问题 选择题

关于机械能守恒,下列说法正确的是( )

A.做自由落体运动的物体,机械能一定守恒

B.人乘电梯加速上升的过程,机械能守恒

C.物体必须在只受重力作用的情况下,机械能才守恒

D.合外力对物体做功为零时,机械能一定守恒

答案

答案:A

题目分析:做自由落体运动的物体,只有重力做功,机械能一定守恒A.人乘电梯加速上升的过程,除重力做功外,还有支持力做功,机械能不守恒B错误;物体除受重力作用外,还可以受到其它力,但其他力不做功的情况下,机械能也守恒C错误;合外力对物体做功为零时,如匀速上升时,机械能不一定守恒D错误.

判断题
单项选择题

Du Bois was a sociological and educational pioneer who challenged the established system of education that tended to restrict rather than to advance the progress of black Americans. He challenged what is called the “Tuskegee machine” of Booker T. Washington, the leading educational spokesperson of the blacks in the U. S. . As a sociologist and historian, Du Bois called for a more determined and activist leadership than Washington provided.

Unlike Washington, whose roots were in southern black agriculture, Du Bois’s career spanned both sides of the Mason-Dixon Line. He was a native of Massachusetts, received his undergraduate education from Fisk University in Nashville, did his graduate study at Harvard University, and directed the Atlanta University Studies of Black American Life in the South. Du Bois approached the problem of racial relations in the United States from two dimensions: as a scholarly researcher and as an activist for civil rights. Among his works was the famous empirical sociological study, The Philadelphia Negro: A Social Study, in which he examined that city’s black population and made recommendations for the school system. Du Bois’s Philadelphia study was the pioneer work on urban blacks in America.

Du Bois had a long and active career as a leader in the civil rights movement. He helped to organize the Niagara Movement in 1905, which led to the National Association for the Advancement of Colored People (NAACP), established in 1909. From 1910 until 1934, Du Bois edited The Crisis, the major journal of the NAACP. In terms of its educational policy, the NAACP position was that all American children and youth should have genuine equality of educational opportunity. This policy, which Du Bois helped to formulate, stressed the following themes: (1) public schooling should be free and compulsory for all American children; (2) secondary schooling should be provided for all youth; (3) higher education should not be monopolized by any special class or race.

As a leader in education, Du Bois challenged not only the tradition of racial segregation in the schools but also the accommodationist ideology of Booker T. Washington. The major difference between the two men was that Washington sought change that was evolutionary in nature and did not upset the social order, whereas Du Bois demanded immediate change. Du Bois believed in educated leadership for blacks, and he developed a concept referred to as the “talented tenth”, according to which 10 percent of the black population would receive a traditional college education in preparation for leadership.

Compared with Booker T. Washington, Du Bois’s stance was()

A. less popular

B. more radical

C. less aggressive

D. more conservative