action line的用法和樣例：
例句
- Click on the center dot to rotate the hand around the action line.
點(diǎn)擊中心圓點(diǎn)，沿著(zhù)作用線(xiàn)旋轉手臂。 - There is an exceptional case, three forces with paralleled lines of action, which will be discussed in Chapter 3.
三個(gè)力的作用線(xiàn)相互平行的例外情況，我們將在第三章討論。
