问题 计算题

如图所示,在水平面上固定一个气缸,缸内由质量为m的活塞封闭一定质量的理想气体,活塞与缸壁间无摩擦且无漏气,活塞到气缸底距离为L0,今有一质量也为m的重物自活塞上方h高处自由下落到活塞上并立即以碰前速度的与活塞一起向下运动,向下运动过程中活塞可达到的最大速度为v,求从活塞开始向下移动到达最大速度的过程中活塞对封闭气体做的功。(被封闭气体温度不变,外界大气压强为p0)  

答案

解:重物自由下落:mghmv02

碰后速度为

以封闭气体为对象,有p1SL0p2S(L0-△L)

初态对活塞:p1Smgp0S=0

最大速度时对活塞与重物:p2Sp0S-2mg=0

以活塞和重物为对象,由动能定理,从碰撞后到最大速度时有:

2mg△Lp0S△LW×2mv2×2m(v0)2

解得:Wmv2mg(L0h)

所以活塞对气体做功为W'=mg(L0h)-mv2

填空题 案例分析题
单项选择题

The willingness of doctors at several major medical centers to apologize .to patients for harmful errors is a promising step toward improving the rather disappointing quality of a medical system that kills tens of thousands of innocent patients a year inadvertently.

For years, experts have lamented that medical malpractice litigation is an inefficient way to deter lethal or damaging medical errors. What they noticed, simply put it, is that most victims of malpractice never sue, and there is some evidence that many patients who do sue were not harmed by a physician’s error but instead suffered an adverse medical outcome that could not have been prevented. The details of what went wrong are often kept secret as part of a settlement agreement.

What is needed, many specialists agree, is a system that quickly brings an error to light so that further errors can be headed off and that compensates victims promptly and fairly. Many doctors, unfortunately, have been afraid that admitting and describing their errors would only invite a costly lawsuit.

Now, as described by Kevin Sack in The Times, a handful of prominent academic medical centers have adopted a new policy of promptly disclosing errors, offering earnest apologies and providing fair compensation. It appears to satisfy many patients, reduce legal costs and the litigation burden and, in some instances, helps reduce malpractice premiums. Here are some examples from colleges of the United States: at the University of Illinois, of 37 cases where the hospital acknowledged a preventable error and apologized, only one patient filed suit; at the University of Michigan Health System, existing claims and lawsuits dropped from 262 in August 2001 to 83 in August 2007, and legal costs fell by two-thirds.

To encourage greater candor, more than 30 states have enacted laws making apologies for medical errors inadmissible in court. That sounds like a sensible step that should be adopted by other states or become federal law. Such laws could help bring more errors to light. Patients who have been harmed by negligent doctors can still sue for malpractice, using other evidence to make their case.

Admitting errors is only the first step toward reforming the health care system so that far fewer mistakes are made. But reforms can be more effective if doctors are candid about how they went astray. Patients seem far less angry when they receive an. honest explanation, an apology and prompt, fair compensation for the harm they have suffered.

From the last paragraph, we can infer that Doctors should describe the way they made mistakes in order to()

A. admit malpractices first

B. make less medical mistakes

C. avoid lawsuits

D. be forgiven