下列情形中物体机械能守恒的是（ ）
A．物体在某恒力的作用下沿水平方向做加速运动
B．自由落体运动
C．匀速降落的跳伞运动员
D．飞机在跑道上加速滑行准备起飞
A、物体在某恒力的作用下沿水平方向做加速运动，动能增加，势能不变，故机械能增加，故A错误；
B、自由落体运动总有重力做功，机械能守恒，故B正确；
C、匀速降落的跳伞运动员，势能减小，动能不变，故机械能减小，故C错误；
D、飞机在跑道上加速滑行准备起飞，动能增加，势能不变，故机械能增加，故D错误；
故选B．
| 完形填空。 | |||
| Once upon a time, a man punished his 5-year-old daughter for 1 the family's only roll of expensive gold wrapping paper. Money was tight then. The child decorated a shoebox with the gold paper and put it 2 the Christmas tree. The next morning the little girl, filled with 3 brought the gift box to her father and said, "This is for you, Daddy! " 4 he opened the box, the father felt sorry for his earlier action. But when he opened it, he found it was 5 and again his anger broke out. "Don't you know, young lady," he said angrily, " when you give someone a present there should be something inside the package ! " The little girl looked 6 at him with tears rolling from her eyes and said," Daddy, it' s not empty. I blew 7 into it until it was all full. " Her father's face turned red. He fell on his 8 and put his arms around his lovely little girl. He begged her to forgive him for his unnecessary anger. An accident 9 the life of the child only a short time later. It is said that the father 10 that little gold box by his bed for all the years of his life. Whenever he was discouraged or faced difficult problems he would open the box , take out an imaginary kiss. There is no more valuable thing anyone could hold. | |||
| ( )1. A. using up ( )2. A. above ( )3. A. interest ( )4. A. After ( )5. A. empty ( )6. A. happily ( )7. A. love ( )8. A. legs ( )9. A. took ( )10. A. put | B. selling B. under B. tears B. As soon as B. a pair of shoes B. angrily B. luck B. feet B. left B. kept | C. losing C. on C. excitement C. If C. full C. up C. kisses C. back C. brought C. laid | D. stealing D. in D. anger D. As D. a roll of gold paper D. down D. wishes D. knees D. carried D. placed |