01-04-2020-Дедушенко И.С.Обучение чтению и устной речи на английском языке (1171845), страница 3
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Generations ofinventors have tried to create a machine, called a perpetualmotion machine that would run forever without fuel. Such amachine is not forbidden by Newton’s laws of motion, which arebuilt around the concepts of force and inertia. Force is free, andcan be multiplied indefinitely with pulleys, gears, or levers. Theprinciple of inertia seems even to encourage the belief that acleverly constructed machine might not ever run down.Here is an example: The magnet draws the ball to the top ofthe ramp, where it falls through the hole and rolls back to thebottom. The example shows one of the innumerable perpetualmotion machines that have been proposed.
The reason thisexample doesn’t work is not much different from the reason allthe others have failed. Consider the machine. Even if we assumethat a properly shaped ramp would keep the ball rolling smoothlythrough each cycle, friction would always be at work. Thedesigner imagined that the machine would repeat the samemotion over and over again, so that every time it reached a givenpoint its speed would be exactly the same as the last time. Butbecause of friction, the speed would actually be reduced a littlewith each cycle, until finally the ball would no longer be able tomake it over the top.Friction has a way of creeping into all moving systems. Therotating earth might seem like a perfect perpetual motionmachine, since it is isolated in the vacuum of outer space withnothing to exert frictional forces on it. But in fact our planet’srotation has slowed drastically since it first formed, and the earthcontinues to slow its rotation, making today just a little longerthan yesterday.
The very subtle source of friction is the tides. Themoon’s gravity raises bulges in the earth’s oceans, and as theearth rotates the bulges progress around the planet. Where thebulges encounter land, there is friction, which slows the earth’srotation very gradually.(2197)16Additional vocabularyMemorize the terminology and use it while discussing thetasks.Energy (E) — a numerical scale used to measure heat, motion,or other properties that would require fuel or physical effort to putinto an object; a scalar quantity with units of joules (J).Power (P) — the rate of transferring energy; a scalar quantitywith units of watts (W).Kinetic energy (KE) — the energy an object possessesbecause of its motion.Heat — the energy that an object has because of itstemperature. Heat is different from temperature because an objectwith twice as much mass requires twice as much heat to increaseits temperature by the same amount.Temperature — what a thermometer measures.
Objects left incontact with each other tend to reach the same temperature.Temperature is essentially a measure of the average kineticenergy per molecule.W — watts, the SI unit of power; equivalent to J/s.Q — the amount of heat transferred into or out of an object.Text 2B. EnergyRead and translate the text with a dictionary.The analysis based on friction is somewhat superficial,however. One could understand friction perfectly well and yetimagine the following situation.Astronauts bring back a piece of magnetic ore from the moonwhich does not behave like ordinary magnets. A normal barmagnet attracts a piece of iron essentially directly toward it, andhas no left- or righthandedness. The moon rock, however, exertsforces that form a whirlpool pattern around it. NASA goes to amachine shop and has the moon rock put in a lathe and machineddown to a smooth cylinder. If we now release a ball bearing on17the surface of the cylinder, the magnetic force whips it aroundand around at ever higher speeds.
Of course there is somefriction, but there is a net gain in speed with each revolution.Physicists would lay long odds against the discovery of such amoon rock, not just because it breaks the rules that magnetsnormally obey but because, like the alchemists, they havediscovered a very deep and fundamental principle of naturewhich forbids certain things from happening.The first alchemist who deserved to be called a chemist wasthe one who realized one day: “In all these attempts to create goldwhere there was none before, all I’ve been doing is sending thesame atoms back and forth among different test tubes.
The onlyway to increase the amount of gold in my laboratory is to bringsome in through the door.”We say that the number of grams of gold is a conservedquantity. In this context, the word “conserve” does not have itsusual meaning of trying not to waste something. In physics, aconserved quantity is something that you wouldn’t be able to getrid of even if you wanted to. Conservation laws in physics alwaysrefer to a closed system, meaning a region of space withboundaries through which the quantity in question is not passing.In our example, the alchemist’s laboratory is a closed systembecause no gold is coming in or out through the doors.A similar light bulb eventually lit up in the heads of thepeople who had been frustrated trying to build a perpetual motionmachine.
