如图，两根长度相等且不可伸长的细绳，一端分别系在竖直杆上A、B两点上，另一端共同系拴住一小球C．当杆以ω转动时，两绳恰好都拉直，此后ω逐渐增大，则（ ）
A．两绳张力都增大
B．AC张力增大，BC张力不变
C．AC张力不变，BC张力增大
D．两绳张力和力总是指向AB中心
设AC绳与杆的夹角为θ，则BC绳与杆的夹角也为θ．
在竖直方向上，有：TAcosθ=mg+TBcosθ
水平方向上有：TAsinθ+TBsinθ=mrω2
知θ角不变，绳子不可伸长，r不变，角速度ω增大，则两绳张力都增大．故A正确，B、C、D错误．
故选：A．
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| Wings of Angel I used to hate myself because I wasn't "normal". Everyone else could play on the monkey bars and ride on a bicycle, but not 1 . I had a severe spinal cord disorder (脊髓病) and I knew I would always be much 2 than others. I hated going to school and I hated people 3 at me. I hated seeing others smiling broadly and standing straight and tall. And most of all, I hated looking in the 4 and seeing an ugly hunchback (驼背). My friends found me 5 because I didn't let others get close to me. I thought I was going to go on like this for the rest of my life 6 Angela appeared. That afternoon, I was sitting by myself in a corner of the school - a spot where no one would 7 me. That's when I first heard her voice. "Hi. Can I sit down?" I raised my head and there she was, with an irresistible smile on her round face. "What are you looking at?" she asked. "Ants." "What are they doing?" "No idea." "I bet they're playing games and making friends. Don't you think so?" That was how our 8 started and it didn't stop. We talked about everything under the sun-the ants, the clouds, my little niche (处境) - until it was sunset. Then suddenly, she saw my 9 . She just stared. My heart sank. What I feared most had happened and I knew for sure she would 10 me now. She stood up, pointed at my back and said, "I know 11 your back is hunched." I closed my eyes like a criminal waiting to be sentenced. I begged in my heart for her to 12 , but she just kept on going. "I know what you've got in there. Do you?" "No," I answered 13 . She bent and whispered in my ears. "Your back is hunched because you've got a pair of wings from the angels." I was 14 . I looked into her eyes and her kindness touched my heart. From that day on, I started to learn to 15 myself because I have the wings of an angel and a kind-hearted friend. | ||||
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