仔细观察图(甲),说明力的作用效果不仅跟力的大小、方向有关,还跟力的____ 有关;观察图(乙)发现,将滑动变为____ 的方式可以减小摩擦;装潢师傅常用重垂线来判断图画挂得是否竖直,如图(丙)所示,这是利用了____ .
Every year,because of the destruction of man,many of the earth's plants and animals are________.
-Qin Shu has just beaten Li Hui in our school ping-pong game!
-It's hard to _________ a girl with only one arm can play ping-pong so well!