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Centripetal/centrifugal force

  1. Aug 19, 2009 #1
    I am slightly confused about an experiment to demonstrate centripetal force.

    Suppose a tennis ball is attached to a piece of string. On the other end of the string is attached a mass hanger and some small masses. In the middle of the string is a small piece of plastic tubing. A person holds the string on the plastic tubing and starts swinging the ball around in a horizontal circle. As the ball speeds up, the mass hanger starts rising through the tubing. I am told that this is just due to the equation F=m(v^2)/r and that the radius must increase if the speed increases and the mass (source of the centripetal force) is staying constant. But surely an upwards force must be acting on the mass hanger through the string to cause the masses to rise? If so, wouldn't this be a centrifugal force? There lies the problem because I am told there is no such thing as a centrifugal force.


  2. jcsd
  3. Aug 19, 2009 #2


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    Homework Helper

    The force exists, being one of the Newton third law pair of forces between string and tennis ball. The tension in the string exerts a centripetal force on the ball, coexistant with the ball exerting a reactive centrifugal force on the string.

    The issue is with the term centrifugal force, not about the existance of a reaction force coexisting with a force that results in acceleration. Wiki includes a reference to reactive centrifugal force.

  4. Aug 19, 2009 #3
    Imagine yourself from the point of view of the rotating ball. All the effects are reversed with an equal and opposite force which is the centripetal from the ball's view and centrifugal from the initial view
  5. Aug 19, 2009 #4

    Doc Al

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    Staff: Mentor

    Of course. The string tension exerts an upward force on the mass hanger (and an inward force on the ball).
    No, it would not be an example of what is called centrifugal force. In standard physics usage, "centrifugal force" is a "fictitious" force that only appears when analyzing motion from a rotating frame. Note that the centrifugal force would act on the ball, not on the string or mass hanger. From the usual inertial frame of reference, you'd never have a centrifugal force.

    Some people do refer to the force that the ball exerts on the string as a "reactive centrifugal force", but do so at your own risk.
  6. Aug 19, 2009 #5

    Doc Al

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    Staff: Mentor


    In the non-inertial rotating frame in which the ball is at rest (and not accelerating), one adds a "fictitious" outward force on the ball--the centrifugal force--in order to make use of Newton's laws (which only apply without correction in inertial frames). In this non-inertial frame, the outward centrifugal force balances the inward centripetal force (the string tension).
  7. Aug 19, 2009 #6
    I replied to "no such thing as a centrifugal force"
    Centripetal force as string tension is a convention. From the point of view of the ball the tubing and masses rotate, so the centripetal force in this case the centrifugal force initially
  8. Aug 19, 2009 #7
    Yes. I too had this doubt before.
    A much simpler example is the conical pendulum- Tie a stone to a string and hang it down. Rotate it by twisting your wrist such that the plane of rotation is making an angle with the string. Alright. Let the picture do the talking.
    By increasing the force of the wrist twist, you see that the stone gradually comes up(angle between the plane of rotation and the string reduces). A simple analysis of the free body diagram will show you why-Doc Al did explain though. You can also see that the angle can never reach zero i.e., the plane of rotation can never be perfectly horizontal. Check! :)
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