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Centrifugal force

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  • Centrifugal force

    Dear all,
    Surprisingly, a lively chat on centrifugal force continues despite
    Gottlieb’s opinion that such a force is a dead thing and is not worthy
    of “scientific discussions”.
    I would like to reiterate some points expressed in other comments and
    add a new one, which relate the topic of inertial forces with another
    thing that, in Gottlieb’s opinion expressed in his numerous
    publications, is also dead - the equilibrium-point hypothesis. In other
    words, I am going to make a bridge between the notion of inertial forces
    and that of motor control, which may be most interesting for students
    and specialists in biomechanics.
    Sorry, Gerry, if at some turns of my thoughts, I centrifugally hit
    your judgment. The purpose of my comments, in addition to what I said
    above, is not to convince Gottlied that he is wrong but rather to warn
    some students (and professors) who might be inclined to take his or a
    similar opinion seriously.

    1. To teach or not to teach students about inertial (in particular,
    centrifugal) forces and about different coordinate systems? Mitnitski’s
    comments correctly imply that the notion of inertial forces and frames
    of reference is a part of the fundamental knowledge of physics. This
    knowledge should be in the list of basic tests offered not only to
    students of mechanics or biomechanics but also to professors who teach
    them. Unlike professor Gottlieb, students cannot afford to be
    wishy-washy about this subject. Also, I would advise, for safety
    reasons, not to sit in a car with the driver who is ambivalent about the
    physical reality of centrifugal forces (the real consequences of such
    forces were nicely illustrated in Bogert’s comments).
    2. Gottlieb’s statement that “the thing called "centrifugal" force is a
    confection of physics and lay terminology that probably should be
    avoided in scientific discussions” advises us to become, in a sense,
    handicapped by forgetting “about the unity of the nature of inertia and
    gravitation” (Einstein 1922) resulting, according to the general theory
    of relativity, from the curvature of the 4-dimensioal space-time
    continuum. We should also forget that the equivalency of inertial and
    gravitational masses directly follows from this theory (see also
    Bogert’s and other messages on this toppic).
    3. The terminology that inertial forces are fictitious or pseudo-forces
    came to us historically since physics initially focused on inertial
    frames of reference in which these forces are non-existent. I think that
    from the present state of knowledge, the terminology referring to
    inertial forces as fictitious is misleading: in non-inertial frames,
    these forces, like gravity, are experimentally measurable and therefore
    are as real as other forces. Examples are plenty (again, see Bogert’s
    4. What causes inertial forces? The acceleration of a mass is determined
    with respect to a physical body with which a frame or system of
    coordinates is associated. In other words, accelerations and forces are
    not absolute quantities. Therefore, if the referent frame, i.e., the
    referent body accelerates, it affects the relative acceleration of the
    mass. The part of the force resulting from the motion of the referent
    frame is called inertial force. There is no mystic in it.
    At this point, I can explain why the notions of frame of reference
    and inertial forces are related to the equilibrium-point hypothesis.
    5. The idea that motor actions are produced by the nervous system by
    shifting spatial frames of reference associated with the environment or
    body parts is a key notion of the most recent formulation of the
    equilibrium-point hypothesis (Feldman & Levin 1995). Specifically, it
    suggests that neural control levels are responsible for the
    specification of parameters that determine the origin, orientation,
    metrics and geometry of the spatial frames of reference for perception
    and action. Changes in neural and muscle activity and, eventually,
    movement or isometric force production emerge from changes in these
    parameters by control levels. Recently, this hypothesis has offered an
    explanation of how multiple skeletal muscles are controlled globally, as
    a coherent whole. The appropriate predictions were verified for several
    movements in humans and monkeys (Feldman et al. 1998; Lestienne et al.
    2000; for references and an illustration of the basic idea, see my web
    site with the address below). Simultaneously, in a set of recent
    studies, especially those from Ostry’s group from McGill, it has been
    demonstrated that the claims of falsification of the equilibrium-point
    hypothesis (including those of Gottlieb’s) are largely flowed. See, for
    example, Feldman et al. (1998) on how the effects of Coriolis force on a
    pointing movement in a rotating room are explained in the framework of
    the EP hypothesis.
    Thus, if control levels change parameters of spatial frames of
    reference for motor actions (which was experimentally established for
    parameter lambda) then muscle forces emerging from this control process
    are all, in essence, inertial. My feeling is that this idea relating the
    motor control and inertial forces is non-trivial, worthy of discussion
    and further elaboration.

    Wishing you Happy Holidays, without problems associated with centrifugal
    forces on snowy and icy road turns,

    Dr. Anatol Feldman
    Neurological Science Research Center
    Department of Physiology
    University of Montreal and
    Rehabilitation Institute of Montreal
    6300 Darlington, Montreal, Quebec, Canada H3S 2J4
    Tel (514) 340 2078 ext. 2192
    Fax (514) 340 2154
    Web Site:

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