题目

宇宙间存在一些离其它恒星较远的三星系统，其中有一种三星系统如图所示，三颗质量均为m的星位于等边三角形的三个顶点，三角形边长为R，忽略其它星体对它们的引力作用，三星在同一平面内绕三角形中心O做匀速圆周运动，万有引力常量为G，求： （1）.每颗星所受的合外力； （2）每颗星做圆周运动的线速度；    （3）.每颗星做圆周运动的周期。My grandfather came from Hungary and was the only one in his family who settled down in the United States. The rest of his family remained in Europe. When World War I broke out, he seemed to have become another man , downhearted. Such obvious change was not born out of concern for his welfare, but out of fear: if his only son, my uncle, had to go to war, it would be cousin fighting against cousin. One day in 1918, my Uncle Milton received his draft notice. My grandparents were very upset. But my mother, at the age of 10, felt on top of the world about her soldier brother going off to war. Realizing how he was regarded by his little sister and all of her friends, my uncle bought them all service pins, which meant that they had a loved one in the service. All the little girls were delighted. The moment came when my uncle and the other soldiers, without any training but all in uniforms, boarded the train. The band played and the crowd cheered. Although no one noticed. I'm sure my grandmother had a tear in her eye for the only son. The train slowly pulled out, but not about a thousand yards when it suddenly paused. Everyone stared in wonder as the train slowly returned to the station. There was a dead silence before the doors opened and the men started to step out. Someone shouted, "The war is over!" For a moment, nobody moved, but then the people heard someone bark orders at the soldiers. The men lined up in two lines, walked down the steps, and with the band playing, marched down the street, as returning heroes, to be welcomed home. My mother said it was a great day, but she was just a little disappointed that it didn't last a tiny bit longer. What the grandfather was most worried about was _________ . A. the spread of the world war             B. the safety of his two cousins C. a drop in his living standards            D. his relatives killing each other The underlined phrase "draft notice" means _________ . A. order for army service                  B. train ticket for Europe C. letter of rejection                      D. note of warning What did the "service pins"  (in Para. 2)  stand for in the eyes of the little girls? A. Strength.         B. Courage.      C. Victory.        D. Honour. Which of the following words can best describe the ending of the story? A. Disappointing.  　  B. Unexpected.       C. Uncertain.      D. Inspiring. Which of the following statements is True according to the passage? A. The grandfather felt downhearted because he was afraid of receiving a draft notice. B. The train was traveling fast to the front when the news came that the war was over. C. Few soldiers on the train had training in advance. D. Most people including the mother were disappointed that the war didn't last a bit longer.