问题 计算题

如图所示,细绳CO与竖直方向成一定角度,A、B两物体用跨过轻滑轮的细绳相连,两绳夹角为60°,不计绳与滑轮间的摩擦。已知物体B的重力为100N。

(1)若地面对物体B的支持力为80N,求物体B所受到的摩擦力大小;

(2)若物体B与水平面之间的动摩擦因数为μ=0.5,欲使物体B在水平面上不滑动,物体A的重力最大不能超过多少?(设最大静摩擦力等于滑动摩擦力)

(3)若在满足(2)中所述情景下,试求CO绳所受的拉力大小。

答案

(1)  (2)   (3)

题目分析:首先选择定滑轮为研究对象受力分析,由于滑轮受力平衡可以解出BC绳子与水平方向的夹角,然后对B物体受力分析,建立直角坐标系,把绳子拉力分解到坐标轴上,最后水平方向与竖直方向分别列方程求解即可.

解:(1)首先确定绳子与水平方向的夹角,选择滑轮为研究对象,受力分析如图:

由于滑轮受力平衡,所以OC的拉力应与另外两绳AC、BC的拉力合力为零,而AC、CB实际为同一绳子,所以拉力大小相同,根据力的合成AC、BC两力的合力大小等于OC的拉力大小,方向沿OC向外,即四边形ACBD是菱形,∠DCA=30°。

对B受力分析如图所示:

因为∠ACB=60°,所以F拉和水平方向的夹角为θ=30°

由于B物体受力平衡:水平方向:Tcosθ="f"    ①

竖直方向:Tsinθ+FN=GB  ②

已知物体B所受重力为100N,水平地面对物体B的支持力FN为80N,

由②解得:T="40N" ③

把③代入①解得:f=

(2)由于B物体受力平衡:

水平方向:Tcosθ="f"    ④

竖直方向:Tsinθ+FN=GB  ⑤

物体受到的最大静摩擦力为:,代入数值可得:由于物体A受力平衡,所以重力等于竖直向上的绳子拉力T=

(3) 绳子OC的拉力大小等于AC、BC两绳子拉力的合力:FCD=2×T×cos30°=

点评:此题难点在于确定倾斜绳子与水平方向的夹角,仍然需要通过受力分析,根据受力平衡来解决夹角问题.

完形填空

第二节:完形填空(共20小题;每小题1分,满分20分)

Not too long ago, an incident that happened at Walt Disney touched me greatly. A guest __21__ out of our Polynesian Village resort(度假胜地) at Walt Disney was asked how she __22__ her visit. She told the front-desk clerk she had had a(n) __23__ vacation, but was heartbroken about __24__ several rolls of Kodak color film(柯达胶卷) she had not yet __25__. At that moment she was particularly __26__ over the loss of the pictures she had shot at our Polynesian Luau, __27__ this was a memory she especially treasured.

Now, please understand that we have no written service rules __28__ lost photos in the park. __29__, the clerk at the front desk __30__  Disney’s idea of caring for our __31__. She asked the woman to leave her a couple rolls of __32__ film, promising she would try her best to make up for her loss.

Two weeks later the guest received a __33__ at her home. In it were photos of all the actors of our show, __34__ signed by each performer. There were also __35__ of the public procession (游行队伍) and fireworks in the park, taken by the front-desk clerk in her own __36__ after work. I happened to know this __37__ because this guest wrote us a letter. She said that __38__ in her life had she received such good service from any business.

Excellent __39__ does not come from policy(政策性的) handbooks. It comes from people who __40__ —and from a culture that encourages and models that attitude.

21A. working           B. checking    C. trying     D. staying

22A. expected       B. realized     C. paid      D. enjoyed

23A.disappointing        B. wonderful C. uncomfortable   D. important

24A. taking       B. dropping   C. losing        D. breaking

25A. developed     B. taken      C. washed      D. produced

26A. silly         B. nervous     C. calm       D. sad

27A. when        B. where        C. as         D. which

28A. covering       B. finding      C. making      D. keeping

29A. Excitedly      B. Fortunately C. Therefore D. Quietly

30A. understood    B. reminded   C. trusted       D. discovered

31A. workers           B. guests        C. managers   D. clerks

32A. printed     B. shot      C. unused      D. recorded

33A. film         B. card      C. camera      D. packet

34A. frequently     B. personally  C. alone       D. actually

35A. rules        B. pictures    C. handbooks        D. performances

36A. case          B. work     C. time      D. position

37A. story        B. place     C. photo           D. show  

38A. only         B. almost       C. never           D. nearly

39A. advice      B. experience        C. quality      D. service

40A. care          B. serve     C. like       D. know

多项选择题