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At the conclusion of this chapter, the student should be able to:

  • 1. Name, define, and use the following terms properly as they relate to rotary motion: eccentric force, torque, couple, lever, moment of inertia, and angular momentum.
  • 2. Solve simple lever and torque problems involving the human body and the implements it uses.
  • 3. Demonstrate an understanding of the effective selection of levers by relating speed, range of motion, and mechanical advantage to the properties of given lever systems.
  • 4. Explain the analogous kinetic relationships that exist between linear and rotary motion.
  • 5. State Newton’s laws of motion as they apply to rotary motion.
  • 6. Explain the cause-and-effect relationship between the forces responsible for rotary motion and the objects experiencing the motion.
  • 7. Define centripetal and centrifugal force, and explain the relationships that exist between these forces and the factors influencing them.
  • 8. Identify the concepts of rotary motion that are critical elements in the successful performance of a selected motor skill.
  • 9. Using the concepts that govern rotary motion, perform a mechanical analysis of a selected motor skill.

Eccentric Force

The effect forces have on an object depends on the magnitude, point of application, and direction of each force. When force is applied in line with a freely moving object’s center of gravity, linear motion occurs. When the direction of force is not in line, a combination of rotary and translatory motion is likely to occur. This relationship between force application and direction and the resulting motion are apparent when a book is pushed along a table. Linear motion occurs when sufficient force is applied in line with the book’s center of gravity, and a combination of linear and rotary motion results from a force directed left or right of center. Similarly, an object with a fixed axis, like a door or one of the body’s limbs, rotates when the force is applied off center but does not rotate when the force is in line with the axis of rotation. A force whose direction is not in line with the center of gravity of a freely moving object or the center of rotation of an object with a fixed axis of rotation is called an eccentric force. There must be an eccentric force for rotation to occur. Some examples of the application of eccentric force are shown in Figure 13.1.


The turning effect of an eccentric force is called torque, or moment of force. The torque about any point equals the product of the force magnitude and the perpendicular distance from the line of force to the ...

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