I'm going to Beijing this summer vacation with my family. We're going there by train. We're going to stay in a hotel. We're going to see the Great Wall, the Bird's Nest. We're going to see Tianan'men square, too. I'm
观察洋葱表皮细胞和观察蜗牛爬行用到的工具分别是( )
在短周期元素中,原子最外电子层只有1个或2个电子的元素()
建筑工人要向楼上运送货物,为了省力,他在一个斜面上安装了一个滑轮组,做成如图所示的机械装置,斜面的倾角为30°且固定在水平面上.工人用400N的力拉绳子,重为500N的货物沿斜面匀速向上运动6m所用时间为20s,求此过程中