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如图所示，是一种小型千斤顶的示意图，当手往下压动摇臂时，能把重物抬高一段较小的距离。工人在2s时间里，用100N的力，竖直向下压动摇臂50cm，把重1000N的重物匀速抬高4cm，则所做的有用功是     J，千斤顶的机械效率是       

 

答案

40J   80%

题目分析：此机械目的是提升重物，克服物重做功为有用功，人的压力做功为总功。

有用功：W有用=G物h=1000N×0.04m=40J

总功：W总=Fs=100N0.5m=50J

机械效率：

单项选择题

Attacking an increasingly popular Internet business practice, a consumer watchdog group Monday filed a complaint with the Federal Trade Commission, asserting that many online search engines are concealing the impact special fees have on search results by Internet users. Commercial Alert, a 3-year-old group founded by consumer activist Ralph Nader, asked the FTC to investigate whether eight of the Web’s largest search engines are violating federal laws against deceptive advertising.
The group said that the search engines are abandoning objective formulas, to determine the order of their listed results and selling the top spots to the highest bidders without making adequate disclosures to Web surfers. The complaint touches a hot-button issue affecting tens of millions of people who submit search queries each day. With more than 2 billion pages and more than 14 billion hyperlinks on the Web, search requests rank as the second most popular online activity after E-mail.
The eight search engines named in Commercial Alert’s complaint are. MSN, owned by Microsoft; Netscape, owned by AOL Time Warner; Directhit, owned by Ask Jeeves; HotBot and Lycos, both owned by Terra Lycos; Altavista, owned by CMGI; LookSmart, owned by LookSmart; and Iwon, owned by a privately held company operating under the same name.
Portland, Ore. -based Commercial Alert could have named more search engines in its complaint, but focused on the biggest sites that are auctioning off spots in their results, said Gary Ruskin, the group’s executive director.
"Search engines have become central in the quest for learning and knowledge in our society. The ability to skew (扭曲) the results in favor of hucksters (小贩) without telling consumers is a serious problem," Ruskin said. By late Monday afternoon, three of the search engines had responded to The Associated Press’ inquiries about the complaint. Two, LookSmart and AltaVista, denied the charges. Microsoft spokesman Matt Pilla said MSN is delivering" compelling search results that people want."
The FTC had no comment about the complaint Monday. The complaint takes aim at the new business plans embraced by more search engines as they try to cash in on their pivotal (关键的) role as Web guides and reverse a steady stream of losses. To boost revenue, search engines in the past year have been accepting payments from businesses interested in receiving a higher ranking in certain categories or ensuring that their sites are reviewed more frequently.

The consumer group complained about ______.

A．special fees that Internet users were charged

B．Federal Trade Commission

C．Commercial Alert

D．online search engines
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单项选择题

MSC和SGSN之间的接口称为（）。

A.Gn

B.Gi

C.Gs

D.Gr
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