At the conclusion of this chapter, the student should be able
- 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
- 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
- 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.
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.
Examples of the application of eccentric force.
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