In perpetual motion machine a, consider the motion ofone of the balls. It performs a cycle of rising and falling. On theway down it gains speed, and coming up it slows back down.Having a greater speed is like having more money in yourchecking account, and being high up is like having more in yoursavings account. The device is simply sending funds back andforth between the two. Having more balls doesn’t changeanything fundamentally.Not only that, but friction is always draining off money into athird “bank account” heat.
The reason we rub our hands together18when we’re cold is that kinetic friction heats things up. Thecontinual buildup in the “heat account” leaves less and less forthe “motion account” and “height account,” causing the machineeventually to run down.These insights can be distilled into the following basicprinciple of physics: the law of conservation of energy. It ispossible to give a numerical rating, called energy, to the state of aphysical system. The total energy is found by adding upcontributions from characteristics of the system such as motion ofobjects in it, heating of the objects, and the relative positions ofobjects that interact via forces. The total energy of a closedsystem always remains constant. Energy cannot be created ordestroyed, but only transferred from one system to another.The moon rock story violates conservation of energy becausethe rock cylinder and the ball together constitute a closedsystem.
Once the ball has made one revolution around thecylinder, its position relative to the cylinder is exactly the sameas before, so the numerical energy rating associated with itsposition is the same as before. Since the total amount of energymust remain constant, it is impossible for the ball to have agreater speed after one revolution. If it had picked up speed, itwould have more energy associated with motion, the sameamount of energy associated with position, and a little moreenergy associated with heating through friction.
There cannot bea net increase in energy.(3436)Text 2C. MomentumRead and render the text. Write a short summary.It should also be noted that conservation of momentum is nota consequence of Newton’s laws, as is often asserted intextbooks. Newton’s laws do not apply to light, and thereforecould not possibly be used to prove anything about a concept asgeneral as the conservation of momentum in its modern form.19Einstein played a role in two major changes in themomentum concept in the 1900’s. First Einstein showed that theequation p = mv would not work for a system containing objectsmoving at very high speeds relative to one another.
He came upwith a new equation, to which mv is only the low-velocityapproximation.The second change, and a far stranger one, was the realizationthat at the atomic level, motion is inescapably random. Theelectron in a hydrogen atom doesn’t really orbit the nucleus, itforms a vague cloud around it.
It might seem that this wouldprove nonconservation of momentum, but in fact the randomwanderings of the proton are exactly coordinated with those ofthe electron so that the total momentum stays exactly constant. Inan atom of lead, there are 82 electrons plus the nucleus, allchanging their momenta randomly from moment to moment, butall coordinating mysteriously with each other to keep the vectorsum constant. In the 1930s, Einstein pointed out that the theoriesof the atom then being developed would require this kind ofspooky coordination, and used this as an argument that there wassomething physically unreasonable in the new ideas.Experiments, however, have shown that the spooky effects dohappen, and Einstein’s objections are remembered today only as ahistorical curiosity.Example 1: The rifle and bullet have zero momentum andzero kinetic energy to start with.
When the trigger is pulled, thebullet gains some momentum in the forward direction, but this iscanceled by the rifle’s backward momentum, so the totalmomentum is still zero. The kinetic energies of the gun and bulletare both positive scalars, however, and do not cancel.The total kinetic energy is allowed to increase, becausekinetic energy is being traded for other forms of energy. Initiallythere is chemical energy in the gunpowder. This chemical energyis converted into heat, sound, and kinetic energy. The gun’s“backward” kinetic energy does not refrigerate the shooter’sshoulder!20Example 2: As the moon completes half a circle around theearth, its motion reverses direction.
This does not involve anychange in kinetic energy, and the earth’s gravitational force doesnot do any work on the moon. The reversed velocity vectordoes, however, imply a reversed momentum vector, soconservation of momentum in the closed earth-moon systemtells us that the earth must also change its momentum.
In fact,the earth wobbles in a little “orbit” about a point below itssurface on the line connecting it and the moon. The two bodies’momentum vectors always point in opposite directions andcancel each other out.(2451)Additional vocabularyMemorize the terminology and use it while discussing thetasks.Periodic motion — motion that repeats itself over and over.Period — the time required for one cycle of a periodic motion.Frequency — the number of cycles per second, the inverse ofthe period.Amplitude — the amount of vibration, often measured fromthe center to one side; may have different units depending on thenature of the vibration.Simple harmonic motion — motion whose x–t graph is a sinewave.T — period;f — frequency;A — amplitude.Discussion1. If all the air molecules in the room settled down in a thinfilm on the floor, would that violate conservation of momentumas well as conservation of energy?212.
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