题干

小明同学在“探究滑动摩擦力与哪些因素有关”的实验时,用弹簧测力计匀速拉动木块,如图甲所示,图乙是他两次拉动同一木块得到的速度随时间变化的图象.下列说法正确的是(   )

A:木块第一次受到的摩擦力较大

B:两次木块具有的动能一样多

C:前后两次,木块受到的拉力相等

D:木块对木板的压力和木板对木块的支持力是一对平衡力

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C

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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.