问题 选择题

如图所示,倾角为q的光滑斜面足够长,质量为m的物体在沿平行斜面向上的恒力F作用下,由静止从斜面底端沿斜面向上做匀加速直线运动,经过时间t,力F做功为40J,此后撤去力F,物体又经过相同的时间t回到斜面底端,若以地面为重力零势能参考面,则下列说法正确的是

A.物体回到斜面底端的动能为40J

B.恒力F=2mgsinθ

C.撤去力F时,物体的重力势能为30J

D.撤去力F前的运动过程中,物体的动能一直在增加,撤去力F后物体的动能一直在减少

答案

AC

题目分析:对物体从斜面底端沿斜面向上运动经过时间2t回到斜面底端全过程研究,重力做功为零,根据动能定理得,,所以物体回到斜面底端的动能为,故A正确;设撤去F时物体的速度大小为,回到斜面底端时的速度大小为,撤F前物体的加速度大小为,撤F后物体的加速度大小为,撤F前后两个时间内发生的位移大小相等,取沿斜面向上方向为正方向,于是有:,解得:,根据牛顿第二定律得:,解得:,故B错误;根据动能定理得:撤F前有,即,所以撤去力F时,物体的重力势能为,故C正确;撤去力F前的运动过程中,物体向上做匀加速直线运动,撤去F后,物体先向上做减速运动,速度减为零之后,向下加速运动,所以撤去力F后的运动过程中物体的动能是先减小后增加,故D错误。所以选AC。

多项选择题

On Apr. 27, the Dean of Duke’s business school had the unfortunate task of announcing that nearly 10% of the Class of 2008 had been caught cheating on a take-home final exam. The scandal, which has cast yet another pall over the leafy, Gothic campus, is already going down as the biggest episode of alleged student deception in the business school’s history.

According to the author, what are the "split messages" sent by the academia and corporations (para. 4)

Almost immediately, the questions started swirling. The accused MBAs were, on average, 29 years old. They were the cut-and-paste generation, the champions of Linux. Before going to the business school, they worked in corporations for an average of six years. They did so at a time when their bosses were trumpeting the brave new world of open source, where one’s ability to aggregate (or rip off) other people’s intellectual property was touted as a crucial competitive advantage.
It’s easy to imagine the explanations these MBAs, who are mulling an appeal, might come up with. Teaming up on a take-home exam: That’s not academic fraud, it’s postmodern learning, wiki style. Text-messaging exam answers or downloading essays onto iPods: That’s simply a wise use of technology. One can understand the confusion. This is a generation that came of age nabbing music off Napster and watching bootlegged Hollywood blockbusters in their dorm rooms. "What do you mean" you can almost hear them saying. "We’re not supposed to share"
That’s not to say that university administrators should ignore unethical behavior, if it in fact occurred. But in this wired world, maybe the very notion of what constitutes cheating has to be reevaluated. The scandal at Duke points to how much the world has changed, and how academia and corporations are confused about it all, sending split messages.
We’re told it’s all about teamwork and shared information. But then we’re graded and ranked as individuals. We assess everybody as single entities. But then we plop them into an interdependent world and tell them their success hinges on creative collaboration.
The new culture of shared information is vastly different from the old, where hoarding information was power. But professors-and bosses, for that matter-need to be able to test individual ability. For all the talk about workforce teamwork, there are plenty of times when a person is on his or her own, arguing a case, preparing a profit and loss statement, or writing a research report.
Still, many believe that a rethinking of the assessment process is in store. The Stanford University Design School, for example, is so collaborative that "it would be impossible to cheat," says D-school professor Robert I. Sutton. "If you found somebody to help you write an exam, in our view that’s a sign of an inventive person who gets stuff done. If you found someone to do work for free who was committed to open source, we’d say, ’Wow, that was smart. ’ One group of students got the police to help them with a school project to build a roundabout where there were a lot of bike accidents. Is that cheating"
That’s food for thought at a time when learning is becoming more and more of a social process embedded in a larger network. This is in no way a pass on those who consciously break the rules. With countries aping American business practices, a backlash against an ethically rudderless culture can’t happen soon enough. But the saga at Duke raises an interesting question. In the age of Twitter, a social network that keeps users in constant streaming contact with one another, what is cheating

单项选择题