问题
单项选择题
图示两物体重力的大小分别为P和Q，用绳子连接，绳子跨过一滑轮，滑轮半径为r，其重力不计。开始时两物体的高度差为c，且Q＞P，则由静止释放后，两物体达到相同高度时所需的时间为（）。
A.A
B.B
C.C
D.D
答案
参考答案：B
解析：
根据动量矩定理求加速度，a=ra
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图示两物体重力的大小分别为P和Q，用绳子连接，绳子跨过一滑轮，滑轮半径为r，其重力不计。开始时两物体的高度差为c，且Q＞P，则由静止释放后，两物体达到相同高度时所需的时间为（）。
A.A
B.B
C.C
D.D
参考答案：B
解析：
根据动量矩定理求加速度，a=ra
My father brought home a sailboat when I was ten, and almost each Sunday in summers we would go sailing. Dad was quite skilled in sailing, but not good at ___36___. As for me, I ___37___ both before twelve because of living close to Lake Ontario. The last time Dad and I set sail together is really ___ 38___. It was a perfect weekend after I graduated from university. I came home and ___39___Dad to go sailing. Out we set soon on the ___40___lake. Dad hadn’t sailed for years, but everything ___41___well with the tiller（舵柄）in his hands. When we were in the middle of the lake, a ___ 42___ wind came all of a sudden. The boat was hit ___43___. Dad was always at his best in any___44___, but at this moment he___45___. "John!___46___!" he shouted in a trembling voice, with the tiller still in his hands. In my memory he could fix any___ 47___. He was the one I always___48___for strength and security. Before I could respond, a___ 49___ of water got into the boat. I rushed to the tiller ___50___it was too late. Another huge wall of water ___ 51___the boat in a minute. We were thrown into the water, and Dad was struggling aimlessly. At that moment, I felt fiercely ___52___of him. I swam to Dad ___ 53___and assisted him in climbing onto the hull（船壳）of the boat. Upon sitting on the hull, Dad was a little awkward about his flash of ___ 54___. "It's all right, Dad. We are safe now," I comforted him. That was the first time Dad had counted on me in a moment of emergency. More importantly, I found it was my turn to start___55___for my father.
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