题目

My heart sank when the man at the immigration counter gestured to the back room. I was born and raised in America, and this was Miami, where I live, but they weren’t quite ready to let me in yet.“Please wait in here, Ms Abujaber,” the immigration officer said. My husband, with his very American last name, accompanied me. He was getting used to this. The same thing had happened recently in Canada when I’d flown to Montreal to speak at a book event. That time they held me for 45 minutes. Today we were returning from a literary festival in Jamaica, and I was startled that I was be用如图所示的装置做“验证机械能守恒定律”实验．已知打点计时器所用电源的频率为50Hz．查得当地的重力加速度g=9.80m/s2，某同学选择了一条理想的纸带，用刻度尺测量各计数点到O点的距离如图所示．图中O点是打点计时器打出的第一个点，A，B，C，D分别是每隔一个点取的计数点，则重物由O点运动到B点时，重力势能的减小量等于    mJ，动能的增加量为    mJ（保留三位有效数字，其中重物质量m已填好），产生误差的主要原因是    ．（写出一条即可）