Chapter 3

After completing this chapter, you will be able to:

• Define and identify common units of measurement for mass, force, weight, pressure, volume, density, specific weight, torque, and impulse.
• Identify and describe the different types of mechanical loads that act on the human body.
• Identify and describe the uses of available instrumentation for measuring kinetic quantities.
• Distinguish between vector and scalar quantities.
• Solve quantitative problems involving vector quantities using both graphic and trigonometric procedures.

When muscles on opposite sides of a joint develop tension, what determines the direction of joint motion? In which direction will a swimmer swimming perpendicular to a river current actually travel? What determines whether a push can move a heavy piece of furniture? The answers to these questions are rooted in kinetics, the study of forces.

The human body both generates and resists forces during the course of daily activities. The forces of gravity and friction enable walking and manipulation of objects in predictable ways when internal forces are produced by muscles. Sport participation involves application of forces to balls, bats, racquets, and clubs, and absorption of forces from impacts with balls, the ground or floor, and opponents in contact sports. This chapter introduces basic kinetic concepts that form the basis for understanding these activities.

Understanding the concepts of inertia, mass, weight, pressure, volume, density, specific weight, torque, and impulse provides a useful foundation for understanding the effects of forces.

### Inertia

In common usage, inertia means resistance to action or to change (Figure 3-1). Similarly, the mechanical definition is resistance to acceleration. Inertia is the tendency of a body to maintain its current state of motion, whether motionless or moving with a constant velocity. For example, a 150 kg weight bar lying motionless on the floor has a tendency to remain motionless. A skater gliding on a smooth surface of ice has a tendency to continue gliding in a straight line with a constant speed.

###### Figure 3-1

A static object tends to maintain its motionless state because of inertia.

Although inertia has no units of measurement, the amount of inertia a body possesses is directly proportional to its mass. The more massive an object is, the more it tends to maintain its current state of motion and the more difficult it is to disrupt that state.

A skater has a tendency to continue gliding with constant speed and direction due to inertia.

### Mass

Mass (m) is the quantity of matter composing a body. The common unit of mass in the metric system is the kilogram (kg), with the English unit of mass being the slug, which is much ...

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