问题 计算题

(8分)如图所示,雪橇在与水平雪面成角拉力F的作用下,沿直线匀速移动了一段距离s.

(1)试求作用在雪橇上的拉力和摩擦力所做的功.

(2)外力对雪橇做的总功是多少.

答案

(1)拉力做的功,摩擦力做的功。(2)0。

题目分析:(1)我们将拉力沿水平方向分解为Fcosα,由于雪撬在该方向上通过的距离为s,而在竖直方向上没通过距离,故拉力做功:;(2分)

又困为雪撬做匀速直线运动,故摩擦力与水平方向的拉力大小相等,故(1分),其方向与物体运动的方向相反,故摩擦力所做的功:(2分)。

(2)由于物体做匀速直线运动,故物体受到的力为平衡力,(1分),

故外力对雪橇做的总功:。(2分)

阅读理解

When I first got an email account ten years ago, I received communications only from family, friends, and colleagues. Now it seems that every time I check my e-mail, I have an endless series of advertisement and other correspondence that do not interest the at all. If we want e-mail to continue to be useful, we need special laws that make spamming(发送垃圾邮件) a crime.

If lawmakers do not do something soon to prohibit spam, the problem will certainly get much worse. Computer programs allow spammers to spend hundreds of millions of e-mails almost instantly. As more and more advertisers turn to spam to sell their products, individual e-mail boxes are often flooded with spam e-mails. Would people continue to use e-mail if they had to deal with an annoying amount of spam each time?

This problem is troubling for individuals and companies as well. Many spam emails contain computer viruses that can shut down the entire network of a company. Companies rely on e-mail on for their employees to communicate with each other. Spam frequently causes failures in their local communications networks, and their employees are thus unable to communicate effectively. Such a situation results in a loss of productivity and requires companies to repeatedly repair their networks. These computer problems raise production costs of companies, which are, in the end, passed on to the consumer.

For these reasons, I believe that lawmakers need to legislate (立法) against spam. Spammers should be fined, and perhaps sent to prison if they continue to disturb people. E-mail is a tool which helps people all over the world to communicate conveniently, but spam is destroying this convenience.

小题1:What does the underlined word “correspondence” in the Paragraph 1 probably mean?                                                          

A.messages

B.ideas

C.connections

D.programs小题2:According to the text, what is the major cause of the flooding spam?       

A.Companies rely on e-mail for communications.

B.More people in the world communicate by e-mail.

C.Many computer viruses contain spam e-mail.

D.More advertisers begin to promote sales through spam.小题3:According to Paragraph 3,who is the final victim of spam?         

A.The business

B.The advertiser

C.The employee

D.The consumer小题4:What is the purpose of the text?        

A.To inform

B.To educate

C.To persuade

D.To instruct

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