如图甲所示,完全相同的木块 A 和 B 叠放在水平桌面上,在 12N 的水平拉力 F 1 作用下, A 、 B 一起作匀速直线运动,此时木块 B 所受的摩擦力为 ________ N ;若将 A 、 B 紧靠着放在水平桌面上,用水平力 F 2 推 A 使它们一起匀速运动(如图乙所示),则推力 F 2 =________N .(不写单位)
【简答题】用attend,join,take part in填空 1. The students are busy preparations for the lecture because they will the competition. 2. The old lady had a good doctor to her. 3. He is proud of his the Party. 4...