问题 多选题

已知万有引力恒量,在以下各组数椐中,根椐哪几组可以测地球质量(  )

A.地球绕太阳运行的周期及太阳与地球的距离

B.月球绕地球运行的周期及月球离地球的距离

C.地球半径、地球自转周期及同步卫星高度

D.地球半径及地球表面的重力加速度

答案

A、已知地球绕太阳运行的周期及太阳与地球的距离,可以求出太阳的质量而求不出地球质量,故A错误;

B、已知月球绕地球运行的周期及月球离地球的距离,可以求出地球质量,故B正确;

C、已知地球半径、地球自转周期及同步卫星高度,可以求出地球质量,故C正确;

D、已知地球半径及地球表面的重力加速度,可以求出地球质量,故D正确;

故选BCD.

单项选择题
单项选择题

An analysis of workplace trends shows that employee perks, a reliable indicator of job market strength, are beginning to make a comeback. While not as Extravagant as those offered in the late 1990s, companies clearly are shifting their focus from workforce reduction to workforce retention. Firms realize that they require a foundation of experienced, trained, and motivated workers. These employers are improving and/ or adding perks to prevent an exodus of workers that could occur as the economy continues improving. They also may be looking further down the road when severe labor shortages are expected to return.

An analysis of perks offered in today’s workplace shows that many of the 1990s-style benefits, such as game rooms and luxury car leases, have been abandoned. The perks that remain popular with employers and employees are those that help workers stay healthy, career focused, and financially stable. Perhaps the most appreciated are those that help individuals maintain work-life balance.

Work-life balance is just one part of the growing concern companies have about the overall emotional and physical health of their workers. Employees who are stressed out or depressed because they do not feel as if they are giving enough attention to the nonwork aspects of their lives ultimately are unproductive. More and more companies also are learning that workers desire the opportunity to grow professionally in the workplace. HewlettPackard, for instance, has boosted its employee education and development budget by 20%.

Nationwide Insurance, based in Columbus, Ohio, established a career-planning website in 2003. The site provides information on company job opportunities, career development, and an in-house mentoring program. Nationwide also is helping to educate its employees in financial matters, acknowledging that workers distracted by such issues on the job are not giving their full attention to the company’s priorities. It is adding classes and seminars on personal finance issues and 401(k) investments.

Sometimes perks simply are about keeping employee morale elevated. Knowing that an improving economy might prompt valued employees to seek new opportunities, the owner of Ticketcity. com has lavished his best performers with tickets to the Masters golf tournament (锦标赛), access to country clubs, and invitations to a management retreat in Sedona, Ariz.

Moreover, even companies that cannot afford to institute costly perks can find ways to make sure current employees are happy. Doug Dorman, vice president of human resources for the Greenville (S.C.) Hospital System explains that there is a definite sense of urgency when it comes to employee retention, knowing that labor shortages are returning. Dorman notes, however, that they have not focused on perks, "but rather on creating a culture of recognition and appreciation. Employees stay when they have good two-way communication with management and are truly appreciated and recognized for their contributions.

Which of the following is not the reason why companies have begun to shift their attention on the retention of workforce()

A. the constant improvement of economy in the US

B. the demand of experienced and well-trained employees

C. the demonstration of its market strength indicator

D. the possible lack of workforce in the near future