题目

阅读理解;根据短文的内容, 从A、B、C、D四个选项中选择一个最佳答案。 My grandfather is an eighty-year-old man. He always complains about how fast things have changed, and he often says that life used to be better.   Families aren't families the way they used to be. A lot of families have broken down. If husband and wife have problems with their marriage, they no longer stay together. And mothers used to stay at home and take care of their children, but now not anymore. Everyone's working. No one has time to look after children at home.   And the cars! No one 如图所示，ABDO是处于竖直平面内的固定光滑轨道，AB是半径为R＝15 m的圆周轨道，半径OA处于水平位置，BDO是半径为r＝7.5 m的半圆轨道，D为BDO轨道的中点．一个小球P从A点的正上方距水平半径OA高H处自由下落，沿竖直平面内的轨道通过D点时对轨道的压力等于重力的倍．g取10 m/s2．求： (1)H的大小． (2)说明此球能否达到BDO轨道的O点， (3)小球沿轨道运动后再次落到轨道上的速度大小是多少？