两个物体的相互作用力总是大小相等，方向相反，同时出现或消失且作用于同一直线上。
To every action there is always opposed an equal reaction； or， the mutual actions of two bodies upon each other are always equal， and directed to contrary parts.
Man is born free， and everywhere he is in chains.
书籍是连接我们彼此的桥梁。