Chapter 13: The Conditions of Rotary Motion

Outline

• Rotary Force

  Eccentric Force

  Torque

• The Lever

  External Levers

  Anatomical Levers

  Lever Arms

  Classification of Levers

  The Principle of Levers

  Relation of Speed to Range in Movements of Levers

  Selection of Levers

  Mechanical Advantage of Levers

  Identification and Analysis of Levers

• Newton's Laws and Rotational Equivalents

  Moment of Inertia

  Acceleration of Rotating Bodies

  Action and Reaction

  Transfer of Momentum

• Centripetal and Centrifugal Forces

• The Analysis of Angular Motion

• Laboratory Experiences

Objectives

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.

Rotary Force

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.