English for students of physics. Часть 1 (110

VII. A. You are giving a lecture on the Archimedes' principle. Tell the students about the discovery and the forces that act on the submerged object.

B. You are giving a small lecture on naval construction to the students in engineering. What factors should be taken into account while making a ship draft?

Unit VI. Transformation of Energy: An Ideal System

A simple example of a system in which energy is being converted from one form to another is provided in the tossing of a ball with mass m into the air. When the ball is thrown vertically from the ground, it’s 1... and thus its 2... energy decreases steadily until it comes to rest momentarily at its highest point. It then reverses itself, and its speed and kinetic energy increase steadily as it returns to the ground. The kinetic energy Ek of the ball at the instant it left the ground (point 1) was half the product of the mass and the square of the 3..., or 1/2mv12, and decreased steadily to zero at the highest point (point 2). As the ball rose in the air, it gained gravitational 4... energy Ep. Potential in this sense does not mean that the energy is not real but rather that it is stored in some latent form and can be drawn upon to do work. Gravitational potential energy is energy that is stored in a body by virtue of its position in the gravitational field. Gravitational potential energy of a mass m is observed to be given by the product of the mass, the height h attained relative to some reference height, and the acceleration g of a body resulting from the Earth's gravity pulling on it, or mgh. At the instant the ball left the ground at height h1 its potential energy Ep1 is mgh1. At its highest point, its potential energy Ep2 is mgh2. Applying the law of 5 ... of energy and assuming no friction in the air, these add up to form the following equations:

Ek1 + Ep1 = Ek2 + Ep2, or

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1/2mv12 + mgh1 = 0 + mgh2

In this idealized example the kinetic energy of the ball at ground level is converted into work in raising the ball to h2 where its gravitational potential energy has been increased by mg (h2 - h1). As the ball falls back to the ground level h1, this gravitational potential energy is converted back into kinetic energy and its total energy at h1 again is 1/2mv12 + mgh1. In this chain of events the kinetic energy of the ball is unchanged at h1; thus the work done on the ball by the force of gravity acting on it in this cycle of events is zero. This system is said to be a conservative one.

Varying degrees of conversion in real systems

Although the total amount of energy in an isolated system remains unchanged, there may be a great difference in the 6 ... of different forms of energy. Many forms of energy, in theory, can be transformed completely into work or into other forms of energy. This is true for mechanical energy and electrical energy. The random motions of constituent parts of a material associated with thermal energy, however, represent energy that is not available completely for conversion into directed energy.

The French engineer Sadi Carnot described (in 1824) a theoretical power cycle of maximum 7 ... for converting thermal into mechanical energy. He demonstrated that this efficiency is determined by the 8 ... of the temperatures at which heat energy is added and waste heat is given off during the cycle. A practical engine operating on the Carnot cycle has never been devised, but the Carnot cycle determines the maximum efficiency of thermal energy conversion into any form of directed energy. The Carnot criterion renders 100 percent efficiency impossible for all heat engines. In effect, it constitutes the basis for what is now the second law of thermodynamics.

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I. Use the terms to fill in the gaps (1 – 8): Conservative, quality, conservation, magnitude, potential, efficiency, kinetic, velocity, speed.

II. Decide whether the statements are true or false:

1.To show how energy is converted you should throw the ball horizontally.

2.The kinetic energy Ek of the ball increased steadily at the highest

point.

3.Potential energy is stored in hidden form and can be drawn upon to

do work.

4.In the ideal system we consider the friction of the air.

5.As the ball falls back to the ground level, gravitational potential energy is converted back into kinetic energy.

6.Although the total amount of energy in an isolated system remains unchanged, there may be a great difference in the quantity of energy.

7.A practical engine operating on the Carnot cycle has never been

created.

8.The Carnot cycle determines the maximum efficiency of conversion of any type of energy into any form of directed energy.

III. Think of the following:

- Explain what the basic difference between kinetic and potential energy is.

- Why does the ball stay at rest at the highest point? - What does the law of conservation of energy state?

- What is a conservative system? Why is the system described called a conservative one? Can you give any more examples of conservative systems?

- In what way are the real and the ideal systems different?

- Why is the example with the ball supposed to be idealized? - According to Carnot, what is efficiency determined by?

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IV. Summarize the article in 7 – 8 sentences.

