题干

游乐场中的一种滑梯如图所示.小朋友从轨道顶端由静止开始下滑,沿水平轨道滑动了一段距离后停下来,则(    )

A:下滑过程中支持力对小朋友做功

B:下滑过程中小朋友的重力势能增加

C:整个运动过程中小朋友的机械能守恒

D:在水平面滑动过程中摩擦力对小朋友做负功

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D

同类题4

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    I have never forgotten the firm belief I had in the arrival through the chimney (烟囱) of the little Santa Claus (圣诞老人), the kind, white-bearded old man who at midnight left in my tiny shoe a present I found the next morning.

    Midnight! It's the perfect hour children don't know. What great efforts I made not to go to sleep before the little old man appeared!

    I had both a great desire and great fear of seeing him, but could never stay awake until then, and the next day my first look was for my shoe. He never failed to carefully wrap (包装) every present for me. I would run barefoot to get my treasure. It never was a very expensive present, for we were not rich. It was a cookie, an orange, or very simply a fine red apple. But it felt so precious that I barely dared to eat it. What a great role imagination plays in a child's life!

    I do not at all agree with the idea that the lies about Santa Claus should be stopped. Doubts and truth come all too soon on their own. I very well remember the first year, when I was five or six, I doubted that it was not Santa Claus who put the cookie in my shoe. Since then, it has never tasted as good as the first few years of my life.

    I noted that my son believed in him longer; boys are more naïve (天真的) than little girls. Like me, he made great efforts to stay awake until midnight. Like me, he loved the cookie baked in Heaven's kitchens. And like me, the first year of his doubts was the last year of the visit of the good old man.