问题 计算题

(10分)如图,质量m=2kg的物体静止于水平地面的A处,A、B间距L=20m。用大小为30N,沿水平方向的外力拉此物体,经t0=2s拉至B处。(已知cos370=0.8,sin370=0.6。取g=10m/s2)

(1)求物体与地面间的动摩擦因数μ;

(2)用大小为30N,与水平方向成37°的力斜向上拉此物体,使物体从A处由静止开始运动并能到达B处,求该力作用的最短时间t。

答案

(1)0.5(2)1.03s

题目分析:(1)根据匀变速直线运动的位移公式可以求得物体的加速度的大小,在根据牛顿第二定律可以求得摩擦力的大小,进而可以求得摩擦因数的大小;

(2)当力作用的时间最短时,物体应该是先加速运动,运动一段时间之后撤去拉力F在做减速运动,由运动的规律可以求得时间的大小.

(1)物体做匀加速运动  

,由牛顿第二定律F-f=ma,f=30-2×10=10N        

所以 

即物体与地面间的动摩擦因数μ为0.5;

(2)设F作用的最短时间为t,小车先以大小为a的加速度匀加速t秒,撤去外力后,以大小为a′的加速度匀减速t′秒到达B处,速度恰为0,

由牛顿定律  Fcos37°-μ(mg-Fsin37°)=ma 

由于匀加速阶段的末速度即为匀减速阶段的初速度,因此有

at=a′t′,

即该力作用的最短时间为1.03s.

点评:分析清楚物体的运动的过程,分别对不同的运动的过程列示求解即可得出结论.

阅读理解

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It is said that e-schoolbags are going to be brought into use in Chinese middle schools soon.An experiment with several hundred e-schoolbags will begin in seven cities: Beijing, Shanghai, Dalian, Shenzhen, Chongqing, Taiyuan and Shijiazhuang.And the e-schoolbags are going to cover all over China if the experiment shows to be successful.In fact, the e-schoolbag should perhaps be called an e-notebook.It is a small hand-held computers for school students.

Heavy schoolbags have long been a serious problem for school students.The average schoolbag of middle school students weighs up to 5 kilos.But the e-schoolbag will change everything.It is much lighter than a usual schoolbag, weighing under 1 kilo.Also, it is no bigger than a usual book, but it can still hold all the things for study, such as a textbook, a notebook and exercise book.They could be made into chips that are as small as a stamp.The students can read the text page by page on the screen.They can still take notes using a special electronic pen.If they want to know the meaning or the pronunciation of a new word, or even e-mail their teachers, it’s just a press of a button.

In some foreign countries, it is becoming common.But it is hard to tell when people will receive this new form of study.Some say that e-textbooks can easily be broken, some say that it is not good to students’ eyesight to look at the screen for long.But only time will tell.

69.The e-schoolbag has many functions EXCEPT ________.

A.It can pronounce the words.                B.It can send e-mails.

C.It can be used as a dictionary.              D.It can phone somebody.

70.According to the passage, the e-schoolbags ­­­________.

A.are not heavy as before but still large 

B.are used as an experiment in Tianjin

C.are made of metal                 

D.are going to cover China if it shows to be successful

71.Which of the following statement is TRUE?

A.All the students in China will use this new schoolbag.

B.We can do a lot of work based on the programmes of the chips.

C.The e-notebooks are very perfect.

D.We will not make notes because the e-notebook can make all by itself.

填空题