首页
登录
职称英语
An airliner travelling from London to New York may take from five to fifteen
An airliner travelling from London to New York may take from five to fifteen
游客
2024-12-23
27
管理
问题
An airliner travelling from London to New York may take from five to fifteen hours to cross the Atlantic, while a space capsule makes one complete circuit of the earth in about ninety minutes. The sequence of events is very similar in both types of flight: the vehicle must take off, climb to a suitable height, fly in the right direction at a relatively constant speed for an appropriate time, descend, and land at the destination. Yet although flights to New York are routine affairs which almost anyone may safely undertake, a flight into space is a hazardous adventure for which only a few selected men are at present considered suitable.
The most obvious difference between an aircraft and a space vehicle is that of speed, but this alone cannot account for the greater stamina required of astronauts. The human body is unaffected by speed alone and we are normally quite unconscious of the earth’s rotation on its axis, or of its rapid motion around the Sun. Of much greater importance is the rate at which the final speed is achieved, for the body is extremely sensitive to alterations of velocity, or accelerations, especially if they are sudden. An airliner can take a comparatively long time to reach its cruising speed of, say, 400mph, and its passengers will experience acceleration only to a mild degree. The space capsule, however, must be hurled through the atmosphere to reach its final speed of 18,000 mph as quickly as possible, and the acceleration applied by the launching rocket must be correspondingly high. The first problem of manned space flight, therefore, is to match the performance of the rocket to the body’s tolerance for
acceleration, and this naturally involves a study of the physiological effects of acceleration.
Like all other accelerations, gravity acts upon objects to produce a force, and this force is experienced as weight, or as pressure. It is usual and convenient to regard the earth’s gravity as a standard unit, referred to as lg, and also to use the expressions "force" and "acceleration" as interchangeable.
Most of our knowledge of the physiological effects of acceleration has come from studies on human centrifuges, in which acceleration is produced by rotation instead of by changing speed. It has been found that human tolerance is greatly affected by the direction in which the force acts. When the acceleration is applied in line with the long axis of the body, the early symptoms are merely of difficulty in lifting the arms and legs, and of being thrust down into the seat. If the acceleration is raised to 3g or so, vision becomes slightly misty or veiled. As the stress is increased further, the field of view contracts from the edges, until at about 4.5g only a small patch of central vision remains. With yet higher accelerations, even this small area is lost, and this is the state well known to fighter pilots as "black-out". Finally, at about 5.5g to 6g consciousness is lost.
The remedy follows logically: if tolerance depends upon the ability of the heart to push blood to the head, it should be possible to reduce the load by shortening the distance between heart and brain. Crouching, or bending the head forward, would be one solution, but an even more satisfactory result can be achieved by placing the body across the line of thrust. The effort needed to pump blood to the brain is then quite small, for the heavy fluid does not have to be lifted very far. In this position men have. withstood an acceleration of 17g for a period of three or four minutes without loss of consciousness.
Gravity and acceleration become important once more during the re-entry of the space capsule through the earth’s atmosphere. In this phase, all the speed acquired at the cost of so much fuel during the launch must be lost. Deceleration has exactly the same properties and physiological effects as acceleration, and the same precautions must be taken to avoid exceeding the limits of tolerance. This is why the American plan involves turning the whole capsule round shortly before re-entry, so that the man is again pressed back into his protective couch.
The highest, and shortest, deceleration of the entire flight comes at the moment of impact with the land or water. Here the last remnants of the speed must be lost very suddenly, and forces of up to 30g can easily accompany descent to an unyielding surface. The duration of this final insult is so short, however, that its physiological effects are negligible. No doubt the astronaut would regard the jolt as a welcome indication of his return to a normal 1g environment. [br] Which of the following statements is NOT true about acceleration?
选项
A、Acceleration can be expressed by lg, a standard unit for the earth’s gravity.
B、The body’s tolerance for acceleration depends only on velocity.
C、Acceleration is important during both the launch and the re-entry stages.
D、By changing the body’s position, we can increase the body’s tolerance for acceleration.
答案
B
解析
第三段第二句谈到地心引力可用1g来表示一个标准单位,而且“加速度”和“地心引力”两个术语可互换(interchangeable),因此A表述正确。第六段第一句说明加速度在宇航舱重返地球时再次变得重要,因而C正确。第五段讲述了减轻加速度带来的不良影响的方法是变换体位,因此D也正确。同时这也说明人体对加速度带来的不良影响的忍受力不仅仅取决于速度,也受到体位的影响,因此B不正确。
转载请注明原文地址:https://tihaiku.com/zcyy/3881613.html
相关试题推荐
[originaltext]London(dpa)-TheQueenhasimposedapalacebanontheuseo
[originaltext]London(dpa)-TheQueenhasimposedapalacebanontheuseo
London’sleisureindustryhopesforabonanzainJulyandAugust,thanksto
London’sleisureindustryhopesforabonanzainJulyandAugust,thanksto
London’sleisureindustryhopesforabonanzainJulyandAugust,thanksto
London’sleisureindustryhopesforabonanzainJulyandAugust,thanksto
WhatisthecapitalofBritain?A、LondonB、LiverpoolC、ManchesterD、BirminghamA
OnJuly7th,IwastravelinginLondon.Iwashavingbreakfastatahotelve
AnairlinertravellingfromLondontoNewYorkmaytakefromfivetofifteen
AnairlinertravellingfromLondontoNewYorkmaytakefromfivetofifteen
随机试题
Robert______whathewanted,buthegaveupatthelastminute.A、mightgetB、ma
ProfessorBhattandRobMeakinhavedevelopedamethodto______.[br]Howdoes
Everyboyandeverygirl______.(有权获得九年义务教育)isentitledto(或hastherighttor
甲公司许可乙公司使用其注册商标“阿凡”,乙公司必须()A.在使用“阿凡”商标
女,4岁,烧伤总面积70%,1小时后送入当地卫生院并准备转送上级医院治疗,当地卫
下列关于马可维茨投资组合分析模型的说法,不正确的是()。A、投资组合具有一个特
既能利尿通淋,又能清解暑热,收湿敛疮的药物是A.茯苓 B.猪苓 C.滑石
运用泊松分布设计的控制图是()。 A.单值X图B.p控制图 C.s控制图
利用概率样本数据推断全国居民人均消费支出,适用的统计学方法是()A.参数估计
下列关于从业人员安全培训的表述中,不符合《生产经营单位安全培训规定》规定的是(
最新回复
(
0
)