问题 问答题

水平面上有一带圆弧形凸起的长方形木块A,木块A上的物体B用绕过凸起的轻绳与物体C相连,B与凸起之间的绳是水平的.用一水平向左的拉力F作用在物体B上,恰使物体A、B、C保持相对静止,如图.已知物体A、B、C、的质量均为m,重力加速度为g,不计所有的摩擦,则拉力F应为多大?

答案

设绳中张力为T,A、B、C共同的加速度为a,与C相连部分的绳与竖直线夹角为α,由牛顿运动定律,对A、B、C组成的整体有

F=3ma…①

对B有

F-T=ma…②

对C有

Tcosα=mg…③

Tsinα=ma…④

联立①②式得:

T=2ma…⑤

联立③④式得:

T2=m2(g2+a2)…⑥

联立⑤⑥式得:a=

3
3
g…⑦

利用①⑦式得F=

3
mg…⑧

拉力F应为

3
mg.

单项选择题
完形填空

回答问题(共3小题;每小题2分,满分6分)

阅读下面短文,根据第58至第60小题的具体要求,尽可能简要回答问题,并将答案转写到答题卡上。

In China, more and more middle school students are getting shorter sleeping time than before. More students sleep less than nine hours every night, because they have much homework to do. Some homework is given by their teachers, and some by their parents. Also, some students don’t know how to save time. They are not careful enough while they do their homework, so it takes them a lot of time. Some students spend too much time watching TV or playing computer games. They stay up very late. Some students have to get up early every morning on weekdays to get to school on time by bus of by bike . It can be a long way from home to school. It will probably make them sleepy during school hours.

Schools and parents should cut down some of the homework so that our children can enjoy more than nine hours of sleep every night for their health. For children, they should make the most (best use) of their time. When they have enough time for sleeping they will find it much better for both their study and health.

58. What make most students sleep less than nine hours every night? (回答词数不超过5个)

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59. According to the article, who else gives children some homework to do except their teachers?

(回答词数不超过5个)

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60. What is the possible result if the students have enough time for sleeping? (回答词数不超过14个)

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