已知地球半径为R，地球表面重力加速度为g，不考虑地球自转的影响。
(1)推导第一宇宙速度v1的表达式；
(2)若卫星绕地球做匀速圆周运动，运行轨道距离地面高度为h，求卫星的运行周期T。
解：(1)设卫星的质量为m，地球的质量为M
在地球表面附近满足
得GM=Rg ①
卫星做圆周运动的向心力等于它受到的万有引力
②
①式代入②式，得到
(2)考虑①式，卫星受到的万有引力为
③
由牛顿第二定律
④
③④式联立解得
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| In the doorway of my home, I looked closely at my 23yearold son, Daniel.In a few hours he would be flying to France to __1__ a different life.It was a transitional (过渡的) time in Daniel's life.I wanted to __2__ him some words of significance.But nothing came from my lips, and this was not the __3__ time I had let such moments pass. When Daniel was five, I took him to the bus stop on his first day of kindergarten.He asked, "What is it going to be like, Dad?Can I do it?" Then he walked __4__ the steps of the bus and disappeared inside.The bus drove away and I said nothing.A decade later, a similar __5__ played itself out. I drove him to college.As I started to leave, I tried to think of something to say to give him __6__ and confidence as he started this new stage of life.Again, words __7__ me. Now, as I stood before him, I thought of those __8__ opportunities.How many times have I let such moments __9__?I don't find a quiet moment to tell him what he has __10__ to me.Or what he might __11__ to face in the years ahead.Maybe I thought it was not necessary to say anything. What does it matter in the course of a lifetime if a father never tells a son what he really thinks of him?__12__ as I stood before Daniel, I knew that it did matter.My father and I loved each other.Yet I always __13__ never hearing him put his __14__ into words.Now I could feel my palms sweat and my throat tighten.Why is it so __15__ to tell a son something from the heart? My mouth turned dry, and I knew I would be able to get out only a few words clearly."Daniel, "I said, "if I could have picked, I would have picked you." That's all I could say.He hugged me.For a moment, the world __16__, and there were just Daniel and me.He was saying something, but tears misted my eyes, and I couldn't understand what he was saying.All I was __17__ of was the stubble (短须) on his chin as his face pressed __18__ mine.What I had said to Daniel was __19__.It was nothing.And yet, it was __20__. | ||||
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