问题 选择题

如图所示,一小孩从公园中粗糙的滑梯上自由加速滑下,其能量的变化情况是(  )

A.重力势能减小,动能不变,机械能减小,总能量减少

B.重力势能减小,动能增加,机械能减小,总能量不变

C.重力势能减小,动能增加,机械能增加,总能量增加

D.重力势能减小,动能增加,机械能守恒,总能量不变

答案

小孩加速滑下,高度不断减小,重力做正功,重力势能减小.速度增大,动能增大.

小孩在下滑过程中与滑梯摩擦生热,一部分机械能转化成内能,导致机械能总量减小.而总能量不变,故B正确,ACD错误.

故选:B

判断题
单项选择题

Studies from 10 nations reveal that the rates of depression among women are twice as high as they are among men. Do women have a biological bent for depression, or are social double standards the major cause

Mental health workers have long noticed among the clinically depressed women take up a bigger proportion. Until recently, though, it was unclear whether more women than men were ill or, instead, whether more women sought help. In fact, a mounting collection of studies has confirmed that major depression is twice as common among women as it is among men. "This is one of the most consistent findings we have ever had," says Myrna M. Weissman of Columbia University.

Scientists searching for explanations are challenged by the fact that a variety of cues prompt depression in different people. Sorting out which factors might have a greater influence on women has not proved easy. Both sexes stand an equal chance of inheriting major depression, so genes are most likely not to blame. Yet hormones and sleep cycles--which differ dramatically between the sexescan alter mood. Also, many workers have proposed that social discrimination might put women under high levels of stress.

In 1990 an international group examines mood disorders. In the 10 nations reviewed so far, the team has found that among generations reaching maturity after 1945, depression seems to be on the rise and occurs at a younger age. Although overall incidence varies regionally, "everywhere the rates of depression among women are about twice as high as they are among men," Weissman says.

In contrast, lifetime rates for manic-depressive illness do not differ according to sex or culture. Mark S. George and his colleagues at the National Institute of Mental Health (NIMH) recently studied which regions of the brain have greater blood flow during periods of depression. George found that "the brain activity of the men and women of depression looked very different. " He has since compared feelings of anger, anxiety and happiness, finding no such big a difference.

Because one in five American women has a history of depression, especially as they often pursue therapy from other sources, sometimes on top of an anti-depressant clinic. Says Leibenluft: "It is remarkable how little work has been done on this subject.

Which one of the following is true about Myrna M. Weissman’s comment()

A. Women have a biological bent for depression

B. The chance for women to have a major depression is twice as high as it is for men

C. Social double standards are the major cause for women’s depression

D. More women sought help, and this makes it seem like more women suffer from depression