长为L的细线一端固定于O点，如图所示，另一端拴一质量为m的小球．把小球拉至最高点A，以水平速度v0抛出．求当v0=
时，球运动到最低点C时细线的拉力大小．gL 2
由于v0=
，小于小球做圆周运动通过最高点时的最小速度gL 2
，故小球做平抛运动，设小球运动到如图所示的B点时细线绷紧，此时细线与竖直方向的夹角为θ，由平抛运动规律有：Lsinθ=v0t；gL
L（1-cosθ）=
gt21 2
解得：θ=90°，说明B与O在同一直线上，vBx=
，由于细线瞬时绷紧，使小球水平速度突然变为零，所以此绷紧过程有机械能损失，然后小球以gL 2
的速度从B点开始做圆周运动到C点，由机械能守恒有：2gL
mvBy2+mgL=1 2
mvC21 2
在最低点C有：T-mg=m v 2C L
解得小球在最低点C时绳的拉力T=5mg．
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| A professor (教授) told his students to go into the city slums (贫民窟) to study the life of 200 boys. He asked them to 1 reports about each boy's life and future. Every one of the students wrote, "He doesn't have any hope. " Twenty-five years 2 , another professor read about the earlier study. He told his students to 3 what had happened to the boys. They tried very hard and found that 176 of the boys had become successful 4 doctors, teachers and scientists. The professor was very 5 and decided to study further. Luckily, all the men were living near the place and was able to ask each one,"What made you 6 ?"Each one answered,'There was a teacher. " The teacher was 7 living there so the professor found her and asked the old woman 8 she had used to pull those boys 9 the slum, and change them into successful people. The teacher's 10 began to shine and said with a sweet smile. "It's really very easy.I loved those boys. " | |||
| ( )1. A. read ( )2. A. later ( )3. A. search ( )4. A. for ( )5. A. glad ( )6. A. wonderful ( )7. A. already ( )8. A. what ( )9. A. out ( )10. A. face | B. study B. after B. find out B. With B. angry B. successful B. yet B. who B. into B. eyes | C. write C. ago C. look for C. as C. surprised C. rich C. even C. which C. away C. hair | D. take D. before D. see D. like D. worried D. popular D. still D. when D. out of D. mouth |