照样子，把句子写具体。

例：小溪流向远方。  清澈的小溪缓缓地流向远方。  

1．同学们听老师讲课。                                                                                                             

2．小鸟站在大树上唱着歌。                                                                                                      

3．天上飘着白云。                                                                                                                     

如图，用100牛的力把一个重为10牛的物体压在竖直墙壁上，物体处于静止状态，这时物体受到的摩擦力大小为______N；当压力减为50牛时，物体沿竖直墙壁匀速下滑，则物体下滑时受到的摩擦力大小是______N．