Page 22 - Dynamics of Mechanical Systems

0593_C01_fm Page 3 Monday, May 6, 2002 1:43 PM

Introduction 3

1.4 Basic Terminology in Mechanical Systems
Particular terminology is associated with dynamics, and specifically with mechanical
system dynamics, which we will use in the text. We will attempt to define the terms as
we need them, but it might also be helpful to mention some of them here:

• A space is a region or geometric entity occupied by particles where, for our
purposes, dynamic events will occur.
• A reference frame may be regarded as a coordinate axis system containing and
locating the points of a space. Typical reference frames employ Cartesian axes
systems.
• A particle is a small body whose dimensions are either negligible or irrelevant
in the description of its motion and of its response to forces applied to it. “Small”
is, of course, a relative term. A body considered as a particle may be small in
some contexts but not in others (for example, an Earth satellite or an automobile).
Particles are generally identified with points in space, and they generally have
finite masses.
• A rigid body is a set of particles whose distances from one another remain fixed,
or constant, such as a sandstone. The number of particles in a body is usually
quite large. A reference frame may be regarded, for kinematic purposes, as a
rigid body whose particles have zero masses.
• A degree of freedom is defined as a way in which a particle, body, or system can
move. The number of degrees of freedom possessed by a particle, body, or system
is defined as the number of geometric parameters (for example, coordinates,
distances, or angles) needed to uniquely describe the location, orientation, and/
or configuration of the particle, body, or system.
• A constraint is a restriction on the motion of a particle, body, or system. Con-
straints can be either geometric (holonomic) or kinematic (nonholonomic).
• A machine is an arrangement of a system of bodies designed for applying, trans-
mitting, and/or changing forces and motion.
• A mechanism is a machine intended primarily for the transmission of motion.
The three general categories of machines are:
1. Gear systems, which are toothed bodies in contact whose objectives are to
transmit motion between rotating shafts.
2. Cam systems, which are bodies with curved profiles in contact whose objec-
tives are to transmit motion between a rotating member and a nonrotating
member. The term “cam” is sometimes also used to describe a gear tooth.
3. Linkages, which are multibody systems intended to provide either a desired
motion of a rigid body or the motion of a point of a body along a curve.
• A link is a connective member of a machine or a mechanism. A link maintains
a constant distance between two points of a mechanism, although links may be
one way, such as cables.
• A driver is an “input” link that stimulates a motion.
• A follower is an “output” link that responds to the input stimulus of the driver.