题目

声音在海水中的传播速度约为1530m/s，为了开辟新航道，探测船的船底装有回声探测仪，探测水下有无暗礁，探测船发出的信号经0.8s被探测仪接收，求障碍物到探测船的距离．（要求写出公式，代数计算） 答案：解：声音从探测船到障碍物的时间： t= {#mathml#}12{#/mathml#} ×0.8s=0.4s，∵v= {#mathml#}st{#/mathml#} ，∴s=vt=1530m/s×0.4s=612m．答：障碍物与探测船间的距离是612m　　To make an estimate means to guess the number of things in a group or guess a measurement of some kind. It sometimes means to make a calculation（计算）about the future, which is based not on exact data but on past experience. This kind of estimate has some basis, but it generally has to be changed and corrected later. 　　When you estimate a number, a size, a temperature or a weight, you do not count things one by one or use a measuring tool. You look at a group of things without counting them and try to get a rough, general idea of their number, you try to weigh something in your hand instead of on a balance. You feel hot and say“My temperature must be 38 degrees”or you look at a room and say that you estimate its area as 10×8 feet. 　　But in many situations in ordinary life we do not have time to measure things and must depend on estimates. Drivers do not have time to measure the distance between their own car and a car which is coming near. They must guess it; the guess or estimate will be based on their past experience. Experienced cooks do not trouble to measure the amount of salt they put into the soup or sugar they mix with flour. 　　 Suppose a man comes into the house and says“There are two inches of snow on the road”,his statement is only an estimate, but we believe it because we think that it is roughly correct. We put our boots on before we go into the snow. 　　For practical purposes in some situations it is safe to make estimates of quantity, but in other situations it is obviously dangerous. If it is your job to weigh drugs or make airplanes, do not depend on guessing but use an accurate instrument. (1) In our daily life the estimate of a value or an amount is usually made ________. [　　] A．by further data B．by instruments C．by counting D．according to one’s past experience (2) According to the passage, measuring instruments are not always used in measuring things in everyday life because ________. [　　] A．people generally do not carry them along B．it is more convenient to guess than to measure C．they are not always ready when people need them D．a general idea of things is-enough (3) We can know from the man’s statement on snow that ________. [　　] A．he wants to inform people that it’s snowing heavily B．it is necessary to depend on estimates C．his estimate is quite exact D．he has measured the level of the snow (4) The writer thinks that depending on estimates ________. [　　] A．may save us both time and money B．may bring us a sense of safety C．does not always keep us out of danger D．is practical but dangerous