问题 填空题

如图所示,从地面上A点发射的一枚远程弹道导弹,在引力作用下沿ACB椭圆轨道飞行击中地面目标B,C为轨道的远地点,距地面高度为h.已知地球半径为R,地球质量为M,引力常量为G.设距地面高度为h的圆轨道上卫星运动周期为T0,则导弹在C点的加速度等于______,导弹从A点运动到B点的时间______(填大于、等于、小于)T0

答案

导弹在C点受到的万有引力F=

GMm
(R+h)2
,根据牛顿第二定律知,导弹的加速度a=
F
m
=
GM
(R+h)2

设导弹运动的周期为T,由于导弹的半长轴小于卫星的轨道半径R+h,根据开普勒第三定律

a3
T2
=k知道:导弹运动的周期T<T0,则导弹从A点运动到B点的时间一定小于T0

故答案为:

GM
(R+h)2
,小于.

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 mantie 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

填空题