搜题请搜索小程序“爱下问搜题”
问题 选择题

如图所示，自动卸货车静止在水平地面上，车厢在液压机的作用下，θ角缓慢增大，在货物相对车厢仍然静止的过程中，下列说法正确的是（  ）

A．货物受到的支持力变小

B．货物受到的摩擦力变小

C．货物受到的支持力对货物做负功

D．货物受到的摩擦力对货物做负功

答案

答案：A

题目分析：货物受重力、支持力、摩擦力的作用，根据平衡条件得，，θ角缓慢增大的过程中，支持力变小，摩擦力增大，所以A正确；B错误；根据做功的公式知，支持力做正功，故C错误；摩擦力不做功，所以D错误。

单项选择题

书报灭菌宜选用

A．环氧乙烷
B．高压蒸汽
C．漂白粉
D．过氧乙酸
E．苯扎溴铵
查看答案
填空题


The United States counts its population every ten years, and each census reveals that the racial and ethnic mix is changing dramatically, so by the year 2050, the "average" American will not be descended from (1) , but the majority of U. S. residents will trace (2) ancestry to Africa, Asia, the Hispanic world, the Pacific Islands, or the Middle East. Once the United States was a microcosm of European nationalities, (3) the United States is a microcosm of the world. The United States is (4) considered a "melting pot" society by many of its residents. (5) , many people prefer the term "salad bowl." They use this (6) to describe American society. American society will soon be (7) nonwhite. "Melting pot" implies that the different ethnic groups blend together (8) one homogeneous (9) , "salad bowl" (10) that nationalities, like the ingredients in a mixed green salad, retain their cultural (11) . Earlier generations of (12) believed they had to learn English quickly not only to survive but also for (13) .
Now, many immigrant groups do not (14) the same need. Because there are many places in America (15) you can work, shop, get medical care, marry, divorce and die without knowing English. (16) , Chinatown in San Francisco and New York. Also, Los Angeles has many Vietnamese immigrants and immigrants from Mexico. (17) , many immigrant groups want their children to know their (18) culture. Many Hispanics, for instance, want their children to learn (19) English and Spanish and study the Spanish language in school. They are fighting for the (20) to bilingual education in many communities. In many communities they are in the majority.
查看答案