Law of action and reaction
When we apply force to an object, that object also exerts the same amount of force on us. At this time, our force is ‘action,’ and the force exerted on us by the object is ‘reaction.’
Let’s say you’ve wired two boats together. When I pull the line, the two boats come close to each other. As much as I act on other boats, other vessels apply the same amount of force to my boat through a string. The two forces are equal in magnitude and opposite in direction.
The law of action-reaction applies not only when an object is stationary but also when it is moving. And not only when they are attached, but also on forces that work apart from each other (electric, magnetic, gravitational force).