8.13. Dispersion of Waves.
Waves Propagation in the Medium with Dispersion
The dependence of the phase velocity of harmonic waves in a medium on frequency is called the wave dispersion. The medium that possesses such properties is said to be the medium with the dispersion. The dispersion of sound waves in the infinite medium depends on the medium properties and is always accompanied by sound absorption.
The wave absorption is the transformation of the wave energy into other forms of energy when propagating in a medium. Elastic waves absorption in the homogeneous medium is caused mainly by the process of internal friction and thermal conductivity. The amplitude A and the intensity J of the plane wave which propagates in the positive direction of the axis OX, are described by exponential law:
Here A0 and J0 are the wave amplitude and the wave intensity in the point x = 0; is a linear coefficient of elastic waves absorption and depends on the medium properties and wave frequency.
8.14. Sound
Sound waves are the waves with the frequencies between 50 and 20000 Hz which human ear can hear. These waves are longitudinal. They can propagate in solid bodies, liquids and gases in the form of pressure oscillations. Each particle of a medium vibrates about its equilibrium position. The displacement of matter in this case does not take place. The waves with frequencies >20000 Hz are called ultra-sound and with <16 Hz – infra-sound.
The part of physics in which sound waves and their properties are studied is called acoustics.
For sound waves all objective laws for longitudinal waves are valid.
8.15. The Doppler Effect in Acoustics
As an automobile traveling at high speed and sounding its horn passes, the pitch heard by pedestrian drops sharply. When a source of sound is moving toward the observer or the observer toward a sound source, the pitch heard is higher than the normal pitch. When the source moves away from the observer or the observer from the source, the pitch is lowered. The fact that the frequency received by the observer is different than the frequency emitted by the source whenever either is moving relative to the other is known as the Doppler effect.
Figure 8.24
In the general case when the source and the receiver are moving in arbitrary directions (Fig.8.24):
where 0 is the frequency of the source vibrations; v1 is the velocity of the source; v2 is the velocity in a medium; is the frequency that is registered
TEXT 2.2.: THE FUTURE OF THE
ENGINEERING PROFESSION
Among various recent trends in the engineering profession computerization is the most widespread. The trend in modern engineering offices is also towards computerization. Computers are increasingly used for solving complex problems as well as for handling, storing, and generating the enormous volume of data modern engineers must work with.
Scientific methods of engineering are applied in several fields not connected directly to manufacture and construction. Modern engineering is characterized by the broad application of what is known as systems engineering principles.
Engineers in industry work not only with machines but also with people, to determine, for example, how machines can be operated most efficiently by workers. A small change in the location of the controls of a machine or of its position with relation to other machines or equipment, or a change in the muscular movements of the operator, often results in greatly increased production. This type of engineering work is called time-study engineering.
A related field of engineering, human-factors engineering, also known as ergonomics, received wide attention in the late 1970s and 1980s when the safety of nuclear reactors was questioned following serious accidents that were caused by operator errors, design failures, and malfunctioning equipment.
Human-factors engineering seeks to establish criteria for the efficient, human-centred design of, among other things, the large, complicated control panels that monitor and govern nuclear reactor operations.
General understanding:
1) What is the most widespread trend in the engineering profession?
2) What are computers used for in modern engineering?
3) What approaches are used in modern engineering?
4) What is “ergonomics“?
5) What does human-factors engineer deal with?
HOME – READING
THREE LAWS OF MOTION
Newton's Laws of Motion help to describe the interaction of forces in physical systems — from the motions of objects, animals, and humans on Earth to the motions of rockets in space. Sir Isaac Newton (1642-1727) was the first to state the laws in a published form that future scientists could use, although other scientists had made similar discoveries before Newton. In order to understand the Laws of Motion it is necessary to understand speed, velocity, and acceleration. Velocity adds to speed the dimension of direction—a change in direction or a change in speed is a change in velocity. Acceleration refers to any change in velocity—therefore it can mean slowing, stopping, or turning as well as speeding up. Since acceleration is a measure of the change in velocity, it is expressed in units of distance per unit of time (velocity — m/s) per unit of time (acceleration — m/s2).
Gravity and friction are two forces important in the workings of Newton's ' Laws. Friction occurs between touching surfaces and always acts opposite to the direction of motion. Friction depends on the nature of the touching surfaces and the amount of force pressing the surfaces together. It does not depend appreciably on the amount of surface area in contact.
Gravity is a force that all objects exert on all other objects, although the pull of Earth is by far the greatest gravitational force experienced on Earth.
Newton's First Law states that an object at rest tends to stay at rest, and an object in motion tends to stay in motion in a straight line and at a constant speed unless acted upon by a net force.
The Second Law of Motion states that the acceleration of an object depends on the net force acting on that object and on the mass of that object.
Newton's Third Law of Motion states that whenever an object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.
LESSON 3