题干

将甲、乙两砝码以细线相连并跨过定滑轮,使两砝码距离底面相同高度,如图所示。当由静止自由释放后,甲砝码下降,乙砝码上升。假设细线及定滑轮的重量不计,且细线与定滑轮间无摩擦力,在砝码运动的过程中甲的动能____(填“增加”“减小”“不变”)。甲的机械能____(填“增加”“减小”“不变”或“无法判断”)

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增加,减小

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    I returned home the other night, tired. My husband asked me how my evening was, “Great.” I told him. I had spent 90 minutes in a gym with 10 Ping-Pong tables and all kinds of players, all playing a little ball over the net. By 9 pm, I was excited, tired, satisfied. I had beaten two young men half my age and lost battles against other competitors. To an observer, the night was common. To me, it was a lucky thing that I hadn't expected.

    I had taken up Ping-Pong during college, and in my 30s took more advanced lessons. However, a serious accident hurt my leg, which made me unable to take exercise. Months later, I tried to play Ping-Pong but my leg pained for a week. I put the game out of my mind.

    When I was 53, one day my bad leg was working a little bit better. Could Ping-Pong be possible for me, now---in my condition, at my age? I tried to play Ping-Pong again

    Ping-Pong is a sport which requires endurance(耐力). Players need quick foot work and upper body movements to return balls, requiring faster response time than tennis.

    Playing Ping-Pong offers benefits for the brain. A study of 164 women aged 60 and older showed that Ping-Pong improved cognitive(认知)function more than dancing, walking or gymnastics. “The great thing about our sport is that it can be played by anyone,” said Jimmy Butler, a four-time national USA Table Tennis Association winner. “I see 90-year-olds and 10-year-olds.”

    Years passed and my endurance improved. People started to praise my shots. I won a game. Then I won agin. These days, I feel wonderful, I believe this sport is the fountain (源泉)of youth.