Page 8 - 4624

fundamental laws of motion and the law of universal gravitational
attraction. Shortly after these laws were postulated, important
techniques for their application were developed by such notables as
Euler, D’Alembert, Lagrange, and others.

2 Fundamental Concepts

Before we begin our study of engineering mechanics, it is
important to understand the meaning of certain fundamental concepts
and principles.
Basic Quantities. The following four quantities are used
throughout mechanics.
Length is used to locate the position of a point in space and
thereby describe the size of a physical system. Once a standard unit of
length is defined, one can then use it to define distances and geometric
properties of a body as multiples of this unit.
Time is conceived as a succession of events. Although the
principles of statics are time independent, this quantity plays an
important role in the study of dynamics.
Mass is a measure of a quantity of matter that is used to compare
the action of one body with that of another. This property manifests
itself as a gravitational attraction between two bodies and provides a
measure of the resistance of matter to a change in velocity.
Force. In general, force is considered as a “push” or “pull”
exerted by one body on another. This interaction can occur when there
is direct contact between the bodies, such as a person pushing on a
wall, or it can occur through a distance when the bodies are physically
separated. Examples of the latter type include gravitational, electrical,
and magnetic forces. In any case, a force is completely characterized
by its magnitude, direction, and point of application.
Idealizations. Models or idealizations are used in mechanics in
order to simplify application of the theory. Here we consider three
important idealizations.
A particle has a mass, but a size can be neglected. For example,
the size of the earth is insignificant compared to the size of its orbit,
and therefore the earth can be modeled as a particle when studying its
orbital motion. When a body is idealized as a particle, the principles of

8