V. Additional questions:

1.What is understood by mechanical energy? What are the conditions for its being constant?

2.Describe how energy is converted on the example of water behind a dam flowing to lower levels through turbines (hydroelectricity).

3.Why is the mechanical energy of the Earth-Moon system nearly

constant?

4.How does the energy change back and forth when you are riding a roller-coaster? What energy prevails when you are:

Unit VII. Vibrations

AVibration is periodic back-and-forth motion of the particles of an elastic body or medium, commonly resulting when almost any physical system is displaced from its 1 _____ condition and allowed to respond to the forces that tend to restore equilibrium.

BVibrations fall into two categories: 2 ____ and forced. Free vibrations occur when the system is disturbed momentarily and then allowed to move without restraint. A classic example is provided by a weight suspended from a spring. In equilibrium, the system has minimum energy and the weight is at rest. If the weight is pulled down and released, the system will respond by vibrating vertically.

CThe vibrations of a spring are of a particularly simple kind known as simple 3 ____ motion (SHM). This occurs whenever the disturbance to the

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system is countered by a restoring force that is exactly proportional to the degree of disturbance. In this case, the restoring force is the tension or 4 ____ in the spring, which (according to Hooke's law) is proportional to the displacement of the spring. In simple harmonic motion, the periodic 5 ____ are of the mathematical form called sinusoidal.

D Most systems that suffer small disturbances counter them by exerting some form of restoring force. It is frequently a good approximation to suppose that the force is proportional to the disturbance, so that SHM is, in the limiting case of small disturbances, a generic feature of vibrating systems. One characteristic of SHM is that the 6 ____ of the vibration is independent of its amplitude. Such systems therefore are used in regulating clocks. The oscillation of a 7 ______, for instance, approximates SHM if the amplitude is small.

E A universal feature of free vibration is 8 _____. All systems are subject to frictional forces, and these steadily sap the energy of the vibrations, causing the 9 _____ to diminish, usually exponentially. The motion is therefore never precisely 10 _____. Thus, a swinging pendulum, left undriven, will eventually return to rest at the equilibrium (minimum-energy) position.

F 11 _____ vibrations occur if a system is continuously driven by an external agency. A simple example is a child's swing that is pushed on each downswing. Of special interest are systems undergoing SHM and driven by sinusoidal forcing. This leads to the important phenomenon of 12 ______. Resonance occurs when the driving 13 ______ approaches the natural frequency of free vibrations. The result is a rapid take-up of energy by the vibrating system, with an attendant growth of the vibration amplitude. Ultimately, the growth in amplitude is limited by the presence of damping, but the response can, in practice, be very great. It is said that soldiers marching across a bridge can set up

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resonant vibrations sufficient to destroy the structure. Similar folklore exists about opera singers shattering wine glasses.

G Electric vibrations play an important role in electronics. A circuit containing both inductance and capacitance can support the electrical equivalent of SHM involving sinusoidal current flow. Resonance occurs if the circuit is driven by 14 ______ current that is matched in frequency to that of the free oscillations of the circuit. This is the principle behind tuning. For example, a radio receiver contains a circuit, the natural frequency of which can be varied. When the frequency matches that of the radio transmitter, resonance occurs and a large alternating current of that frequency develops in the 15_______. In this way, resonating circuits can be used to filter out one frequency from a mixture.

H In musical instruments, the motion of strings, membranes, and air columns consists of a superposition of SHM's; in engineering structures, vibrations are a common, though usually undesirable, feature. In many cases, complicated periodic motions can be understood as the superposition of SHM at many different frequencies.

I Fill in the terms: Frequency, pendulum, equilibrium, alternating, free, forced, compression, amplitude, harmonic, damping, circuit, resonance, sinusoidal, oscillations, period. Think of the title for each paragraph.

II Say whether the statements are true or false:

-The vibrations of a spring are immensely complex.

-Free vibrations occur when the system moves without restraint.

-A restoring force is directly proportional to the degree of disturbance.

-The frequency of the vibration is independent of its amplitude.

-Friction is the reason for damping.

-A child’s swing is an example of free vibration.

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-Resonance occurs with the growth of the vibration amplitude.

-Resonance helps to separate one frequency from a mixture.

-In many cases vibrations should be avoided when it is possible.

