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  • I was reading https://en.m.wikipedia.org/wiki/Centrifugal_force and found this in the intro:

    Confusingly, the term has sometimes also been used for the reactive centrifugal force, a real frame-independent Newtonian force that exists as a reaction to a centripetal force.

    Then this long-established article: https://en.m.wikipedia.org/wiki/Reactive_centrifugal_force :

    In classical mechanics, a reactive centrifugal force forms part of an action–reaction pair with a centripetal force.
    ...
    In accordance with Newton's first law of motion, an object moves in a straight line in the absence of a net force acting on the object. A curved path ensues when a force that is orthogonal to the object's motion acts on it; this force is often called a centripetal force, as it is directed toward the center of curvature of the path. Then in accordance with Newton's third law of motion, there will also be an equal and opposite force exerted by the object on some other object, and this reaction force is sometimes called a reactive centrifugal force, as it is directed in the opposite direction of the centripetal force.
    ...
    Unlike the inertial force known as centrifugal force, which exists only in the rotating frame of reference, the reactive force is a real Newtonian force that is observed in any reference frame.

    Is my brain creaking more than I thought, or is this just wrong? I don't get the jump from 'an object' to the key bit of Newton's third law, 'mutual actions of two bodies upon each other are always equal'. Neither do I understand the discussion on the talk page.

    Edit: there are hundreds of pages elsewhere with claimed graduates arguing about this. I think it comes down to the authors fundamentally not getting what reference frames are, especially rotating (i.e. accelerating) ones.

  • I thought the middle bit of the Wikipedia article was reasonably clear:

    In the case of a ball held in circular motion by a string, the centripetal force is the force exerted by the string on the ball. The reactive centrifugal force on the other hand is the force the ball exerts on the string, placing it under tension.

    IOW:

    1. The second form of Newton's 3rd law is

      mutual actions of two bodies upon each other are always equal, and directed to contrary parts

    2. The two bodies are the ball and the string

    3. The centripetal force is the string pulling the ball towards the centre of rotation

    4. The reactive force is the ball causing the tension in the string

    5. Both of those are visible in an inertial reference frame, although the directions are constantly changing

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