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

A

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Do not remember only grammar when you are learning English. Try to read stories in English and speak in English as often as you can. A few days before the exam you should go to bed early. Do not study late at night. Before you start the exam, read carefully over the exam paper. Try to understand the exact meaning of each question before you pick up your pen to write. When you have finished your exam, read your answers again. Correct(改正) the mistakes if there are any and make sure you have not missed anything out.

小题1:If you want to do well in an exam, you must       .

A.work hard every day in the year

B.study hard a few days before the exam

C.get up early a few days before the exam

D.keep doing your homework day and night小题2:It is      to go to bed late at night before the exam.

A.useful

B.important

C.not helpful

D.difficult小题3:Try your best to get the meaning of each question       .

A.after you write down your answers

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A.think it hard

B.be important

C.be wrong

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A.try to read as many English stories as you can

B.learn grammar well

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D.A, B and C

单项选择题