When I was looking for a Christmas present for my daughter in a toy store, a nicely dressed little girl, with some money in her little hand, was looking at some beautiful dolls. When she saw a doll she liked, she would ask her father if she had enough money. He usually said yes.
At the same time, a boy, with old and small clothes, was looking at some video games. He, too, had money in his hand, but it looked no more than five dollars. Each time he picked up one of the video games and looked at his father, he shook his head.
The little girl had chosen her doll, a very beautiful one. However, she noticed the boy and his father. She saw the boy give up a video game with disappointment and walk to another corner of the store.
The little girl put her doll back to the shelf and ran over to the video game. After she talked to her father, she paid for the video game and whispered(耳语) to the shop assistant.
So the boy got the video game that he wanted for free—he was told it was a prize from the store. He smiled happily, although he felt it was so incredible. The girl saw all this happen. She smiled, too.
When I walked out of the store to my car, I heard the father ask his daughter why she had done that. I would never forget their short talk. " Daddy, didn’t Grandma want me to buy something that would make me happy"
He said, "Of course, she did. "
"Well, I just did!" With that, the little girl started skipping towards their car happily.
Which of the following is TRUE according to the story
A．The little girl was kind and helpful.
B．The video game was a prize from the store.
C．The writer paid for the video game for the boy.
D．The boy bought the video game himself from the store.
＝37°。现有一质量为m＝1kg的小球穿在滑轨上，以Ek0＝24J的初动能从B点开始水平向左运动，小球与两段直轨道间的动摩擦因数均为μ＝
，设小球经过轨道连接处均无能量损失。（g＝10m/s2，sin37°＝0.6，cos37°＝0.8）求：