题干

如图所示,福娃妮妮正在通过定滑轮将一重物拉上去,她所用的拉力为25N,所拉物体的质量为2kg,物体向上升高了8m,(g取10N/kg),则下列说法中错误的是(   )

A:妮妮作用在绳子上的拉力做功为400J

B:绳与滑轮间的摩擦没有被忽略

C:克服被拉物体的重力做的功为有用功,其大小为160J

D:此次使用定滑轮的过程中,其机械效率为80%

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答案(点此获取答案解析)

A

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    She sat at the picnic table alone. Recess(休息) was in full swing. She remained awkward around her classmates. She seemed unsure of what to do or say, yet I could see her eyes longing for acceptance. Many students had already decided that her friendship would not be worth the energy required to overcome the awkwardness. Others teased her. Most ignored her — except for one.

    Brianna, the class clown, was making the other students laugh, as usual. “Brianna, do you see Molly down there? Would you mind walking down there and inviting her to come up here with the rest of us?”

    Brianna sighed. I could tell she didn't want to sacrifice precious minutes of her own recess to do what I was asking of her, but I also knew her heart. She often thought of others before herself — a rare character for anyone, much less a kid.

    Knowing this choice was hard for her, I pulled out a D-buck, our class currency. Though bribery(行贿) was not the ideal way to handle this situation, I needed her cooperation.

    “Here, I'll pay you for your time.”

    She offered an insincere smile, grasped the green paper, and headed down the hill.

    As the rest of the children screamed and laughed, my eyes locked on Brianna as she neared the picnic table. Molly could be difficult, and I wouldn't have been surprised if she sent Brianna back alone, refusing her invitation. When she encouraged herself to a standing position, I sighed with relief.

    A minute later, 1 felt tap on my shoulder. “Here, Mrs. D.” She handed me the D-buck.

    “ Why?” I asked.

    “I shouldn't keep this.” Her eyes fell to her feet, guilt radiating from her quiet voice. “I don't want Molly to think 1 only went to get her so I could earn a muck. She's my friend.”

    A moment later, they were all laughing again, and who should I see among them, laughing for the first time that week? Molly.