题目

一个重40 N的物体在推力为18 N作用下在水平桌面上做匀速直线运动，它的质量是___kg，在运动过程中物体所受摩擦力为________N。 答案：【答案】4 18 【解析】由G=mg可得，物体的质量：m===4kg，因物体在水平桌面上做匀速直线运动时，水平方向受到的推力和摩擦力是一对平衡力，所以，运动过程中物体所受摩擦力：f=F=18N.Tim just moved to Greenburg. In fact, he was in Lincoln School just a few days. On the fourth day he was there, Bill and his friends asked Tim to have lunch with them. That made Tim really happy. Bill was a big wheel at school.It wasn’t long, however, before Tim began to see that being with Bill and his friends didn’t go well. They were always in trouble. They made others angry by asking them to do many things in the school or on the school bus. And they made troubles in class; Mrs. Jordon and the others began to worry when they saw Tim in class. They thought Tim would be a troublemaker too, just because he went around with Bill and his friends.There were some other students in the class whom Tim really wanted to get to know. Jack had some model planes and Ted liked the subject science as Tim did. On the other hand, it was a lot more fiin to be with Bill and his friends. It was hard for Tim to choose what to do.(    )    1. From the passage we can know_.A. Tim didn’t like the new placeB. Tim liked science very muchC. Tim was a troublemaker in classD. Mrs. Jordon didn’t like Tim(    )2.    The underlined word “wheel” in the passage means_.A. a teachers’ helper    B. a good driverC. an important person    D. a helpful monitor(    )    3.    One thing that made Tim pleased was_.A. he had lunch with Bill and his friendsB. he was in the new school just a few daysC. his new teacher liked him a lotD. he had many new friends (    )    4.    In Tim’s class,_.A. everyone went around BillB. there was someone who liked model planesC. Jack and Ted were his best friendsD. all the students liked to have lunch together(    )5. We can infer (推断）from the reading that_•A. Tim would move again after a few days in Lincoln  SchoolB. Mrs. Jordon was not interested in Tim at allC. Bill and his friends would not make others angry any moreD. Tim had to leave Bill and his friends if he wanted to make other friends