问题 选择题

为了求高层建筑的高度,从楼顶上自由下落一光滑小石子,除了知道当地的重力加速度以外,还需要知道下述哪个量①第一秒末的速度;②第一秒内的位移;③最后一秒的位移; ④最后一秒的初速度(  )

A.①或②

B.①或③

C.③或④

D.②或④

答案

①、知道第一秒末的速度,无法得到运动的总时间,也无法求出最后的速度,故无法求出高度,故①错误;

②、自由落体运动第一秒内的位移h=

1
2
gt2=5m;无法得到运动的总时间,也无法求出最后的速度,故无法求出高度,故②错误;

③、知道最后一秒的位移,由于最后一秒的加速度、时间也是已知的,先根据位移时间关系公式x=v0t+

1
2
at2求初速度,然后根据vt=v0+gt求出末速度,最后再根据v2=2gh求高度,故③正确;

④、知道最后一秒的初速度,由于最后一秒的加速度、时间也是已知的,故先根据vt=v0+gt求出末速度,然后再根据v2=2gh求高度,故④正确;

故选C.

多项选择题
阅读理解

Hobbs was an orphan(孤儿). He worked in a factory and every day he got a little money. Hard work changed him thin and weak. He wanted to borrow a lot of money to learn to paint pictures, but he did not think he could pay off the debts.One day the lawyer said to him, “One thousand dollars, and here is the money.” As Hobbs took the package of notes, he was very dumbfounded(目瞪口呆的). He didn’t know where the money came from and how to spend it. He said to himself, “I could go to find a hotel and live like a rich man for a few days; or I give up my work in the factory and do what I’d like to do: painting pictures I could do that for a few weeks, but what would I do after that? I should have lost my place of the factory and have no money to live on. If it were a little less money, I would buy a new coat, or a radio, or give a dinner to my friends. If it were more, I could give up the work and pay for painting pictures. But it’s too much for one and too little for the other.”

“Here is the reading of your uncle’s will(遗嘱),” said the lawyer, “telling what is to be done with this money after his death. I must ask you to remember one point. Your uncle has said you must bring me a paper showing exactly what you did with his money, as soon as you have spent it.”

“Yes, I see. I’ll do that.” said the young man.

小题1:He wanted to borrow money because he wanted to_____.

A.study abroad

B.work abroad

C.pay for the debts

D.learn to paint pictures小题2:Hobbs was dumbfounded on receiving the money because he thought_____.

A.the money was too much

B.the money was too little

C.he would be dismissed

D.the lawyer meant to punish him小题3:With the money he got, first Hobbs_____.

A.planned to have a happy life for a few days

B.decided to give up his work in the office

C.was to give a dinner to his friends

D.had no idea what to do小题4:What Hobbs was asked to do was_____.

A.to tell the lawyer what he did with the money after spending it

B.to read his uncle’s will

C.to tell the lawyer what was to be done with the money

D.nothing but to buy some pictures小题5:What Hobbs really loved to do was_____.

A.working in the factory

B.living in a fine hotel

C.painting pictures

D.saving the money