问题 问答题

物理世界

        荡秋千的技巧与能量转化首先让我们回顾一下荡秋千的情景,秋千荡来荡去,幅度越来越小,最终停了下朱要想秋千不停下来,就要有人不断地推它. 其实,荡秋千时完全可以不用别人来推,荡秋千的人自己就可以不停地荡下去,并且可以使秋千越荡越高.当然这要掌握一定的技巧,那么应该如何做呢?说起来也很简单,荡秋千的人,在秋千往高处荡的时候,要站起来,往下荡的时候,要蹲下去,这其中的道理,从能量转化的角度,不难理解, 在上面的问题中,机械能的转化情况是:在向上荡的过程中,动能转化为重力势能,在向下荡的过程中,重力势能转化为动能,但机械能的总量却在不断地减小,原因是在上述过程中,秋千要克服摩擦阻力做功,消耗掉一部分机械能,使之转化为其他形式的能量(内能).因此,随着机械能的减小,秋千摆动的幅度越来越小,最终停了下来,而要使秋千继续荡下去,就要不断地补充能量.

问题:

(1).我们在做滚摆实验时,发现滚摆每一次上升和下降的幅度都比前一次小,最终会停止下来.滚摆在动能和势能相互转化的过程中,它的机械能总量为什么不守恒呢?

(2).向地板上抛乒乓球,要使乒乓球弹跳起来的高度比原来抛球的位置高,应该    [ ]

A.让乒乓球自由落下    

B.用力向上抛    

C.用力向下抛          

D.无法确定

答案

(1)摩擦消耗能量; 

(2)C

阅读理解

阅读理解

       One day I went out and left my key at home. When I came back, I couldn't get in because my

parents were away. I had to break a window, so I found a big stone(石头). There was a great noise

when I broke the glass(玻璃), and I had to be careful when I reached through to turn the handle (把手).

I was just climbing in when a deep voice said, "What are you doing?"

       It was a policeman. My neighbor heard the breaking and telephoned the police.

       It took me quite a long time to explain(解释) that I wasn't a thief (贼).

1. One day the writer couldn't get into his house because       .

A. he forgot to take his key with him and his parents weren't at home

B. his parents were staying with his uncle

C. he left his key in his office

D. his parents were angry with him and didn't let him in

2. The writer             with a big stone.

A. broke the door      

B. knocked at the door

C. broke the window  

D. went away

3. The writer heard a policeman's voice when he was             .

A. turning the handle            

B. breaking the door

C. climbing (爬) in through the window  

D. knocking at the door

4. Why did the policeman come?

A. Because the writer wanted his help.

B. Because the writer's neighbor telephoned the police.

C. Because the writer phoned the police.

D. Because the policeman heard the noise.

5. At last the policeman knew that              .

A. the writer was a thief

B. the writer's neighbor played a joke (玩笑) on him

C. the writer's window is broken

D. the writer was the master (主人) of the house

单项选择题 B型题