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问题 选择题

质量为m的人造卫星在地面上未发射时的重力为G0，它在离地面的距离等于地球半径R的圆形轨道上运行时的（　　）

A．周期

2mR/G0

B．速度为

2G0R/M

C．动能为

1
4
GoR

D．重力为0

答案

A：由G

Mm
r2
=mr
4π2
T2
，解得：T=
4π2mr3
GMm
，其中r=2R，又GM=gR2，带入得：T=
32π2mR
G0
，故A错误

B：由G

Mm
r2
=m
v2
r
，解得：v=
GMm
r
，带入r=2R，又GM=gR2，得：v=
G0R
2
，故B错误

C：由G

Mm
r2
=m
v2
r
解得：
1
2
mv2=
GMm
2r
=
G0R
4
，故C正确

D：在次轨道上时，仍受地球引力作用，故而其重力不为零，故D错误

故选C

选择题

下列词语中划线的字，读音全都正确的一组是（    ）

A．剔透（tī ）誊写 （téng ）畏葸（ xǐ ） 泥古不化（ní ）

B．疮痍（chuāng）宽宥（yǒu ）筵席（yán）前倨后恭（jù）

C．焗油（jū）氛围（fēn）自诩（xǔ ）身陷囹圄（yǔ）

D．莅临（ lì ）桑梓 （zǐ） 绰刀（chāo ）自怨自艾（yì ）
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填空题

Of all the employed workers in the United States, 12.5 million are part of a temporary workforce. The United States Bureau of Labor Statistics counts 9.2 percent of workers as those who have______(1) they term "alternative employment arrangements." The government predicts that temporary staffing agencies______(2) experience growth of 49 percent by 2010. If numbers increase at the rate ______ (3), these companies will add 1.9 million new jobs by 2010. That’’s more than any______(4) industry.Temporary workers were the first ones to be affected by the recent economic recession, but they are______(5) to be the first to regain positions as the economy picks up and companies work to rebuild. Employers use temporary workers as a resource because_______(6) offer flexibility and come at a low cost. Many temporaries are categorized as "independent contractors" instead of employees,______(7) allows the employer to avoid______(8) certain taxes. Because they are not required to offer benefits to temporary workers, companies can save a great deal of money by hiring temps. Usually, temporary workers can qualify_______(9) benefits if they work for a staffing company for a certain length of time. Most temps, however, do not continue with one company long______(10) to qualify for them because they regard the position as a step on the ladder to a______(11) position later.According to Richard Wahlquist, president of the American Staffing Association, 75_______(12) of temporary workers hope for transition out of the temporary staffing category______(13) a period of time. Wahlquist finds that temporary workers spend about 11 weeks on various assignments before their______(14) out of the temporary workforce. Many use temporary positions to gain experience and skills______(15) they move on to better jobs. Wahlquist says that the temp workforce as a whole turns______(16) 400 percent each year.Tom Dilworth, research director at the Employment Policies Institute, explains______(17) some workers like having temporary jobs______(18) it affords them a great deal of elasticity with time and everything. Some temps only have a limited______(19) of time to work and temp agencies can help coordinate jobs to fit their schedules. Other sometimes-employees use temporary jobs to get an______(20) to a company from the inside, to get a foot in the door of a certain business or career._______(21) others take temporary positions in hopes their employers will change them over to permanent positions eventually.Nearly all the industries in the United States use temporary workers.______(22) to Tim Costello, coordinator of the North American Alliance for Fair Employment, the growth of temporary labor threatens______(23) job security of permanent workers who fear replacement, as well as the temps who are more accustomed to turnover. He predicts that there will continue to______(24) a gradual shift from permanent employees to contingent staffing, and that such a_____(25) in workplace demographics will "lead to lower wages, poorer working conditions, and more instability."
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