在质量为M的电动机上，装有质量为ｍ的偏心轮，偏心轮转动的角速度为ω，当偏心轮重心在转轴正上方时，电动机对地面的压力刚好为零；则偏心轮重心离转轴的距离多大? 在转动过程中，电动机对地面的最大压力多大?
解：设偏心轮的重心距转轴ｒ
偏心轮等效为用一长为ｒ的细杆固定质量为ｍ（轮的质量）的质点，绕转轴转动
轮的重心在正上方时，电动机对地面的压力刚好为零
则此时偏心轮对电动机向上的作用力大小等于电动机的重力
即：F=Mｇ①
根据牛顿第三定律，此时轴对偏心轮的作用力向下
大小为F=M ｇ
其向心力为：F+mg=m ω2r ②
由①②得偏心轮重心到转轴的距离为：
r= （M+m）g/ （mω2） ③
当偏心轮的重
心转到最低点时，电动机对地面的压力最大
对偏心轮有：F'-mg=mω2r ④
对电动机，设它所受支持力为FN，FN=F'+Mg ⑤
由③、④、⑤解得FN=2（M+m）g
由牛顿第三定律得，电动机对地面的最大压力为2（M+m）g
| 完形填空。 | ||||
| One will feel happy when others flatter (奉承) him in his face. It is said that the best way of flattering someone is to give him a "top hat" 1 . A student was going to leave the capital to become 2 official (官员) in a city far away. Before he started, he came to say 3 to his teacher. "It is 4 job to be a good official." his teacher said. "You must be strict 5 yourself and never be careless." "Don't worry about me, sir." The student answered. "I have already 6 anyone hundred top hats, which will 7 those people quite happy." "But we are really gentlemen! 8 could a real gentleman do such a thing?" His teacher was a bit 9 . "Never forget 10 I taught you in class!" " 11 are always right, sir. I also hate such things. But, sir, 12 no one really gentleman like you can be seen in the world now." said the student. It seemed that he had to do so. 13 hearing this, the teacher was pleased. "What you said is true!" "I have 14 one top hat already. Now I have ninety-nine 15 ." the student said to his friend later on when he asked the student what he had talked with the teacher about. | ||||
|