问题 选择题

如图所示,绝缘光滑的半圆轨道位于竖直平面内,并处于竖直向下的匀强电场中,在轨道的上缘有一个质量为m,带电荷量为+q的小球,由静止开始沿轨道运动.下列说法正确的是(     )

A.小球运动过程中机械能守恒

B.小球在轨道最低点时速度最大

C.小球在最低点对轨道的压力为mg+qE

D.小球在最低点对轨道的压力为3(mg+qE)

答案

BD

分析:物体机械能守恒的条件是只有重力或者是弹力做功,根据机械能守恒的条件分析物体的受力的情况,即可判断物体是否是机械能守恒.

小球向下运动的过程中,不仅是重力做功,还有电场力做功,由功和能的关系,可以判断速度的变化,小球做的是圆周运动,由向心力的公式可以分析球与轨道的作用力.

解答:解:A、物体机械能守恒的条件是只有重力或者是弹力做功,小球在运动的过程中还有电场力对求做功,所以机械能不守恒,故A错误.

B、小球向下运动到轨道最低点的过程中,重力和电场力对小球都做正功,根据动能定理可以知道,小球在轨道最低点时速度最大,所以B正确.

C、对小球由动能定理可得,(mg+qE)R=mV2,在最低点时,由向心力的公式可得,N-mg-qE=m,联立以上两个方程可得N=3(mg+qE),由牛顿第三定律可知,小球在最低点对轨道的压力为3(mg+qE),所以C错误.

D、根据C的分析,可知D正确.

故选BD.

多项选择题
填空题

World Water Shortage


A new study warns that about thirty percent of the world’s people may not have enough water by the year 2025.
A private American organization called (1) Action International did the new study. It says (2) than three-hundred-thirty-five-million people (3) enough water now. The people live in twenty-eight (4) . Most of the countries are in Africa or the (5) East.
P-A-I researcher Robert Engelman says (6) the year 2025, about three-thousand-million people (7) lack water. At least 18 more countries are (8) to have severe water problems. The demand (9) water keeps increasing. Yet the amount of water on Earth (10) the same.
Mr. Engelman says the population in countries that lack water is (11) faster than in other parts of the world. He says (12) growth in these countries will continue to (13) .
The report says lack of water in the future may (14) in several problems. It may increase health (15) . Lack of water often means drinking (16) not safe. Mr. Engelman says there are problems (17) over the world because of diseases, such as cholera, (18) are carried in water. Lack of water may also result (19) more international conflict. Countries may have to (20) for water in the future. Some countries now get sixty percent of their fresh water from other countries. This is true of Egypt, the Netherlands, Cambodia, Syria, Sudan, and Iraq. And the report says lack of water would affect the ability of developing to improve their economies. This is because new industries often need a large amount of water when they are beginning.