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CF / CP Release

CF / CP release refers to two different ways of releasing the club. ‘CF’ and ‘CP’ are implying centrifugal and centripetal force which are hence thought to be a characteristic element in the release action of the club. Release is the action of the rapid overtaking of the club relative to the arms through the bottom part of the down swing and requires a torque acting on the club.

Logically one can hence assume that this particular torque is being generated either by a centrifugal or a centripetal force depending on the type of release. However in a body, free to rotate, these two forces, even becoming often quite large, can’t exert a torque as their line of action is along a line going both through the center of rotation and the center of mass of the rotating body. For any force to generate a torque its line of action must have an offset relative to the center of mass, Fig1.

Centripetal force is defined as the force required to make a body follow a curved path. It is always directed orthogonal to the velocity of the body, toward the instantaneous center of curvature of the path. Centrifugal force is the reactive counterpart of centripetal force, forming an action reaction pair of forces. This definition for centripetal force is valid for a single mass but in multibody mechanics one associates radial forces with generalized coordinates.

Let's get a bit more specific. In Fig2a a rigid body rotates around a fixed point. In Fig2b again a rotating rigid body consisting of a slender bar bent at point B and with masses M1 and M2 added at B and C. Fig2c appears like Fig2b but is quite different as there are two bodies connected through a hinge at B. The red arrows represent centripetal force vectors. Gravity forces are ignored.

In Fig2a the rigid body M rotates around a fixed point. All mass elements describe circles around the fixed center. Therefore the instantaneous center of curvature of the path of each of these small mass elements is the fixed center. The line of action for the centripetal force acting on any arbitrary small mass element dm of M will always pass through the mass element and the center of rotation as shown.

In Fig2b we have again a rigid mass M - slender bar ABC, masses M1 and M2. The trajectories of M1 and M2 are circles with A as center. Hence, as in Fig2a, the lines of action of the centripetal forces acting on the masses M1 and M2 are through the masses M1 and M2 and the center A. Due to the angle between the line of action AC and bar BC the centripetal force F2 exerts a torque on the bar BC. The centripetal force F1 acting on M1 along the line of action AB does not exert any torque on the upper segment but it does on the lower segment. Hence both centripetal forces, F1 and F2, generate an torque on the lower segment BC of the ensemble.

In Fig2c we have a hinge at point B. Very different behavior. It becomes a multi-body problem and the Lagrangian mechanics convention for radial forces is used. The line of action for the centripetal force F2 acting on M2 is now aligned with bar BC and hence this centripetal force does not exert any torque on the lower segment whatsoever. However the centripetal force F1 acting on M1 has an offset angle with BC and does exert a torque on the lower segment.

Fig2b and Fig2c portray the situation of the arm/club ensemble in the down swing. For a very short time, from the top,it is like shown in Fig2b but for the remaining of the down swing it is like shown in Fig2c. Hence a radial force, either centripetal or centrifugal force, associated with the club, doesn’t play a role in the release action of the club. However F1 does exert a torque on the lower segment, either when swinging (CF) or hitting (CP).

The centripetal torque for the case depicted in Fig2b can become quite large. Anyone having seriously tried to prevent release to occur in a full swing will be aware of this. Hence there is a paradox. When the arm club ensemble is behaving like one solid mass potentially a large centripetal release torque is developed due to force F2. However as soon as release starts this particular centripetal torque vanishes.

The centripetal torque generated in the arm/club ensemble, initially from the top, and the forward bending of the shaft through impact are similar in the way the torque is being developed as illustrated in Fig3 and Fig4 for the forward bending of the shaft through impact. Simply imagine BC a bit larger for the arm/club ensemble.

Even nowadays there are still golf instructors either referring to or actually using a tethered ball to illustrate centrifugal force. Subsequently one is readily enticed to think that there is also a centrifugal force acting on a line of action from the inner pivot to the clubhead. The swinger is supposed to use this centrifugal force for releasing the club on the ball and the hitter actually resisting this centrifugal force.

Form discussion above it is evident that the frequent use of the expression ‘‘CP / CF release’’ does not quite correspond with reality. In a future post I will look more in detail into the release action in swinging and hitting. It will be also interesting to analyze the 'swinging left' action of the CP release.

mandrin