1.9 Problem Solving in Statics
We study statics to obtain a quantitative description of forces which
act on engineering structures in equilibrium. Mathematics establishes
the relations between the various quantities involved and enables us to
predict effects from these relations. We use a dual thought process in
solving statics problems: We think about both the physical situation and
the corresponding mathematical description. In the analysis of every
problem, we make a transition between the physical and the mathematical.
One of the most important goals for the student is to develop the
ability to make this transition freely.
Making Appropriate Assumptions
We should recognize that the mathematical formulation of a
physical problem represents an ideal description, or model, which approximates
but never quite matches the actual physical situation.
When we construct an idealized mathematical model for a given engineering
problem, certain approximations will always be involved.
Some of these approximations may be mathematical, whereas others
will be physical.
For instance, it is often necessary to neglect small distances, angles,
or forces compared with large distances, angles, or forces. Suppose a
force is distributed over a small area of the body on which it acts. We
may consider it to be a concentrated force if the dimensions of the area
involved are small compared with other pertinent dimensions.
We may neglect the weight of a steel cable if the tension in the cable
is many times greater than its total weight. However, if we must calculate
the deflection or sag of a suspended cable under the action of its
weight, we may not ignore the cable weight.
Thus, what we may assume depends on what information is desired
and on the accuracy required. We must be constantly alert to the various
assumptions called for in the formulation of real problems. The ability to
understand and make use of the appropriate assumptions in the formulation
and solution of engineering problems is certainly one of the most important
characteristics of a successful engineer. One of the major aims of
this book is to provide many opportunities to develop this ability through
the formulation and analysis of many practical problems involving the
principles of statics.
Using Graphics
Graphics is an important analytical tool for three reasons:
1. We use graphics to represent a physical system on paper with a
sketch or diagram. Representing a problem geometrically helps us
with its physical interpretation, especially when we must visualize
three-dimensional problems.
2. We can often obtain a graphical solution to problems more easily
than with a direct mathematical solution. Graphical solutions are
both a practical way to obtain results, and an aid in our thought
processes. Because graphics represents the physical situation and
its mathematical expression simultaneously, graphics helps us make
the transition between the two.
3. Charts or graphs are valuable aids for representing results in a form
which is easy to understand.
The Free-Body Diagram
The subject of statics is based on surprisingly few fundamental concepts
and involves mainly the application of these basic relations to a
variety of situations. In this application the method of analysis is all
important. In solving a problem, it is essential that the laws which apply
be carefully fixed in mind and that we apply these principles literally
and exactly. In applying the principles of mechanics to analyze forces
acting on a body, it is essential that we isolate the body in question from
all other bodies so that a complete and accurate account of all forces acting
on this body can be taken. This isolation should exist mentally and
should be represented on paper. The diagram of such an isolated body
with the representation of all external forces acting on it is called a freebody
diagram.
The free-body-diagram method is the key to the understanding of
mechanics. This is so because the isolation of a body is the tool by which
cause and effect are clearly separated, and by which our attention is
clearly focused on the literal application of a principle of mechanics. The
technique of drawing free-body diagrams is covered in Chapter 3, where
they are first used.
Numerical Values versus Symbols
In applying the laws of statics, we may use numerical values to
represent quantities, or we may use algebraic symbols, and leave the
answer as a formula. When numerical values are used, the magnitude
of each quantity expressed in its particular units is evident at each
stage of the calculation. This is useful when we need to know the magnitude
of each term.
The symbolic solution, however, has several advantages over the
numerical solution. First, the use of symbols helps to focus our attention
on the connection between the physical situation and its related
mathematical description. Second, we can use a symbolic solution repeatedly
for obtaining answers to the same type of problem, but having
different units or numerical values. Third, a symbolic solution
enables us to make a dimensional check at every step, which is more
difficult to do when numerical values are used. In any equation representing
a physical situation, the dimensions of every term on both
sides of the equation must be the same. This property is called dimensional
homogeneity.
Thus, facility with both numerical and symbolic forms of solution is
essential.
Solution Methods
Solutions to the problems of statics may be obtained in one or more
of the following ways.
1. Obtain mathematical solutions by hand, using either algebraic
symbols or numerical values. We can solve most problems this
way.
2. Obtain graphical solutions for certain problems.
3. Solve problems by computer. This is useful when a large number of
equations must be solved, when a parameter variation must be
studied, or when an intractable equation must be solved.
Many problems can be solved with two or more of these methods. The
method utilized depends partly on the engineer’s preference and partly
on the type of problem to be solved. The choice of the most expedient
method of solution is an important aspect of the experience to be gained
from the problem work. There are a number of problems in Vol. 1 Statics
which are designated as Computer-Oriented Problems. These problems
appear at the end of the Review Problem sets and are selected to
illustrate the type of problem for which solution by computer offers a
distinct advantage.