## Principle Of Spin

It is clear that fluid forces affect the motion of objects through a fluid. Lift is a key fluid force that can be modified by imparting spin on a projectile. The Principle of Spin is related to using the spin on a projectile to obtain an advantageous trajectory or bounce. Kinesiology professionals can use the principle of spin to understand the most successful techniques in many activities. The upward lift force created by backspin in a golf shot increases the distance of a drive (Figure 8.17a), while the backspin on a basketball jump shot is primarily used to keep the ball close to the hoop when impacting the rim or backboard (Figure 8.17b). The bottom of a basketball with backspin is moving faster than the center of the ball be cause the ball is rotating. This increases the friction force between the ball and the rim, decreasing the horizontal velocity of the ball, which makes the ball bounce higher. In applying the spin principle, professionals should weigh the trajectory and bounce effects of spin changes.

Applying spin to projectiles by throwing or striking have a key element in common that can be used to teach clients. The body or implement applies force to the ball off-center, creating a torque that produces spin on the projectile. The principles of torque production can be applied to the creation of spin, in that a larger force or a larger moment arm will increase the torque and spin produced. Coaching athletes to project or hit balls with minimal spin to create erratic trajectories requires that the object be in contact with a force in line with the ball's center of gravity. In volleyball, for example, the athlete is taught to strike through the center of the ball with minimal wrist snap. The flat impact through the ball's center of gravity and minimal torque from wrist rotation ensures that the ball will have minimal spin.

Unfortunately, the linear speed of the projectile is inversely proportional to the spin created. In other words, the more spin produced in the throw or hit comes at a cost

Figure 8.17. The principle of spin is used on a golf ball to create lift forces (FL) that affect ball trajectory, while spin on a basketball is primarily used to modify ball rebound to increase the chance of a made basket.

of lower ball speed. In tennis the lateral break of a ball with slice (sidespin) will not travel as fast as a flat serve (minimal spin) hit with the same effort. Much of the art of teaching and coaching is being able to evaluate a person's performance, diagnosing the factors related to spin and speed production that are appropriate for a specific situation.

There is one more advantage of imparting spin to a projectile that is not related to fluid or contact forces on a surface. This third advantage of projectile spin is related to Newton's laws and conservation of angular momentum. Any object in angular motion without external-acting torques (like a projectile) will conserve angular momentum. This inertia in a rotating object can be used to keep the projectile in a certain orientation. A pass in American football does not create significant lift force, but the spin stabilizes the flight of the ball in a streamlined position. Divers and gymnasts

(human body projectiles) can overcome this inertia and move body parts relative to an axis of rotation with internal muscle forces. In these situations athlete can transfer angular momentum from one axis to another (e.g., add a twist in the middle of somersaults) by asymmetric motions of body parts. Coaches of these sports need to be familiar with this interesting application of the spin principle (see Yeadon, 1991, 1997).

Knowing what advantage of spin in a particular situation is important and how to mechanically create it are critical, but this knowledge must be integrated with knowledge from other kinesiology disciplines. A physical educator could ask a junior high student to "use an eccentric force" or "increase the effective moment arm" and may be mechanically correct, but a good teacher selects an appropriate cue that communicates the essential correction without using such technical language. Think about what would be good cues for hitting a sport ball to create topspin, backspin, right or left sidespin. Deciding whether cues about the ball (target) or body action (technique) are most relevant depends on the situation. This is another example of how a biome-chanical principle must be integrated in an interdisciplinary fashion with other kinesi-ology disciplines (e.g., motor development, motor learning, psychology).

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