Wave motion and sound

Wave motion. One of the most important phenomena in nature is the transformation of energy from one point to another by wave motion. This kind of motion is illustrated in many ways. When a stone is dropped into a pool of still water, the surface of the water is covered with circular wavelets which widen out from the central point where the stone fell. The water does not really move outward from the central point, but it rises and then falls again. That is seen by observing a floating leaf or piece of wood. It does not move forward but returns again and again to its former position. Hence, the water on which the leaf rests must have this same kind of upward and downward motion rather than a forward motion. When one end of a rope is fastened to a rigid wall and the free end moved up and down rapidly, each jerk travels along the rope, each portion of the rope communicating the jerk to the next portion. Each particle of the rope imparts its upward and downward motion to its neighbours. The jerk moves forward, but the particles of the rope move only up and down. Motions of this kind are wave motion. In all these cases it is evident that there is a vibrating centre which produces motions in those portions of the medium immediately in contact with it, and that these portions impart their motions to the neighbouring portions.

Transverse waves. If part of a stretched string is drawn aside, the tension in the string tends to bring it back to its position of equilibrium. Since the string has inertia, the force that causes the displacement requires time to produce its full effect so that a wave can travel across the string with a definite velocity. Waves of his kind are easily produced in a rope fixed at one end and held in the hand at the other. If the rope is tightly stretched, a jerk imparted to the end of the rope travels down the rope as a wave. The more tightly the rope is stretched, the more rapidly the jerk travels down it. Of a series of to-and-fro movements is imparted to the end of the rope a series of waves travel down the rope. Such waves are known as transverse waves, because the particles of the medium in which the waves travel are more perpendicular to the direction of the wave motion. They can be easily represented by plotting the displacements on the vertical axis and the distance from the source in a given direction on the horizontal axis. Light and other forms of electromagnetic waves are excellent illustrations of transverse waves.

Ex. 1. Answer the following questions

1. How is energy transmitted? 2. Where does each particle of the rope impart its upward and downward motion to? 3. How does the jerk move? 4. What does the tension in the string tend to do? 5. When can a wave travel along the string with a definite velocity? 6. Where are transverse waves easily produced? 7. What waves are known as transverse waves?

Ex. 2. Translate the sentences into Russian paying special attention to the different meanings of the verb “to be”.

1. One of the most important phenomena in nature is the transmission of energy from one point to another by wave motion. 2. This kind of motion is illustrated in many waves. 3. He is in the laboratory. 4. He is making an experiment now. 5. They are to study some material about sound. 6. The masses in the two pans will then be equal. 7. His aim is to study this material. 8. Many hydroelectric power stations were built in our country. 9. This view has been supported by a number of experiments. 10. There have been many theories to explain the nature of heat. 11. Heat was considered to be an invisible substance. 12. Electrical energy can be transformed into heat energy.

Ex. 3. Make up all possible questions to the following sentences.

In contrast to a transverse wave, a longitudinal wave is one in which the vibrations occur in the same direction as the direction of travel of the wave. The displacements of the particles cause regions of high density and of low density to be formed. These regions move along with the speed of the wave.

Ex. 4. Retell this dialogue in the form of monologue

• In nature there exists the transmission of energy by wave motion. So, a wave allows energy to be transferred, doesn’t it?

• Yes, it does. A wave carrying energy travels along the medium from one point to another.

• All right. For example, sound waves carry energy from the source to the ear by disturbance of the air.

• Yes, you are right.

• What sound waves are there in nature?

• There are some of them. For example, a wave which is propagated by vibrations perpendicular to the direction of travel of the wave is called a transverse wave.

• I’m sorry, but I don’t remember the definition of a longitudinal wave. Will you be so kind to give it?

• Of course. In contrast to a transverse wave a longitudinal wave is one in which the vibrations occur in the same direction as the direction of travel of the wave.

Ex. 5. Translate into Russian paying attention to the infinitives.

1. To help navigation at sea, small radars are used on small commercial ships. 2. Ground-based radar is employed to control flights of aircraft. 3. To determine the exact location of any target is possible through accurate measurements of range, altitude and azimuth. 4. To understand something of the properties of electrons it is necessary to know a little about the atomic structure of matter. 5. The number of electrons must always equal the number of protons to keep the atom electrically neutral. 6. To put Tsiolkovsky's dream to life the efforts of all peoples were necessary. 7. Our scientists had to solve many important problems to launch a first artificial satellite. 8. K. Tsiolkovsky was the first to propose the idea of creating artificial satellites of the earth. 9. Modern space technology has made it possible to build orbiting scientific "laboratories" to conduct observations at and beyond the earth's atmosphere. 10. The progress of chemistry and the chemical industry helps to provide all citizens with adequate food and means to combat deseases.

Ex. 6. Rewrite these sentences using the infinitives.

Model: They came to Moscow. They want to study at the University.

They came to Moscow to study at the University.

I. Early experimenters "rubbed a galosh against a fur coat". They tried to produce electric charges. 2. We want to electrify a hard-rubber rod. We can rub it with a piece of fur or wool. 3. We want to get the corresponding result. We can use a glass rod and a silk cloth. 4. We want to confirm Gilbert's conclusions. These experiments are usually done. 5. We want to complete this qualitative picture. We need one more experiment. 6. We want to rewrite Coulomb's law. We can find the force in newtons between two one-coulomb charges 1 m apart. 7. We magnetize iron, steel or cobalt. We want to form strong magnets. 8. We place an iron or steel rod near a magnet We want to magnetize it.