题干

A、B两列火车,在同一轨道上同向行驶,A车在前,其速度vA=10m/s,B车在后,速度vB=30m/s,因大雾能见度很低,B车在距A车x0=75m时才发现前方有A车,这时B车立即刹车,但B车要经过180m才能停止,问:B车刹车时A车仍按原速率行驶,两车是否会相撞?若会相撞,将在B车刹车后何时相撞?若不会相撞,则两车最近距离是多少?

上一题 下一题 0.0难度 选择题 更新时间:2018-05-16 10:05:08

答案(点此获取答案解析)

解:假设不相撞,依题意画出运动过程示意图,如图所示

B车做匀减速运动的加速度 <

同类题1

阅读理解

    About 30 years ago, I left Cuba for the United States with my son. After getting settled finally in Brunswick, New Jersey, I enrolled(注册) my son in kindergarten. Several weeks later, my son’s teacher asked me to meet him at his office.

    In the teacher’s office, and exchange of greetings was followed by his questions: “Is your son mentally retarded(弱智的)?Does he suffer from any kind of mental disability?”

    Was he talking about my wonderful Scola? NO, no, it can’t be. What a helpless, lonely moment! I told him that Scola was a quiet, sweet little boy, instead. I asked him why he was asking me all these questions.

    My son could not follow the teacher’s directions, he told me, and thus, Scola was disrupting the class, Didn’t he know my son did not speak English yet?

    He was angry; “Why hasn’t your son been taught to speak English? Don’t you speak English at home?”

    No, I didn’t speak English at home, I replied. I was sure my son would learn English in a couple of months, and I didn't want him to forget his native language. Well, wrong answer! What kind of person would not speak in English to her son at home and at all time? “Are you one of those people who come to this country to save dollars and send them back to their country, never wanting to be a part of this society?”

    Needless to say, I tried to tell him I was not one of “those people.” Then he told me the meeting was over, and I left.

    As I had expected, my son learned to speak English fluently before the school year was over. He went on to graduate from college and got a job, earning close to six figures. He travels widely and leads a well-adjusted, contented life. And he has benefited from being bilingual(双语的)。

    Speaking more than one language allows people to communicate with others; it teaches people about other cultures and other places—something very basic and obviously lacking in the “educator” I met in New Jersey.