问题 计算题

(14分)某煤矿运输部有一新采购的水平浅色足够长传送带以4.0m/s的恒定速度运动,若使该传送带改做加速度大小为3.0m/s2的匀减速运动,并且在传送带开始做匀减速运动的同时,将一煤块(可视为质点)无初速度放在传送带上。已知煤块与传送带间的动摩擦因数为0.10,重力加速度取10m/s2,求煤块在浅色传送带上能留下的痕迹长度和相对于传送带运动的位移大小。 (计算结果保留两位有效数字)

答案

  

题目分析:由运动情况作出传送带和煤块的v-t图像,如图所示。因煤块与传送带间的动摩擦因数为μ,则煤块在传送带上运动的加速度,(2分)

由图可得:  (2分)

解得 (1分)

        (1分)

此过程中煤块相对于传送带向后滑动,划线的长度为

 (2分)

当煤块与传动带间的速度相等以后,两者都做匀减速直线运动,煤块相对于传送带又向前滑动,划线的长度为     (2分)

因为,煤块在传送带上留下的划线长度为   (2分)

煤块相对于传送带的位移为    (2分)

阅读理解

阅读理解。

     When Nancy Lublin received $5,000 from her grandfather in 1996, she never once considered taking a

vacation or paying off student loans (贷款). Instead, the 24-year-old New York University law student

began thinking about helping low-income women get better jobs. "If a woman goes for a job interview (面试)

poorly dressed, she won't get the job, " Lublin says."But without a job, she can't afford suitable clothes."

     So, with the money Lublin founded "Dress for Success' and began collecting women's clothes which were

still in good condition but which their owners no longer needed. "So many women have clothes lying around

that they will never wear again," one of Lublin's assistants says "Nancy's idea is so simple and yet so important

to women."

     Many women come to Lublin's office before going to a job interview. Here, they receive a suit, shoes and

any other things they need, Since it was set up, more than 1,000 women have turned to "Dress for Success"

for help. Many of them have won job. Some have round jobs after being out of work for many years. Jenny,

a 32-year-old woman who was recently hired as a law-firm office manager says, "I made a good impression

(印象) because of 'Dress for Success'."

1. What can we learn from the text?

A. Nancy's grandfather lent her the money to set up the office.

B. Nancy's office gathers used clothes from women

C. Low-income women can get jobs at "Dress for Success"

D. Nancy set up "Dress for Success" to make money

2. "Dress for Success" attracts many women because they _____.

A. like to wear different clothes

B. like the design of the dresses

C. prefer buying clothes at a low price

D. need to look smart when looking for a job

3. From what Jenny says, we know that ______.

A. she got a lot of good ideas from Nancy

B. she is working happily at her present job

C. clothes from Nancy's office helped to get her a job

D. "Dress for Success" has a good business relation with her firm

4. What would be a good title for the text?

A. Keep Your Old Clothes

B. A Helping Hand for Women

C. Nancy: A Successful Law Student

D. A Successful Clothing Business

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