My name is Sam. I am a junior high school student. My school is close to my home, so I always go to school on foot. Classes start at 8 a.m., and I am seldom late. I like Geography best. I enjoy learning about different places in the world.

    In the morning, we usually study Chinese, Maths and English. We have our morning break at 9:50 a.m..  When the bell rings, I run to the playground with my best friends Tom and Jack. We often play games. Break ends at 10:10 a.m..

Lunch is from 11:50 a.m. to 12:30 p.m.. Afternoon classes end at 3:30 p.m. Then Tom, Jack and I take part in the school bank practice. We make great music together.

    I always have a good time at school.

方程 x² = 3x的根是 （    ）

著名的历史学者李慎之在《“封建”一词不可滥用》一文中说道：“历览前史，中国的封建时代恰恰是人性之花开得最盛最美的时代，是中国人的个性最为高扬的时代”，文中的“封建时代”应该是指

三峡升船机于2015年12月21日进行实船测试．如图所示，旅游船“长江电力”号先进入上游航道，开启活动桥、卧倒门、防撞装置、工作门后，旅游船缓慢驶入承船厢，随后承船厢内相关设施依次关闭．客轮在承船厢中，仿佛进入了一个装水的“大脚盆”，像坐电梯一样，由机械驱动承船厢下降，承船厢以0.2m/s的速度匀速下降，从上游175m水位下降至下游62m水位处，又依次开启活动桥、卧倒门、防撞装置、工作门后，旅游船缓缓驶出承船厢．

①升船机承船厢在下降过程中动能、重力势能和机械能怎样变化？

②升船机承船厢下降过程所用的时间是多少？