Principles of Statics - Structural and Stress Analysis - page 20

Chapter 2 / Principles of Statics

Statics , as the name implies, is concerned with the study of bodies at rest or, in other
words, in equilibrium, under the action of a force system. Actually, a moving body
is in equilibrium if the forces acting on it are producing neither acceleration nor
deceleration. However, in structural engineering, structural members are generally at
rest and therefore in a state of statical equilibrium .

In this chapter we shall discuss those principles of statics that are essential to structural
and stress analysis; an elementary knowledge of vectors is assumed.

2.1 F ORCE

The definition of a force is derived from Newton's First Law of Motion which states
that a body will remain in its state of rest or in its state of uniform motion in a straight
line unless compelled by an external force to change that state. Force is therefore
associated with a change in motion, i.e. it causes acceleration or deceleration.

The basic unit of force in structural and stress analysis is the Newton (N) which is
roughly a tenth of theweight of this topic. This is a rather small unit formost of the loads
in structural engineering so a more convenient unit, the kilonewton (kN) is often used.

1kN = 1000N

All bodies possess mass which is usually measured in kilograms (kg). The mass of a
body is a measure of the quantity of matter in the body and, for a particular body,
is invariable. This means that a steel beam, for example, having a given weight (the
force due to gravity) on earth would weigh approximately six times less on the moon
although its mass would be exactly the same.

We have seen that force is associated with acceleration and Newton's Second Law of
Motion tells us that

force = mass × acceleration

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