题目

自地面垂直向上发射火箭,火箭的质量为m,试计算将火箭发射到距地面的高度为h时所做的功. 答案：思路分析:发射火箭需加外力,它必须克服地球的引力,用以克服地球引力的外力与地球引力大小相等,因此被积函数就是地球对物体的引力,问题得解.解:地球吸引物体的力为f(r)=mg,其中m表示物体的质量,R表示地球的半径,r表示地球中心到物体的距离.将［R,R+h］分成n等份,得Δri=,ri=R+i.故f(ri)=mg.故物体用以克服I met the old man at a cafe. “Did you hear the radio news yesterday?” he asked me. “No,” I replied. “Anything exciting?”“Exciting? NO! Something very sad. A group of hungry dogs killed and ate my best friend.”?“Oh, dear!” I cried. “How did it happen?”“He was working on the hillside when the dogs attacked him. When he didn't return, I went to the hillside and found…”“His body?” I asked. The old fellow drank half of his coffee. “No. I told you they were hungry dogs, didn't I? The big bones were lying every where. But they found this.” He pushed open a match box he was holding in his hand. In it was a man's thumb, lying on some white bloody material.?“This is my friend's right thumb. The dogs ate the rest of him”. The old man began to cry. He finished his coffee quickly and left the cafe. I drank mine and called the waiter. “I'll pay the gentleman's bill. His poor friend—how terrible!”“You've heard the news?” The waiter laughed. “Sure. There's a hole in the bottom of the match box. He put his own thumb through the hole. The blood is red ink, I believe. Is the story worth a cup of coffee, sir?”“But he held the box in his right hand.”“Yes, but listeners look into the box. They just can't take their sight off that terrible thing.”“And when he tells the story, he gets free cup of coffee!” I said, laughing. “Yes, sir, but only from strangers who come to this town, and, of course, he does us no harm!”?1.We can learn from this passage ________.?A.the writer came to the cafe for the first time?B.the old man made a living by telling jokes in the cafe?C.the writer had known about the old man before?D.what had happened to his best friend made the old man mad?2.Having heard the old man's story, the writer _______ .?A.couldn't help laughing immediately?B.showed great mercy upon him?C.didn't believe him at all?D.bought the old man another cup of coffee?3.It turned out that the thumb in the match box was actually __________.?A.the right thumb of his best friend?B.the thumb he stole from the dead body of an unknown person?C.something made of bloody white materials?D.his own right thumb?4.The waiter hadn't let out the truth of the old man's story earlier because _______ .A.he was nobody but the best friend of the old man?B.the old man wouldn't pay for his coffee if he did ?C.the waiter hadn't seen through the old man's trick?D.the old man helped the cafe in some way?5.As suggested by the passage, what might happen in the end?      ??A.The writer refused to pay the old man's bill.?B.The writer decided to make the trick known to the public.?C.More strangers would hear the old man's story.?D.The old man wouldn't visit the cafe any more.