net real force plus the fictitious forces. The net real force acting on the rock is thus ω2r directed inward.

      Note well: There is no centripetal force in this point of view. The rock isn't moving in this point of view.

    4. Dynamics point of view, rotating observer
      The net real force on the rock is the sum of the tensile force from the string and the gravitational force. Reconciling this with the kinematics result yields the exact same results as does the inertial dynamical point of view.


      Note well: There is no centripetal force in this point of view. The rock isn't moving in this point of view.

    Finally, note that there is no "reactive centrifugal force" in any of the points of view.
     
  15. May 26, 2009 #14

    That's a matter of opinion. I brought it up in the hope that it might clear up the confusion. If I was wrong about that, so be it. But, only the OP can judge if I confused him more, or helped him understand better.
     
  16. May 26, 2009 #15
    I know you are aware of the distinction. Did you really doubt that? The reference was obviously for the OP.
     
    Last edited: May 26, 2009
  17. May 26, 2009 #16
    By the way, I have a sincere question about this. Since there seems to be a real distaste to use the term centrifugal force as a real force, what is the preferred terminology to talk about this force? As an electrical engineer working in the field of motors and generators, the term centrifugal force is often used to describe the force that outer parts of a rotating system place on the inner parts. Should I say "reaction force to centripetal force" and avoid the term centrifugal force, or is there an existing term that describes the force without confusion?
     
  18. May 26, 2009 #17
    So, I conclude that one of Newton's laws is not correct. Apparently, the law that every force has an equal and opposite force is not valid? What keeps the string under tension? The string pulls on the stone. Should not the stone pull on the string?

    If you don't like he terminology, fine, I don't like to argue about terminology. What is the correct terminology to describe the real force that the stone puts on the string to keep it under tension? I've gotten into this debate a number of times and am tired of it. Please tell me the correct word to use to avoid arguements.
     
  19. May 26, 2009 #18
    Has the centrigugal source idea been discarded? I have seen some other threads on CF force like: https://www.physicsforums.com/showthread.php?t=231139 and found the discussions interesting. I am so thankful to all of you guys for being helpful, but still I am so much confused.

    elect_engg,
    Thanks for staying here. Your last post gives me the idea that you are doubtful too. I didn't think this theory so hard to understand at first.

    tiny-tim,
    I don't understand your point. Why should I be cheating? It is quite possible to keep the stone rotating within a plane fairly parallel to the Earth, or not?

    DH,
    Quote: "To simplify things, I'll assume one end of the string is attached to a thin rotating vertical pole."
    I have got the instinct that the stone won't rotate as it does when a person rotates it by hand. Are you satisfied with the experimental setup? In my opinion, the result would be that the rope will be wound around the pole in the shape of a coil.

    And why are you using the rotating pole? Definitely a person doesn't keep his hand/arm rotating. Is the rotation idea just to start motion in one particular direction? How would the pole provide centripital force.



    Dear Fellows,
    Which physics book/website do you people recommend to study the up-to-date Physics ideas and concepts. I realize that I have many more misconceptions.
     
    Last edited: May 27, 2009
  20. May 26, 2009 #19
    I'm not doubtful of my understanding of the physics or how to do the calculations. However, I am doubtful of the correct terminology that doesn't offend people. In high school, I was taught that centrifugal force is the reaction force to centripetal force. In college, the ideas of a rotating reference frame (noninertial frame) having ficticious forces due to the accelleration of the frame itself (Coriolis force, Euler force), were made clear. (and also referred to as centrifugal force)

    As an electrical engineer, I don't really have to worry about the terminology. If I need to do a calculation or derivation, I just do it.

    Apparently, there is a desire to classify centrifugal force as only a fictitious force needed in noninertial reference frames. If so, I'm asking what is the accepted terminology for the real force that is the reaction to centripetal acceleration. Mind you, this real force is not a force on the stone in your example. I made this clear in my first post.
     
    Last edited: May 26, 2009
  21. May 26, 2009 #20

    tiny-tim

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    With your hand below the plane, no.
     
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