题干

如图所示,当水平拉力F=50N时,恰好可以使物体A沿水平地面向右做匀速直线运动.已知物体重为200N,所受地面的摩擦力约为80N,假如在5s时间内,物体水平移动了0.6m,不计绳和滑轮的自重,则在此过程中(   )

A:拉力F做功为30J

B:物体重力做功为120J

C:拉力F做功的功率为12W

D:该装置的机械效率约为60%

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答案(点此获取答案解析)

C

同类题2

根据短文理解,完成下列表格。

    Jane was on her way home at around 6 o'clock. It was getting dark. As she drove into her community, she noticed a man.

    She saw the man walking along the sidewalk next to some houses. He didn't live in any of those houses. She had seen this man several times before. He looked like a criminal (罪犯).

    She drove by him. He looked at her car as she drove by. She continued driving and watched him in her rear view mirror (后视镜). She got to the corner and stopped. She continued to watch him walking in her direction. Suddenly, he turned left up a driveway (车道). A red SUV was parked in the driveway right next to the sidewalk. She saw him walk up to the driver's door and put his hand on the handle (把手). Then he returned to the sidewalk and went on to walk in her direction.

    "What is that all about?" Jane wondered. Then she realized that he had tested the door to see if it was locked. "He is a criminal," she thought. "That's what he does. He just walks in our neighborhood looking for cars to break into."

    She called the police and described the man. The police officer said he knew who the man was. The people in the community called two or three times this week to report him walking by. But the police couldn't arrest him for walking around unless they caught him with stolen things. "But he was testing that SUV door to see if it was unlocked," she said.

    "I'm sorry," said the officer. "That's not against the law. If you see him actually steal something, please give us a call."

Is He a Thief

Background Information

It was getting ____. Jane was ____ home when she noticed a man who she had seen several times before.

Details of the story

The man

He was walking in Jane's ____ and then suddenly turned left. He walked up to an SUV, put his hand on the handle of the driver's door and then ____ walking. 

Jane

She ____ the man all the time. She thought he was testing if the door was ____, so she called the police. She ____ the man and his strange behaviour.

The police officer

He said he knew the man because he had been____ two or three times. But the man's behaviour couldn't lead to the ____ of him. He said walking around was not ____ the law.