问题 多选题

为了研究超重与失重现象.某同学把一体重秤放在电梯的地板上,他站在体重秤上随电梯运动并观察体重秤示数的变化情况.下表记录了几个特定时刻体重秤的示数(表内时间不表示先后顺序):若已知t0时刻电梯静止,则 (  )

时间 t0  t1t2t3
体重秤示数(kg)45.050.040.045.0

A.t1和t2时刻该同学的质量并没有变化,但所受重力发生变化

B.t1和t2时刻电梯的加速度方向一定相反

C.t1和t2时刻电梯运动的加速度大小相等,运动方向不一定相反

D.t3时刻电梯可能向上运动.

答案

A、超重和失重时物体的重力和质量是不变的,只是对悬挂物的压力或者支持力变化了,故A错误;

B、根据表格读数分析,t1时刻根据牛顿第二定律分析得知,电梯的加速度方向向上,物体处于超重状态.t2时刻电梯的加速度方向向下,物体处于失重状态,故B正确;

C、根据表格读数分析,t1时刻根据牛顿第二定律分析得知,电梯的加速度方向向上,物体处于超重状态.t2时刻电梯的加速度方向向下,物体处于失重状态;速度方向未知,故C正确;

D、t3时刻电梯可能向上做匀速直线运动;

故选BCD.

选择题
单项选择题

LAST month, America’s National Law Journal told its readers that " employment lawyers are warning lovestruck co-workers to take precautions in the office before locking lips outside". The advice came too late for Harry Stonecipher. The boss of Boeing was forced to resign last weekend--for reasons that will strike many outsiders as absurd--after his board were told of an affair that the 68-year-old married man had been conducting with a female employee "who did not report directly to him".

Inevitably, as the week rolled on, details of the affair rolled out. The other party was reported to be Debra Peabody, who is unmarried and has worked for Boeing for 25 years. The couple were said to have first got together at Boeing’s annual retreat at Palm Desert, California in January. After that much of the affair must have been conducted from a distance: Mr. Stonecipher’s office is at Boeing’s headquarters in Chicago; Ms Peabody runs the firm’s government-relations office in Washington, DC. They exchanged e-mails, it seems, as office lovers tend to do these days, and therein probably lay Mr Stonecipher’s downfall.

Lewis Platt, Boeing’s chairman, said that Mr Stonecipher broke a company rule that says: "Employees will not engage in conduct or activity that may raise questions as to the company’s honesty, impartiality, reputation or otherwise cause embarrassment to the company. " Having an affair with a fellow employee is not, of itself, against company rules; causing embarrassment to Boeing is. It seems that the board judged that the contents of the lovers’ e-mails would have been bad for Boeing had they been made public. Gone are the days when a board considered such matters none of its business, as Citibank’s did in 1991 when its boss, John Reed, became the talk of Wall Street for having an affair with a stewardess on Citi’s corporate jet.

At Boeing, a whistleblower is said to have forwarded the messages to Mr Platt. In general, e-mails are encrypted and not accessible to anyone who does not know the sender’s password. But many firms install software designed to search electronic communications for key words such as, " sex" and " CEO". A study last year of 840 American firms by the American Management Association found that 60% of them check external e-mails ( incoming and outgoing), while 27% scrutinize internal messages between employees. Sweet nothings whispered by the water cooler may travel less far these days than electronic billets doux.

Boeing is particularly sensitive to embarrassment at the moment. Mr. Stonecipher was recalled from retirement only 15 months ago, after the company’s previous boss, Phil Condit, and its chief financial officer, Michael Sears, had left in the wake of a scandal involving an illegal job offer to a Pentagon official.

Mr Stonecipher, a crusty former number two at Boeing, was brought back specifically to raise the company’s ethical standards and to help it be seen in its main (and affectedly puritanical) market, in Washington, DC, as squeaky clean. Verbally explicit extra-marital affairs are inconsistent with such a strategy, it seems, though they are not yet enough to bring down future kings of England.

In corporate life, such affairs are hardly unusual. One survey found that one-quarter of all long-term relationships start at work; another found that over 40% of executives say they have been involved in an affair with a colleague, and that in half of these cases one or other party was married at the time. Many a boss has married his assistant and lived happily ever after. Boeing apparently used to accept this: Mr. Condit’s fourth wife was a colleague before they married.

Mr. Stonecipher had to leave his job because()

A.the strike that happened in his company was absurd to outsiders

B. he shouldn’t be involved in an affair after he got married

C.his behavior has caused big economic loss to the Boeing company

D.the board found his affair leading to embarrassment to Boeing