问题 计算题

(12分)如图所示,有一条沿顺时针方向匀速传送的传送带,恒定速度v=4 m/s,传送带与水平面的夹角θ=37°,现将质量m=1kg的小物块轻放在其底端(小物块可视作质点),与此同时,给小物块沿传送带方向向上的恒力F=10 N,经过一段时间,小物块上到了离地面高为=2.4 m的平台上。已知物块与传送带之间的动摩擦因数μ=0.5,(g取10 m/s2, sin37°=0.6,cos37°=0.8).

问:(1)物块从传送带底端运动到平台上所用的时间?

(2)若在物块与传送带达到相同速度时,立即撤去恒力F,计算小物块还需经过多少时间离开传送带以及离开时的速度?

答案

1.25s     1s   2m/s

题目分析: (1)对物块受力分析可知,物块先是在恒力作用下沿传送带方向向上做初速为零的匀加速运动,直至速度达到传送带的速度,由牛顿第二定律

(1分),计算得:

(1分)   (1分)

物块达到与传送带同速后,对物块受力分析发现,物块受的摩擦力的方向改向

(1分),计算得:

(1分)  (1分)得(1分)

(2)若达到同速后撤力F,对物块受力分析,因为>,故减速上行

(1分),得

设物块还需离开传送带,离开时的速度为,则(1分),(1分)

(1分)  (1分)

选择题
阅读理解

The library is one of the most popular places at a western university. Students turn to it for research, conversations about class, and many other services.

Compared with Chinese libraries, college libraries, college libraries in the US and UK tent to offer more resources. A postgraduate at Yale University can borrow as many as 225 books at a time.

In addition to borrowing books, there are online and electronic resources. These include a database search of popular and academic material, such as LexisNexis Academic, which offer items from newspapers and magazines.

Although books and articles are the items that students ask for most frequently, some libraries provide audio and video recordings, maps and sheet music (活页乐谱). At some schools, teachers and tutors put electronic copies of their teaching PPTs on the library Web to give easier access for students.

Another useful service in Western college libraries is the Interlibrary Loan. This allows a student at one school to borrow books from another school. The loan request is make through the student’s college library, which gets the book, gives it to the user, and arranges for its return.

Technology has brought more services to students and has made libraries attractive. Some universities have services for students to send messages through the computer or mobile texts to ask the library staff for information.

Earlier this year, Harvard University introduced a new Scan and Deliver service, allowing students to make requests for parts of books and articles. Requests made through the system are handled by library staff. The student receives and e-mail with an Internet link to the scanned pages. The service is free and all material comes within four days.

It used to be that libraries didn’t allow food or drinks. But that rule is changing and many of them now contain a café so students can spend as much time as they want in the library.

Actually, some US university libraries are now all-night affairs, or have at least one study room open all night.

1. In Western college libraries a postgraduate can ______.

A. have a free drink or a meal

B. borrow as many books as they want

C. access not only books but other study resources

D. ask questions and get answers from their tutors

2. LexisNexis Academic is the name of ______.

A. a database

B. a library

C. a university

D. a guide

3. How can a college student borrow books from other college’s libraries?

A. By going to these libraries.

B. By getting them through the Internet.

C. By sending messages through the computer or mobile texts.

D. By borrowing them through their own library.

4. Which of the following statements is TRUE?

A. Chinese library services are better than western ones.

B. Library staff play an important role in college library service.

C. Postgraduates can post their own PPTs to the library website.

D. 24-hour services are adopted in all western libraries nowadays.

5. Why are university libraries more attractive now?

A. Because students can eat and drink there at any time.

B. Because students can have a stay-over there all night.

C. Because students can enjoy quicker and more convenient services.

D. Because students can borrow anything they want there.