题目内容
The gravitational pull of the earth and moon is important to us as we attempt to conquer more and more of outer space. Here’s why.As a rocket leaves the earth, the pull of the earth on it becomes less and less as the rocket roars out into space. If you imagine a line between the earth and the moon, there’s a point somewhere along that line, nearer to the moon than to the earth, at which the gravitational pull of both the earth and the moon on an object is just about equal. An object placed on the moon side of that point would be drawn to the moon. An object placed on the earth side of that point would be drawn to the earth. Therefore, a rocket need be sent only to this "point of no return" in order to get it to the moon. The moon’s gravity will pull it the rest of the way.The return trip of the rocket to the earth is, in some ways, less of a problem. The earth’s gravitational field reaches far closer to the moon than does the moon’s to earth. Thus, it will be necessary to fire an earthbound rocket only a few thousand miles away from the moon to reach a point where the rocket will drift to earth under the earth’s gravitational pull.The problem of rocket travel is not so much concerned with getting the rocket into space as it is with guiding the rocket after it leaves the earth’s surface. Remember that the moon is constantly circling the earth. A rocket fired at the moon and continuing in the direction in which it was fired would miss the moon by a wide: margin and. perhaps continue to drift out into space until "captured" in another planet’s gravitational field. To reach the moon, a rocket must be tired toward the point where the moon will be when the rocket has traveled the: required distance. This requires precise calculations of the speed and direction of the rocket and of the speed and direction of the moon.For a rocket to arrive at a point where the moon’s gravity will pull it the rest of the way, it must reach a speed called the velocity of escape. This speed is about 25, 000 m/les (about 40,200 kilometers) per hour. At a speed less than this, a rocket will merely circle the earth in an orbit and eventually fall back to the earth. The velocity of escape is about （）
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Noise constitutes a real and present danger to people’s health. Day and night, at home, at work, and at play, noise can produce serious (62) and psychological stress. No one is (63) to this stress. Though we seem to (64) to noise by ignoring it, the ear, in fact, never closes and the body still (65) sometimes with extreme tension--to a strange sound in the night.The (66) we feel when faced with noise is the most common outward (67) of the stress building up inside us. The more (68) and more serious health hazards (69) with the stress caused by noise traditionally hay been given much (70) attention. (71) , when we are annoyed or made irritable by noise, we should consider, these symptoms fair warning (72) other things may be happening to us, some of which may be damaging our health.(73) many health hazards of noise, hearing loss is the most clearly (74) and measurable by heal professionals. The other hazards are harder to (75) . For many of us, there may be a risk that (76) to the stress of noise increases susceptibility to disease and infection. The more (77) among us may experience noise a (78) factor in heart problems and other diseases. Noise that causes annoyance and irritability in health persons may have more serious consequences for those already ill in mind or body. (79) , the link between noise and many disabilities or diseases has not yet been (80) demonstrated, and we (81) to dismiss annoyance caused by noise as a price to pay for living in the modem world. 70（）
水流流经变径管，两断面直径之比d1/d2=2，流量不变，则两断面雷诺数之比Re1/Re2为( )。
The gravitational pull of the earth and moon is important to us as we attempt to conquer more and more of outer space. Here’s why.As a rocket leaves the earth, the pull of the earth on it becomes less and less as the rocket roars out into space. If you imagine a line between the earth and the moon, there’s a point somewhere along that line, nearer to the moon than to the earth, at which the gravitational pull of both the earth and the moon on an object is just about equal. An object placed on the moon side of that point would be drawn to the moon. An object placed on the earth side of that point would be drawn to the earth. Therefore, a rocket need be sent only to this "point of no return" in order to get it to the moon. The moon’s gravity will pull it the rest of the way.The return trip of the rocket to the earth is, in some ways, less of a problem. The earth’s gravitational field reaches far closer to the moon than does the moon’s to earth. Thus, it will be necessary to fire an earthbound rocket only a few thousand miles away from the moon to reach a point where the rocket will drift to earth under the earth’s gravitational pull.The problem of rocket travel is not so much concerned with getting the rocket into space as it is with guiding the rocket after it leaves the earth’s surface. Remember that the moon is constantly circling the earth. A rocket fired at the moon and continuing in the direction in which it was fired would miss the moon by a wide: margin and. perhaps continue to drift out into space until "captured" in another planet’s gravitational field. To reach the moon, a rocket must be tired toward the point where the moon will be when the rocket has traveled the: required distance. This requires precise calculations of the speed and direction of the rocket and of the speed and direction of the moon.For a rocket to arrive at a point where the moon’s gravity will pull it the rest of the way, it must reach a speed called the velocity of escape. This speed is about 25, 000 m/les (about 40,200 kilometers) per hour. At a speed less than this, a rocket will merely circle the earth in an orbit and eventually fall back to the earth. This short passage is mainly about （）
西方人普遍认为“13”这个数字是凶险或不吉利的，常以( )代替。
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