题干

如图所示,小明用一动滑轮将一袋重物匀速提升2m,分析正确的是(  )

A:手拉绳子的力和绳子拉手的力是一对平衡力

B:手拉绳子的力和重物受到的重力相等

C:重物受到的重力和动滑轮对重物的拉力是一对平衡力

D:重物对动滑轮的拉力小于动滑轮对重物的拉力

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答案(点此获取答案解析)

C

同类题2

完形填空

    Alice was very worried about her dad. For some time now she had noticed he was 1 his hair. One day, she asked him, “Daddy, every day you have less hair.2 is that?” Her father smiled and said, “It's the hair thief. He visits my head during the night when I'm 3. One by one he 4 out my hair, as much as he likes.”

    This worried Alice but she was 5 to help her father. That very night she stayed 6 as long as she could. When she was 7 her dad and mom had fallen asleep, she took a hammer and went straight to their bedroom. She walked carefully in order not to make any 8. She didn't want the hair thief to hear her. Then she began carefully 9 her father's head. When she arrived at her father's side, she began looking at his head,10to catch the hair thief as soon as he appeared. Before long, she saw a shadow on it and 11 all her strength, she swung the hammer down.

    Thwack! Her father 12a loud cry. The lump(肿块) on his head was big and growing.13 with shock, he turned the light on and saw Alice waving her hammer in the 14 with her hands. “Daddy, I 15 got him. I thought I 16 him, but it looks like he escaped!” said Alice. At the same time, the mother woke up. She 17laughing when she knew what had happened.

    So Alice's father had to 18to her that the hair thief didn't exist, and that losing hair is something that just happens 19 to most daddies. And Alice still worried about her daddy, but she no longer waited for the hair thief.20 , she bought her daddy a very nice sleeping hat.