题干

如图甲所示,一个m=3kg的物体放在粗糙水平地面上,从t=0时刻起,物体在水平力F 作用下由静止开始做直线运动.在0~3s时间内物体的加速度a随时间t的变化规律如图乙所示,已知物体与地面间的动摩擦因数处处相等.则(  )

 

A:在0~3s时间内,物体的速度先增大后减小

B:3s 末物体的速度最大,最大速度为10m/s

C:前2s 内物体做匀变速直线运动,力F 大小保持不变

D:2s 末F 最大,F 的最大值为12N

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答案(点此获取答案解析)

B,C

同类题2

阅读下面短文, 然后根据括号内所给汉语意思写出单词的正确形式(每空一次)。

    When I was in the ____ (九) grade, luckily, I was chosen to be the princess in the school play. For weeks my mother had helped me practice my lines. But ____ (一旦) on the stage, every word disappeared from my head. Then my teacher asked me to change my role to be a narrator (解说者) for the play.

    ____ (尽管) I didn't tell my mother what had happened that day, she noticed my unhappiness. She asked if I wanted to take a walk in the garden.

    It was a lovely spring day. We could see several dandelions (蒲公英) ____ (在…中间) the grass. I watched my mother bend down by one of the dandelions. "I think I'll dig up all these weeds (野草)," she said. "From now on, we'll only have roses in this garden."

    "But I like dandelions," I disagreed. "All flowers are beautiful-even dandelions!"

    My mother looked ____ (严肃地) at me. "Yes, every flower gives pleasure in its ____ (自己的) way, doesn't it?" she asked thoughtfully. I nodded. "And that is true of people, too," she added.

    When I ____(意识到) that she had guessed my pain, I started to cry and told her the ____ (真相). "But you will be a wonderful narrator," she said, reminding me of how much I loved to read stories aloud.

    Thanks to my mother's encouragement, I got to know everybody had his or her ____ (价值) in the world. I felt ____ (骄傲的) of the role in the play.