Engineering mechanics - Part one

Engineering mechanics - Part one

The importance of mechanics in the preparation of young engineers for work in specialized fields cannot be overmphasized. The demand from industry is more and more for young men who are soundly grounded in their fundamental subjects rather than for those with specialized raining. There is good reason for this trend: The industrial engineer is continually being confronted by new problems, which do not always yield to routine methods of solution. The man who can successfully cope with such problems must have a sond understanding of the fundamental principles that apply and be familiar with various general methods of attack rather than proficient in the use of any one. It seems evident, then, that university training in such a fundamental subject as mechanics mus seek to build a strong foundation, to acquaint the student with as many general methods of attack as possibile, to illustrate the application of these methods to practical engineering problems, but to avoid routine drill in the manipulation of standardized methods of solution.

The solution of a problem in mechanics usually consists of three steps:

1. The reduction of a complex physical problem to such a state of idealization that it can be expressed algebraically or geometrically;
2. The solution of this purely mathematical problem;
3. The interpretation of the results of the solution in terms of the given physical problem. It is too often the case that the student's attention is called only to the second step so that he does not see clearly the connection between this and the true physical problem. 

Rigid body 

We shall be mostly concerned in engineering mechanics with problems involving the equilibrium of rigid bodies. Physical bodies, such as we have to deal with in the design of engineering structures and machine parts, are never absolutely rigid but deform slightly under the action of loads which they have to carry.

Force

For the investigation of problems of statics we most introduce the concept of force, which may be defined as any action that tends to change the state of rest of a body to which it is applied. There are many kinds of force, such as gravity force, with which we are all familiar, and the simple push or pull that we can exert upon a body with our hands. Other examples of force are gravitational atraction between the sun and planets, the tractive effort of a locomotive, the force of magnetic attraction, steam or gas pressure in a cylinder, win pressure, atmospheric pressure and frictional resistance between contiguous surfaces.

Characteristics of a force

For the complete definitions of a force we must know 1 its magnitude, 2 its point point of application, and 3 its direction. These three quantities, which completely define the force, are called its characteristics or specifications.

The magnitude of a force is obtained by comparing it with a certain standard, arbitarily taken as the poundm which represents the weight of a certain platinum cylinder kept in the Tower of London. The magnitudes of forces are commonly measured by using various kind of dynamometers.

The point of application of a force acting upon a body is thet point in the body at which the force can be assumed to be concentrated. Physically, it will be impossible to concentrate a force at a single points; that is, every force must have some finite area or volume over which its action is disturbed. However, we often find it convenient to think of such disturbed force as being concentraded at a single point of application wherever this can be done without sensibily changing the effect of the force on the contitions of equilibrium. In the case of gravity force disturbed throught the volume of a body, the ooint of application at which the total weight can be assumed to be concentrated is called the center of gravity of the example is always directed vartically downward.