问题 问答题

如图所示,一根长L=1.5m的光滑绝缘细直杆MN,竖直固定在场强为E=1.0×105N/C、与水平方向成θ=30°角的倾斜向上的匀强电场中.杆的下端M固定一个带电小球A,电荷量Q=+4.5×10-6C;另一带电小球B穿在杆上可自由滑动,电荷量q=+1.0×10-6C,质量m=1.0×10-2kg.现将小球B从杆的上端N静止释放,小球B开始运动(静电力常量k=9.0×109N•m2/C2,取g=10m/s2

(1)小球B开始运动时的加速度为多大?

(2)小球B的速度最大时,距M端的高度h1为多大?

答案

(1)开始运动时小球B受重力、库仑力、杆的弹力和电场力,沿杆方向运动,

由牛顿第二定律得

mg-k

Qq
L2
-qEsinθ=ma

代入数据解得a=3.2 m/s2

(2)小球B向下运动,受A的斥力增大,加速度减小,速度增大,

当小球B速度最大时合力减为零,

k

Qq
h12
+qEsinθ=mg

代入数据解得:h1=0.9m

答:(1)小球B开始运动时的加速度为3.2 m/s2

(2)小球B的速度最大时,距M端的高度h1为0.9m.

阅读理解

Clothes play a critical part in the conclusions we reach about who people are, who they are not, and who they would like to be. They tell us a good deal about the wearer′s background, personality, status, mood, and social outlook.

  Since clothes are such an important source of social information, we can use them to influence people′s impression of us. Our appearance takes on particular significance in the initial phases of interaction. An elderly middle-class man or woman may be alienated (疏远) by a young adult who is dressed in an unconventional manner, regardless of the person′s education, background, or interests.

  People tend to agree on what certain types of clothes mean. Adolescent girls can easily agree on the lifestyles of girls who wear certain outfits (套装), including the number of boyfriends they likely have had and whether they smoke or drink. Newscasters and announcers on TV are considered to be more convincing, honest, and competent when they are dressed conservatively. College students who view themselves as taking an active role in their interpersonal relationships say the costumes they wear change the way they feel about themselves and how they act. Perhaps you have used clothing to gain confidence when you face a stressful situation, such as a job interview, or a court appearance.

  In the workplace, men have long had well-defined role models for achieving success. It has been otherwise for women. A good many women in the business world are uncertain about the appropriate mixture of “masculine”(男性)and “feminine”(女性)characteristics they should convey to men.

  Male administrators tend to judge women more favorably for managerial positions when the women display less “feminine” grooming (打扮)— shorter hair, moderate use of make-up, and plain tailored clothing. As one male administrator confessed, “An attractive woman is definitely going to get a longer interview, but she won′t get a job.”

57. According to paragraph two, an elderly middle-class man and     unconventionally-dressed young man are unlikely to____.

A. understand each other's lifestyle

B. share the same background or interests

C. influence their impressions of each other

D. interact in a positive way

58. The author uses the newscasters' example to show the idea that____.

A. certain clothing gives people more confidence

B. clothes have nothing to do with one's abilities

C. we tend to label people by their clothes

D. competent people always dress conventionally

59. Men's professional clothing can best be described as "_____".

A. conventional                B. individualistic

C. out-of-date                 D. changeless

60. How should professional women dress according to the passage?

A. They must be dressed conservatively.

B. They don't have a certain dressing style to follow.

C. They choose either masculine or feminine clothing.

D. They can't wear unconventional outfits.

单项选择题