问题 选择题

如图所示,汽车以速度通过一弧形的拱桥顶端,且汽车对桥面有压力.关于汽车受力的说法中正确的是(  )

A.汽车受重力、支持力、牵引力、摩擦力和向心力的作用

B.汽车的向心力是它所受的重力与压力的合力

C.汽车的向心力是它所受的重力与支持力的合力

D.汽车的向心力是它所受的重力、支持力与摩擦力的合力

答案

A、汽车在桥顶时受到重力、支持力、牵引力和摩擦力四个力的作用.故A错误.

B、汽车在桥顶的向心力由重力和支持力的合力提供.故B错误,C正确,D错误.

故选:C.

单项选择题

In considering how the American family is changing, the starting point is the traditional family. a form which has developed over time on the basis of a number of assumptions. It is assumed that the family is heterosexual (异性的) institution, with prescriptions about how a man and a woman ought to be joined together and live together. The proper family form is assumed to be the nuclear family, that is, a family composed of a married man and woman and their children. It is assumed that the husband is the head of the family, with ultimate authority over wife and children, and that in their clearly separated roles the husband is the income-earner and the wife is the homemaker and provider of child care. It is assumed that the family lives by itself in its own house or residence.

Observers of family life have suggested that numerous changes are taking place in this traditional American family form. Included are the following general observations about trends:

More men and women seem to be living together before getting married.

Women and men seem to be marrying at a later age.

Married couples are having fewer children.

Unmarried women appear to be having more children.

Wives, even mothers with small children, are increasingly likely to be employed outside the home.

Marriages are more likely to end with divorce.

Single-parent families are more prevalent.

Remarriage is likely to follow divorce rather than widowhood.

Remarriage rates are declining, especially for women.

Some trends, viewed in a long-range historical context, appear not to be trends at all. The age at which men and women normally marry is the same now as it was 100 years ago, though it has fluctuated (变动,波动); a short-range increase in the 1940s and 1950s due to widespread postponement of marriage in war time made it appear that a change had occurred. Other trends appear to be following established patterns rather than representing a sharp break with tradition. Divorce rates have been increasing and families have been having fewer children for well over a century. Single-parent families and stepfamilies were very common in the past, although the reason for them was different. Many marriages formerly are dissolved and followed by remarriage because of the death of a parent or partner. Today single parenthood and remarriage are more likely to be a response to divorce.

Some social scientists see in these changes the breakdown of the family, to the detriment of the society. We take the position that family institutions and systems, like all human systems, are open, take many forms, and are constantly changing.

The author holds the position that family institutions and systems are ().

A. likely to breakdown, causing harm to the society

B. likely to stop changing in the near future

C. presenting new problems to the society

D. open and constantly changing

单项选择题