“神舟”六号飞船发射升空后不久，将在离地面某一高度上沿着圆轨道运行，运行中需要进行多次“轨道维持”．所谓“轨道维持”就是通过控制飞船上发动机的点火时间和推力的大小、方向，使飞船能保持在预定轨道上稳定运行．如果不进行轨道维持，由于飞船受轨道上稀薄空气的摩擦阻力，轨道高度会逐渐降低，在这种情况下飞船的动能、重力势能和机械能的变化情况将会是（ ）
A．重力势能逐渐减小，动能逐渐增大
B．重力势能逐渐减小，机械能不变
C．重力势能逐渐增大，动能逐渐减小
D．动能逐渐增大，机械能逐渐减小
轨道高度逐渐降低，即飞船的高度降低、重力势能减少，速度将增大、动能增大，重力势能转化为动能；由于飞船受轨道上稀薄空气的影响，飞船要克服空气阻力做功，飞船的机械能减少，转化为内能．
故选AD．
| 完形填空。 |
| Huandreds of years ago, life was 1 than 2 today. People didn't have modern machines. There 3 modern medicine, either. Life today 4 new problems. One of the biggest is pollution. Water pollution has 5 our rivers and lakes dirty. It kills our fish and affects our drinking water. Noise pollution makes us 6 louder and 7 . Air pollution is the most serious kind of pollution. It affects every living thing in the world. Cars, planes and factories all pollute our air every day.Sometimes the pollution is so 8 that it is like a quilt (被子) 9 a city. This kind of quilt 10 smog (雾). Many countries are making new laws to fight pollution. Factories must now clean their water 11 away. They mustn't blow dirty smoke into the air. We need to do many other things.We can put a waste thing in the dustbin and not throw it on the ground. we can go to work by bus 12 with our friends 13 . If there are fewer people driving, there will be 14 pollution. Laws are not 15 . Every person must help to fight pollution. |
| ( )1.A.more hard ( )2.A.they are ( )3.A.were not ( )4.A. bring ( )5.A.made ( )6.A.say ( )7.A.felt happy more quickly B.get angry much easier C.become angry more easily D.feel sad more slowly ( )8.A. thin ( )9.A.around ( )10.A.calls ( )11.A.before they are thrown B.when they are thrown C.after it is thrown D.before it is thrown ( )12.A.and ( )13.A.in the same car ( )14.A.little ( )15.A.enough | B. more harder B.they were B.was no B.have B.let B.talk B.thick B.on B.is seen B.or B.go to our office B.less B.good enough | C. much harder C.it was C.were C.have got C.taken C.tall C.hard C.in C.is called C.but C.in different cars C.much C.enough good | D.much more harder D.it is D.was D.has brought D.changed D.spoke D.light D.over D.is like D.if D.go home D.more D.good |