题干

用如图4-2-9示装置可以研究动能和重力势能转化中所遵循的规律.在摆锤从A位置由静止开始向下摆动到D位置的过程中(  )

①重力做正功,重力势能增加
②重力的瞬时功率一直增大
③动能转化为重力势能
④摆线对摆锤的拉力不做功
⑤若忽略阻力,系统的总机械能为一恒量

A:①③

B:②④

C:②⑤

D:④⑤

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答案(点此获取答案解析)

D

同类题3

阅读短文,根据短文内容及首字母提示,在空白处填入一个适当的单词,必要时进行词形变化.

    One afternoon,I went into an art museum while waiting for my husband.I hoped to enjoy the works of art quietly.

    A young couple walked in front of me and talked ____the paintings between themselves all the time.The lady did almost all the talking,but the man kept listening to her.I thought the man was very patient ____nobody would like to be bothered (打扰) while enjoying the paintings.

    I met them several ____as I moved through the different rooms of the art museum.Each time I heard her talking,I moved ____quickly.

    I was ____for some gifts at the museum shop when the couple walked slowly to the exit (出口).Before they left,the man ____out a cane (拐杖).Then he tapped (轻敲) his way to the coatroom to ____the jacket for his wife.

    "He is a brave man,"the worker of the shop said."He decided not to change his life after his eyes got hurt.Though he is ____now,he never gives up.He and his wife come here whenever there is a new art show."

    "But what can he get out of the art?He can't see!"I asked.

    "You are wrong.He ____a lot,more than you and I do."The worker said,"His wife describes ____painting so he can see it in his mind."

    I fell silent.Then I realized that I learnt something about patience,courage and love that day.