what is action and reaction

牛頓第三定律 (Newton's Third Law Of Motions)：又稱作用與反作用定律 (law of action and reaction)。這定律說明對於每一個作用力，必會產生一方向相反、大小相等的反作用力。例如：當你推一塊木頭時，你把一作用力加於木頭，與此同時，木頭必然以一大小相等、方向相反的力加於你的手，手所承受的力就是反作用力。

Newton's third law of motion (Law of action and reaction):
To every action, there is an equal but opposite reaction


Action-reaction pair should:
- have the same magnitude
- act in opposite directions
- act on two different objects, exerted by one object on the other


Eg.1. Jumping
[Action] Our legs push against the ground when we jump
[Reaction] The ground exerts an upward reaction on our body


Eg.2. Cat lying on a stool
[Action] Normal reaction force on the cat by the stool
[Reaction] Force on the stool by the cat



PS. action and reaction是屬於Physics物理, 而非化學