问题 选择题

一根细长绳的下端系一金属小球,上端固定,制成一个摆,把小球拉离竖直位置松手,让他摆动起来,如图所示,可以观察到小球摆动过程中摆动的幅度会逐渐减小,最终停下来,对于小球在摆动过程中的能量转化情况,下列说法不正确的是

A.小球从高处向低处摆动过程中,势能部分转化为动能

B.小球从低处向高处摆动过程中,动能部分转化为势能

C.小球摆动过程中机械能守恒

D.小球依次经过最低点时,动能会逐渐减少

答案

答案:C

由于小球摆动过程中摆动的幅度会逐渐减小,最终停下来,说明小球摆动的过程中机械能不守恒,小球的机械能逐渐转化成了内能;故C是不对的。

单项选择题
单项选择题

Marriage, and its many ups and downs, still exercises a powerful hold over newspapers, magazines and the airwaves. Nearly 23m Americans watched Prince William being joined in holy marriage to Kate Middleton. Millions more have indulged in the break-up of Arnold Schwarzenegger’s marriage after revelations that he fathered a son with a maid.

Less delightful are revelations about the sorry state of marriage across the United States. Data from the Census Bureau show that married couples, for the first time, now make up less than half of all households.

The iconic American family, with mom, dad and kids under one roof, is fading. In every state the numbers of unmarried couples, childless households and single-person households are growing faster than those comprised of married people with children, finds the 2010 census. And the trend has a potent class dimension. Traditional marriage has evolved from a near-universal ritual to a luxury for the educated and affluent.

There barely was a marriage gap in 1960: only four percentage points separated the wedded ways of college and high-school graduates(76% versus 72%). The gap has since widened to 16 percentage points, according to the Pew Research Centre.

"Marriage has become much more selective, and that’s why the divorce rate has come down," said Bradford Wilcox, director of the National Marriage Project at the University of Virginia in Charlottesville. The project found that divorce rates for couples with college degrees are only a third as high as for those with a high-school degree.

Americans with a high-school degree or less tell researchers they would like to marry, but do not believe they can afford it. Instead, they raise children out of wedlock. Only 6% of children born to college-educated mothers were born outside marriage, according to the National Marriage Project. That compares with 44% of babies born to mothers whose education ended with high school.

"Less marriage means less income and more poverty," reckons Isabel Sawhill, a senior fellow at the Brookings Institution. She and other researchers have linked as much as half of the income inequality in America to changes in family composition: single-parent families (mostly those with a high-school degree or less) are getting poorer while married couples (with educations and dual incomes) are increasingly well-off. "This is a striking gap that is not well understood by the public," she says.

Do not expect the Democratic Party, however, to make an issue of the marriage gap in next year’s elections. Unmarried women voted overwhelmingly for Barack Obama. "You don’t want to suggest to someone who isn’t married and has children that they should be married," says Ms Sawhill. "That is a scorn on their lifestyle.

The first two paragraphs suggest that()

A. the public like to spy on celebrities’ marriage

B. many celebrities’ marriages are going wrong

C. Americans’ marriage is going downhill

D. people feel sorry for Americans’ marriage