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,
- 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.
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.
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 (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 larger than a