题目

 “Tell me what you were like when you were my age,” I asked my mother one afternoon. She looked up, 16 at my question. After a long time she answered, “I was 17 like you. Then, there was only one 18 for girls — being a hard-working 19. To tell the truth, it is 20 for me to tell you that I 21 my mother when I was growing up. I did so much for her. Besides doing lots of housework, every night I 22 go into my parents’ bedroom, and wash their feet with warm water. Not once did my mother ever said, ‘thank you’. I stared out of the window, remembering 23 I grew up, always wan7．某次“验证机械能守恒定律”的实验中，用6V、50Hz的打点计时器打出的一条无漏点的纸带，如图所示，O点为重锤下落的起点，选取的计数点为A、B、C、D，各计数点到O点的长度已在图上标出，单位为毫米，重力加速度取9.8m/s2，若重锤质量为1kg，计算结果保留3位有效数字．（1）下列器材中选出实验所必须的，其编号为ABEA．打点计时器（包括纸带）   B．重锤    C．天平    D．秒表   E．毫米刻度尺（2）从开始下落算起，打点计时器打B点时，重锤的动能EkB=0.690J；重锤的重力势能减小量为0.691J．（3）根据纸带提供的数据分析，重锤从静止开始到打出B点的过程中，可得出的结论是在误差允许的范围内，垂锤的机械能守恒．