题干

如图(甲)所示,质量不计的弹簧竖立固定在水平面上,t=0时刻,将一金属小球从弹簧正上方某一高度处由静止释放,小球落到弹簧上压缩弹簧到最低点,然后又被弹起离开弹簧,上升到一定高度后再下落,如此反复.通过安装在弹簧下端的压力传感器,测出这一过程弹簧弹力F随时间t变化的图象如图(乙)所示,则(   )

A:t1时刻小球动能最大

B:t2时刻小球动能最大

C:t2~t3这段时间内,小球的动能先增加后减少

D:t2~t3这段时间内,小球增加的动能等于弹簧减少的弹性势能

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答案(点此获取答案解析)

C

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    One October morning, I got off the all-night train in Mandalay, a city in Myanmar. A rough man came up and offered to show me around. The price he asked was less than I would pay for a bar of chocolate at home. So I climbed into his trishaw(三轮车).

    As he was showing me around, he told me how he had come to the city from his village. He'd earned a degree in mathematics. His dream was to be a teacher. But of course, life is hard here, and so for now, this was the only way he could make a living. Many nights, he told me, he actually slept in his trishaw so he could catch the first visitors off the all-night train.

    And very soon, we found that in certain ways, we had so much in common---we were both in our 20s, we were both fascinated by foreign cultures----that he invited me home.

    So we turned off the wide, crowded streets, and came to rough, wild alleyways(小巷). I really lost my sense of where I was, and realized that I could easily get cheated or something even worse.

    Finally, he stopped and led me into a hut. And then he reached under his bed. Something in me froze. I waited to see what he would pull out. And finally he took out a box. Inside it was every single letter he had ever received from visitors from abroad.

    So when we said goodbye that night, I realized he had also shown me the secret point of travel, which is to go inwardly(向内心)as well as outwardly to places you would never go otherwise, to go into uncertainty, even fear.

    At home, its dangerously easy to think we're on top of things. Out in the world, you are reminded every moment that you're not, and you can't get to the bottom of things, either.