问题 选择题

下列说法正确的是(  )

A.平抛运动是曲线运动,所以不可能是匀变速运动

B.平抛运动的加速度不变,所以平抛运动是匀变速运动

C.匀速圆周运动的速度大小不变,所以匀速圆周运动是匀速运动

D.匀速圆周运动的加速度大小不变,所以匀速圆周运动是匀变速运动

答案

A、B、平抛运动是曲线运动,速度方向不断变化,故一定是变速运动,由于受力恒定,加速度恒定,故是匀变速曲线运动,故A错误,B正确;

C、D、匀速圆周运动的速度大小不变、方向时刻,故一定是变速运动;加速度方向变化,大小不变,故加速度也是变化的,故C错误,D错误;

故选B.

单项选择题 案例分析题
单项选择题

Almost every day the media discovers an African American community fighting some form of environmental threat from land fills, garbage dumps, petrochemical plants, refineries, bus depots, and the list goes on. For years, residents watched helplessly as their communities became dumping grounds.

But citizens didn’t remain silent for long. Local activists have been organizing under the mantle of environmental justice since as far back as 1968. More than three decades ago, the concept of environmental justice had not registered on the radar screens of many environmental or civil rights groups. But environmental justice fits squarely under the civil rights umbrella. It should not be forgotten that Dr. Martin Luther King, Jr. went to Memphis on an environmental and economic justice mission in 1968, seeking support for striking garbage workers who were underpaid and whose basic duties exposed them to environmentally hazardous conditions.

In 1979, a landmark environmental discrimination lawsuit filed in Houston, followed by similar litigation efforts in the 1980s, rallied activists to stand up to corporations and demand government intervention.

In 1991, a new breed of environmental activists gathered in Washington, D.C., to bring national attention to pollution problems threatening low-income and minority communities. Leaders introduced the concept of environmental justice, protesting that Black, poor and working-class communities often received less environmental protection than White or more affluent communities. The first National People of Color Environmental Leadership Summit effectively broadened what "the environment" was understood to mean. It expanded the definition to include where we live, work, play, worship and go to school, as well as the physical and natural world. In the process, the environmental justice movement changed the way environmentalism is practiced in the United States and, ultimately, worldwide.

Because many issues identified at the inaugural summit remain unaddressed, the second National People of Color Environmental Leadership Summit was convened in Washington, D. C., this past October. The second summit was planned for 500 delegates; but more than 1,400 people attended the four-day gathering.

"We are pleased that the Summit II was able to attract a record number of grassroots activists, academicians, students, researchers, planners, policy analysts and government officials. We proved to the world that our movement is alive and well, and growing. "says Beverly Wright, chair of the summit. The meeting produced two dozen policy papers that show powerful environmental and health disparities between people of color and Whites.

Paragraph 3 implies that, in 1979, ().

A. the environmental justice issues were first brought to court in Houston

B. environmental activists cooperated in defying the U. S. government

C. the government intervention helped promote environmental justice

D. environmental problems attracted the attention of the government