阅读下面短文，用括号内所给动词的适当形式填空。
A farmer had some dogs to sell. He painted a sign advertising the dogs. A little boy came up to him
and said, "Sir, I want to buy a dog."
"Well," the farmer said," but they (1) ___________ (be) very expensive." The boy (2) ___________
(take) some changes out of his pocket,"I've got thirty-nine cents. Is that enough to take a dog?"
"Sure. I have six dogs. You (3) ___________ (choose) any of them." Then the farmer whistled, "Dolly!"
At once, Dolly ran out of the doghouse followed by four little dogs. Then the little boy's eyes (4) ___________ (dance) with happiness.
But the boy saw another little dog (5) ___________ (move) out slowly. This one was the smallest. She was
trying her best (6) ___________ (catch) up with the others…
"I went that one," the boy said, (7) ___________(point) to the last one. The farmer was surprised. "She
(8) ___________(not run) and play with you like other dogs. Will you still want to buy her?" he said kindly.
Hearing that, the little boy rolled (卷起) up one leg of his trousers. A steel brace (支架) (9) ___________
(tie) on both sides of his leg. Looking up at the farmer, he said, "So you see, Sir. I can't walk very well myself,
I (10) ___________ (understand) her. So we will be good friends."
、
，足够长的PI、AB直轨与水平均成θ=37°，一质量为m的小环套在AB杆上，环与BC、EF、HI水平直杆轨道间的动摩擦因数均为μ=0.1，其中BC=
，其他轨道均光滑，轨道拐弯连接处也光滑，环通过连接处时动能损失忽略不计，现环在AB杆上从距B点
处的地方无初速释放．已知sin37°=0.6，试求：