III. Speaking

-Describe the motion of a pendulum using the pictures:

- In picture C you can see the schematic diagram of a 1924 Anderson-Wood torsion pendulum seismograph, the type used by seismologist Charles F. Richter to define his earthquake magnitude scale. Using any extra sources, describe the principle of its work.

- Get ready with the abstract. Make sure it is short (7 – 8 sentences) and contains only the basic information.

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Unit VIII

I. Arrange the paragraphs below to make two separate articles under the following titles: “What is Sound?” and “Basic Interference Patterns”. Which of the texts should come after the paragraph “Properties of a wave”?

A Waves travel as a transfer of energy within a medium - a wave is essentially a sequence of compressions (moving together) and rarefactions (moving apart) of molecules.

B However, if one of the speakers is moved half a wavelength further away from the listener (in this example, half a metre), then an entirely different effect will be observed. The rarefactions (troughs) of one of the waves will now reach the listener at the same time as the compressions (peaks) of the other. Following the same additive principles as before, the variations in air pressure now cancel each other out. This is destructive interference, when two signals are perfectly out of phase. Noise-cancelling headphones use this technique to reduce unwanted ambient sounds.

C As the distances are equal, the compressions of each wave (peaks) are reaching the listener at the same time. A process of linear superposition then occurs - the combined pattern of the waves is the sum of the individual wave patterns. As the pressure of both waves is waxing at the same time, the pressure fluctuations where the two waves meet exhibits twice the amplitude of the individual waves. This means that the waves are exactly in phase - creating a condition known as constructive interference.

D Put simply, sound is vibration. As such, sound can pass through many different substances - in fact, it requires the presence of a medium. Sound cannot travel in a vacuum. The most common medium within which we perceive sound

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is, of course, air. Various movements around us cause vibrations in air molecules, and this sound energy is transported outwards as waves. Much in the same way as waves move across the surface of a pond, so does sound move through the air. Once the action that caused the waves ceases, then the pond will gradually return to its original position, as if nothing had happened.

E Sound also travels through water, and can travel through solids too, such as wood, brick, iron and so on. The ease with which it can do so depends upon the composition of the medium, and the nature of the sound itself. Different frequencies can move more easily through certain substances than others, and some frequencies travel further than others. Approaching a concert, for example, you may well hear the thumping of the bass drum before all else.

F A practical example can illustrate how sound waves interfere with one another. We can set up two loudspeakers located at a distance of three metres from the listener. The speakers are producing the same tone, with a wavelength of one metre. The speakers’ diaphragms are also moving in synchrony - that is, they both move in and out at the same time.

G Sound, like all waves, rarely occurs in isolation. Every day, the world around us is awash with sounds, from the rustling of leaves to the roaring of engines. All of these sounds interact with one another, and with all the elements and obstacles of their environment. Hence, the same sound sources can sound vastly different depending upon the position of the listener in relation to them.

Properties of a Wave

II. Fill in the gaps with the terms given to you: pitch, amplitude, period, wavelength, frequency.

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A number of properties are commonly used to define a wave. The 1 _____

may be defined as the horizontal distance between two successive equivalent points on the waveform. For convenience, these two points are usually taken at peaks (highest point) or troughs (lowest). The 2 _____ then is the time it takes for the wave to complete one full cycle.

The 3 ______ equates to the height of the wave; loud sounds produce waves of higher amplitude. The loudness or intensity of sound is measured in decibels; however, it must be remembered that this is not a linear or absolute scale of measurement. The lowest threshold of human hearing is set at zero; a decibel is sometimes defined as the smallest change in volume discernable by a human. For a doubling in volume, the decibel level goes up by six. Within this scale, normal speech levels fit in at around 60dB.

The frequency of a wave is the number of cycles that pass a set point in a second, and is measured in Hertz (Hz). Frequency is intimately connected to

_____, although they are not exactly synonymous; the A above middle C is a vibration at a rate of 440 Hz. Lower frequency vibrations are perceived as being lower in pitch, and higher frequencies seem higher in pitch.

III. Discussion:

1.How can you characterize the language of these paragraphs? Is it totally scientific? Explain your viewpoint. Think where this article could be published.

2.Who might need this information and for what purpose?

3.Who else might want to use the information about sound and hearing?

4.Give a scientific definition of sound. What substances can it travel through? What properties are used to define a wave?

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