题干

某摄影组在一大楼边拍摄武打片,要求特技演员从地面飞到屋顶,如图所示。导演从房顶离地高H=8 m处架设了轮轴,轮和轴的直径之比为2∶1。若轨道车从图中A点前进s=6 m到B点时速度vB=5 m/s。已知特技演员质量m=50 kg,人和车均视为质点,取地面为零势能面,g取10 m/s2,则由绕在轮上的细钢丝拉动特技演员

A:上升的高度为6 m

B:在最高点具有竖直向上的速度3 m/

C:在最高点具有的机械能为2900 J

D:钢丝在这一过程中对演员做功1225 J

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C

同类题4

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Only a Ribbon

    In one of the London hospitals,a short time ago,there was a young doctor. He noticed one of the patients,a ten-year-old poor girl called Katie,who had a very painful illness.

    Katie lay in her bed,day after day,with nothing to help her forget her pain and cheer up. One day the doctor noticed her trying to make a doll of her finger, and speaking to it. But she was soon tired of it,and lay back into her bed.

    That afternoon,the young doctor saw a beautiful ribbon in a shop window. He thought that it might help cheer the little girl up,so he went in and bought it for her. When he handed the ribbon to the girl,she was too surprised to speak. There were tears in her eyes. This little gift made her feel rich and happy.

    The nurse later told the doctor that the girl played with the ribbon all day long. She twisted(盘绕)it round her head. She held it in her hand while she slept.

    Some weeks later,Katie's illness became worse,and it was necessary to perform a dangerous operation. When Katie was brought into the operation mom,she cried out for the young doctor. He was brought,and he held one of the girl's hands,while the other tightly held the ribbon.

    During the operation,the ribbon fell off Katie's hand. As soon as she woke up and opened her eyes,she looked at her friend and said in a low voice,“My ribbon.”The doctor gave it to her and she smiled happily.

    It was only a little gift,but it made the girl's stay in the hospital bright with beauty and love.