题目

  Another person’s enthusiasm was what set me moving toward the success I have achieved. That person was my stepmother.   I was nine years old when she entered our home in rural Virginia. My father__16__me to her with these words: “I would like you to meet the fellow who is___17__for being the worst boy in this county and will probably start throwing rocks at you no ___18__than tomorrow morning.”   My stepmother walked over to me, ___19__my head slightly upward, and looked me right in the eye. Then she looked at my father and replied, “You are ___20__. This is not the worst 为了测量木块与木板间的动摩擦因数，某实验小组设计了如图甲所示的实验装置：一个木块放在水平长木板上，通过细线跨过定滑轮与重物相连，木块与纸带相连，在重物牵引下，木块先向左加速运动，重物落地后，木块做减速运动，打出的纸带撕下其中一段如图乙所示，已知交流电频率为50 Hz，重力加速度g取10 m/s2。（1）木块减速阶段的加速度大小为 m/s2（结果保留两位有效数字）。（2）木块与木板间的动摩擦因数μ= （纸带与打点计时器间的摩擦可忽略不计）。