Материал: Андрианова English Thesaurus of Student-Physicist 2013

and forth between a parent and a child moves repetitively, but the motion isn't simple harmonic motion, because the force acting on the ball doesn't take the form of Hooke's law.

The following three concepts are important in discussing any kind of periodic motion. The amplitude is the maximum distance of the object from its equilibrium position. The period is the time it takes the object to move through one complete cycle of motion. The frequency is the number of complete cycles or vibrations per unit of time.

Elastic potential energy

A system of interacting objects has potential energy associated with the configuration of the system. A compressed spring has potential energy that, when allowed to expand, can do work on an object, transforming spring potential energy into the object's kinetic energy. The energy stored in a stretched or compressed spring or some other elastic material is called elastic potential energy.

A valuable example of the relationship between simple harmonic motion and circular motion can be seen in vehicles and machines that use the back-and-forth motion of a piston to create rotational motion in a wheel.

The period T represents the time required for one complete trip back and forth, is also the time it takes the ball to make one complete circular trip on the turntable. The inverse of period is frequency. The units of frequency are cycles per second, or hertz(Hz).

Motion of a pendulum

A simple pendulum is mechanical system that exhibits periodic motion. It consists of a small bob of mass m suspended by a light string of length L fixed at its upper end. (By a light string, we mean that the string's mass is assumed to be very small compared with the mass of the bob and hence can be ignored.) When released, the bob swings to and fro over the same path; but is its motion simple harmonic?

Answering this question requires examining the restoring force - the force of gravity - that acts on the pendulum. The pendulum bob moves along a circular arc, rather than back and forth in a straight line. When the oscillations are small, however, the motion of the bob is nearly straight, so Hooke's law may apply approximately.

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Fig. 13. Thesaurus for Unit 13

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Damped oscillation

The vibrating motions we have discussed so far have taken place in ideal systems that oscillate indefinitely under the action of a linear restoring force. In all real mechanical systems, forces of friction retard the motion, so the systems don't oscillate indefinitely. The friction reduces the mechanical energy of the system as time passes, and the motion is said to be damped.

Shock absorbers in automobiles are one practical application of damped motion. A shock absorber consists of a piston moving through a liquid such as oil. The upper part of the shock absorber is firmly attached to the body of the car. When the car travels over a bump in the road, holes in the piston allow it to move up and down in the fluid in a damped fashion.

Damped motion varies with the fluid used. For example, if the fluid has a relatively low viscosity, the vibrating motion is preserved but the amplitude of vibration decreases in time and the motion ultimately ceases. This is known as underdamped oscillation. If the fluid viscosity is increased, the object returns rapidly to equilibrium after it's released and doesn't oscillate. In this case, the system is said to be critically damped. If the viscosity is made greater still, the system is said to be overdamped.

Waves

The world is full of waves: sound waves, waves on a string, seismic waves, and electromagnetic waves, such as visible light, radio waves, television signals, and x-rays. All of these waves have as their source a vibrating object, so we can apply the concepts of simple harmonic motion in describing them.

In the case of sound waves, the vibrations that produce waves arise from sources such as a person's vocal chords or a plucked guitar string. The vibrations of electrons in an antenna produce radio or television waves, and the simple up-and-down motion.

Interference of waves

Many interesting wave phenomena in nature require two or more waves passing through the same region of space at the same time. Two traveling waves can meet and pass through each other without being destroyed or even altered. For instance, when two pebbles are thrown into a pond, the expanding circular waves don't destroy each other. In fact, the ripples pass through each other. Likewise, when sound waves from two sources move through air, they pass through each other. In the

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region of overlap, the resultant wave is found by adding the displacements of the individual waves.

Reflection of waves

Whenever a traveling wave reaches a boundary, part or all of the waves is reflected. Note that the reflected pulse is inverted. This can be explained as follows: When the pulse meets the wall, the string exerts an upward force on the wall.

1.Give examples of physical phenomena containing periodic mo-

tion.

2.On what condition does the spring exert a force on the object?

3.What do you know about a restoring force?

4.What do you understand under the term simple harmonic motion?

5.Explain the term a simple pendulum.

6.What can you say about the vibrations in the case of sound

waves?

7.Explain the phenomena of reflection of waves.

Exercises

1. Rearrange the letters in the anagrams to form equivalents for the Russian words:

2. Match the words in A with the words in B to form word combinations:

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3. Fill in the gaps with the missing words from the list:

frequency, simple harmonic motion, amplitude, simple pendulum, period

1._____ occurs when the net force along the direction of motion obeys Hooke's law.

2.The _____ is the maximum distance of the object from its equilibrium position.

3.The _____ is the time it takes the object to move through one complete cycle of motion.

4.The _____ is the number of complete cycles or vibrations per unit of time.

5.The period of a _____ doesn't depend on the mass, but only on the pendulum's length and on the free-fall acceleration.

UNIT 14

SOUND

Read the following texts. Study the thesaurus on fig. 14, answer the questions after the texts

Sound waves are the most important example of longitudinal waves. In this Unit we discuss the characteristics of sound waves: how they are produced, what they are, and how they travel through matter. We then investigate what happens when sound waves interfere with each other. The insights gained in this Unit will help you understand how we hear.

Producing a sound wave

Whether it conveys the shrill whine of a jet engine or the soft melodies of a crooner, any sound wave has its source in a vibrating object. Musical instruments produce sounds in a variety of ways. The sound of a clarinet is produced by a vibrating reed, the sound of a drum by the vibration of the taut drumhead, the sound of a piano by vibrating strings, and the sound from a singer by vibrating vocal cords.

Sound waves are longitudinal waves traveling through a medium, such as air. In order to investigate how sound waves are produced, we focus our attention on the tuning fork, a common device for producing pure musical notes. A tuning fork consists of two metal prongs, or tines, that vibrate when struck. Their vibration disturbs the air near them.

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