Lesson 5

I. Read and memorize the following words and word-combinations:

II. Read and translate the following text:

Chief Stages in the Historical Development of Mechanics

It took thousands of years for man to find scientific explanation for mechanical phenomena. The first known attempts of the kind were conducted during the 4-th century B.C. Implements and mechanical devices of the time were extremely simple, knowledge of mechanics was correspondingly limited and the devices known - the lever, pulley, windlass etc., - where studied for the most part from the standpoint of statics to attain an understanding of equilibrium of forces.

Some of the most important work in the field of statics was done by .Archimedes (287 - 212 B.C.) who carried on research on the laws of the lever, centre of gravity, and other phenomena.

After Archimedes, there was little advance in mechanics until the 15-th century A.D., when it began to develop intensively, spurred on by the transition from primitive handicraft to improved methods of production. During this period Leonardo da Vinci (1452 - 1519) made several discoveries in the field of mechanics, while Stevinus (1548-1620) further developed many of Archimedes principles of statics and investigated the mechanical properties of the inclined plane.

In the 17-th century mechanics was further enriched by Galileo Galilei (1564 - 1642). Galileo's work in this sphere was, carried forward by Isaac Newton (1642 - 1727), who improved the formulation of some of Galileo's laws and developed mechanics to the level of a science. The mechanics of Galileo and Newton, known as classical mechanics, formed the foundation for the subsequent intensive growth of that science.

The 13-th century saw the advancement of a new science Sailed analytical mechanics, whose founder was the Russian mathematics and mechanic, Academician L. Euler C1707-17S3).

An outstanding name of the eighteenth century was that of the Russian scholar M.Lomonoaov (1711 - 1765) eminent for his discoveries in various spheres of science, including mechanics; another major contribution was his discovery of the law of the conservation of matter and energy.

Lesson 6

I. Read and memorize the following words and word-combinations:

II. Read and translate the following text: Laws of Static Friction

We know from experience that when two bodies tend to slide to each other a resisting force appears at their surface of contact which opposes their relative motion. This force is called static friction.

Friction is due primarily to minute irregularities on the contacting surfaces which resist their relative motion and to forces of adhesion between contacting surfaces. A detailed examination of the nature of friction is a com­plex physico-mechanical problem lying beyond the scope of theoretical mechanics.

Engineering calculations are based on several general laws deduced from experimental evidence, which reflect the principal features of friction with an accuracy sufficient for practical purposes. These laws, the laws of static friction, can be formulated as follows:

1.When two bodies tend to slide on each other a frictional force is developed at the surface of contact the magnitude of which can have any value from zero to a maximum value Fe which is called limiting friction, or friction of impending motion. Frictional force is opposite in direction to the force which tends to move a body.

2. Limiting friction is equal in magnitude to the product of the coefficient of static friction for friction of rest) fS_o and the normal pressure or normal reaction N.

Fe = S_o N

The coefficient of static friction is a dimensionless quantity which is determined experimentally and depends on the material of the contacting bodies and the conditions of the surfaces (their finish, temperature, humidity, lubrication , etc. ).