10.2 Newton’s Laws in Mechanics 259
10.2 Newton’s Laws in Mechanics
As we mentioned in Chapter 6, physical laws are based on observations. In this section, we will
briefly discuss Newton’s laws, which form the foundation of mechanics. The design and anal-
ysis of many engineering problems, including structures, machine parts, and the orbit of satel-
lites, begins with the application of Newton’s laws. Most of you will have the opportunity to
take a physics, statics, or dynamics class that will explore Newton’s laws and their applications
further.What We Mean by Mechanics
Next, it is important that we define what we mean by the term mechanics. There are three con-
cepts that you should fully understand: (1) Mechanics deals with the study of behavior of objects
or structural members when subjected to forces. As mentioned previously, the tendencies of
mechanical forces are to translate, rotate, squeeze or shorten, elongate, bend, or twist objects.
(2) What do we mean by behavior? In mechanics, by the term behaviorwe mean obtaining
information about the object’s linear and angular (rotational) displacements, velocity, acceleration,
linear and angular deformations, and stresses. As we mentioned in Chapter 6, engineers are book
keepers. Whereas, in accounting practice, accountants keep track of dollars and cents, revenues,
and expenditures, in mechanics, engineers keep track of forces, masses, energies, displacements,
accelerations, deformations, rotations, and stresses. (3) Moreover, an object is defined by its
geometric characteristicsand material properties. Geometric characteristics provide information
such as length, cross-sectional area, and first and second moment of area. On the other hand,
the material properties provide information about material characteristics such as density, mod-
ulus of elasticity, and shear modulus of an object. The object’s interaction with its surrounding
is also defined by boundary and initial conditions, and the manner in which forces are applied. In
the following sections, we will elaborate on each of these concepts and how they contribute to
the way an object behaves when subjected to a force, and provide simple examples to demon-
strate these concepts.Newton’s First Law
If a given object is at rest, and if there are no unbalanced forces acting on it, the object will then
remain at rest. If the object is moving with a constant speed in a certain direction, and if there
are no unbalanced forces acting on it, the object will continue to move with its constant speed
and in the same direction. Newton’s first law is quite obvious and should be intuitive. For
example, you know from your everyday experiences that if a book is resting on a table and you
don’t push, pull, or lift it, then the book will lie on that table in that position until it is disturbed
with an unbalanced force.Newton’s Second Law
We briefly explained this law in Chapter 6, where we said that if you place a book on a smooth
table and push it hard enough, it will move. Newton observed this and formulated his obser-
vation into what is called Newton’s second law of motion. Newton observed that as he increased62080_10_ch10_p251-302.qxd 5/22/10 12:32 AM Page 259
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