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Centripetal and Centrifugal force

  1. Mar 11, 2012 #1
    I just read that Centrifugal Force is a false force. It does not exist. Inertia (which is not a force) is mistakenly called Centrifugal force. Now, here is my question

    According to Newton's 3rd law of motion, forces always exist in pairs; action and reaction. Centripetal force which is the force acting on the body pushing it towards the center must have a pair that is the force acting on the object that is exerting the centripetal force. For example: a yo-yo twirled around a finger is pulled along the string by the finger towards the finger - centripetal force. The force with which the yo-yo pulls on the finger along the string What is that force called then? This has nothing to do with the inertia of the yo-yo which will make it fly along a straight line as soon as the centripetal force is moved.
  2. jcsd
  3. Mar 12, 2012 #2


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    It is an inertial force, not an interaction force. In a rotating frame it "exists" in the same sense as interaction forces do:
    It is not a "mistake" but a practical mathematical concept:

    Note that this law applies only to interaction forces, not inertial forces. But you are correct that the interaction centripetal force must have a equal but opposite force, according to Newtons 3rd.

    Some call it "reactive centrifugal force":

    Note that contrary to the "inertial centrifugal force" in a rotating ref. frame this "reactive centrifugal force" is an interaction force that exists in all ref. frames. See the table in this section for a comparison of the two:

    Here another example to explain all three forces, a rotating space station with two astronauts:

    Last edited: Mar 12, 2012
  4. Mar 12, 2012 #3


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    hi kghosh! :smile:
    there is no centrifugal force for an inertial observer

    there is centrifugal force for an observer in a rotating frame

    centrifugal force is a fictititious (or inertial) force … some observers create it for their convenience :wink:
    just a reaction force
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