开普勒从1609年～1619年发表了著名的开普勒行星运动三定律．第一定律：所有的行星分别在大小不同的椭圆轨道上围绕太阳运动，太阳在这个椭圆的一个焦点上；第二定律：太阳和行星的连线在相等的时间内扫过的面积相等；第三定律：所有行星的椭圆轨道的半长轴的三次方跟公转周期的平方的比值相等．实践证明，开普勒三定律也适用于人造地球卫星的运动．如果人造地球卫星沿半径为r的圆轨道绕地球运动，当开启制动发动机后，卫星转移到与地球相切的椭圆形轨道，如图所示．问在这之后，卫星多长时间着陆？空气阻力不计，地球半径为R，地球表面重力加速度为g．
t=
设卫星在地球表面运行时周期为T卫星由圆轨道转移到椭圆轨道，其半长轴为
，设周期为T′，据开普勒第三定律有
卫星落地时间t=
设卫星在地球表面做圆周运动，则mg=mRω2=
,T2=
所以T′2=
所以t=
.
| 阅读理解。 | ||
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| 1. If a customer (顾客) goes to the biggest Shopping Centre with Shiyan Daily of May 3,2000, he will get a small present ______. | ||
[ ] | ||
| A. every day B. from Shiyan Daily C. on May 8,2000 D. on May 3,2000 | ||
| 2. If an American teacher has worked in China for 3 years and comes to Ying Hua Middle School to ask for the job in June, he will ______. | ||
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| A. get a bad job B. not get the job C. be a good teacher D. be useful |
| 3. When and where will the football match be? |
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| A. In Hankou Stadium on Sunday. B. In Hankou Stadium on February 3. C. In Guo'an on February 8. D. In Shenhua Stadium at 3:00 on February 8. |
| 4. If three adults (成人) and six students want to watch the match, the tickets will cost RMB ______ yuan. |
[ ] |
| A. 165 B. 135 C. 195 D. 225 |
| 5. Which of the following is NOT TRUE? |
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| A. The Shopping Centre is in No. 16 Hong Kong Street. B. The telephone number of Ying Hua Middle School is 8866177. C. Xu Genbao is a coach. D. The match is between Beijing Team and Guo'an Team. |