题目

建筑工地上有一台大型汽车吊，把100t建筑材料从地面吊到15m高处，汽车吊做的有用功为 ____________J．在这过程中，汽车吊消耗柴油的质量为2.0kg，则完全燃烧2.0kg柴油释放的内能为 J，该过程中汽车吊的效率为 %．（柴油的热值为q=4.3×107 J/kg，取g=10N/kg） 答案：【答案】1.5×107；8.6×107；17.4。【解析】由于汽车吊所做的有用功就是把重物提高了，故克服重物重力做的功就是有用功，即W有=Gh=100×103kg×10N/kg×15m=1.5×107J；由柴油的热值与质量，可计算出其完全燃烧放出的热量Q=mq=2.0kg×4.3×107 J/kg=8.6×107J；故汽车吊的效率为η==17.4%。Two years ago,my family moved to a new city and I had to study in a new school. As I had few friends there,I felt lonely. Then I met Tony. The first time I saw him,he was standing in the center of a group of students,telling jokes. The children around laughed from time to time. Tony knew about my problem. He asked me to play basketball with his friends and helped me with my studies. We soon became good friends.About a year ago,however,Tony's father was killed in an accident. As a result,his family had to move to a small house. Tony changed into a different person. He became silent and he even lost his interest in studies. Several times,I invited him to go out and play basketball with me,but he refused. I wanted to help him,but I didn't know what to do.Then something strange happened in my class. Two classmates lost the money in their schoolbags.Last Friday,just before the P. E. lesson,I went back to the classroom to get my running shoes I would use. The door was half open,and I went in. To my astonishment, I saw Tony was searching one of my classmates' schoolbags. I was shocked...(   ) 26. What kind of boy was Tony at the beginning of the story?   A.He was a clever boy.   B.He was a poor boy.   C.He was a popular boy.   D.He was a shy boy.(   ) 27. Why did the writer become friends with Tony?   A.Because Tony helped him do his homework.   B.Because Tony was kind to him.   C.Because Tony liked playing basketball.   D.Because Tony told jokes.(   ) 28. What caused the change of Tony?   A.His father's accident.   B.His strange character.   C.The move of his family.   D.The loss of his interest in playing with his friends.(   ) 29. How did the writer feel when he saw Tony's change?   A.He felt angry.   B.He felt surprised.   C.He felt unhappy.   D.He felt worried.(   ) 30. The underlined word  " astonishment" means        .   A. sadness   B. surprise   C.  disappointment   D. joy