What Is The Law of Action & How Does It Work?
A force in its simplest sense is the action of pushing or pulling; but, looking more closely, Newton realized that a force is not an isolated thing: it is part of a mutual action-of an interaction between two things.
When I lean against a wall, I exert a force on it. But this force is only half of the issue because there must also be a force that pushes me to avoid my fall: this force is what the wall exerts on me. So, in the interaction between the wall and me, there are a couple of forces: a force that I exert on the wall and another that the wall exerts on me.
Observations like these led Newton to formulate what we know today as "law of action and reaction" or "Newton's third law."
The law states as follows:
"When an object exerts a force on another object, the second object exerts on the first an equal force and in the opposite direction."
One force is called the force of action, and the other is reaction force. It does not matter which one of them we call to action and which of reaction, the important thing is that both are part of a single interaction and neither can exist without the other. The forces have the same magnitude, the same line of action but opposite senses and occur at the same time,
It is important to note that the force that I exert on the wall, and the force that the wall exerts on me, constitutes a pair action_reaction. But it is not the forces that act on me in vertical direction: the forces that act on me in this direction are the weight (exerted by planet earth) and the normal (used by the terrace floor) and are equal and placed, but they do not constitute a pair action_reaction because they act on it
When we swim, we interact with the water: we push the water backward and in turn, the water pushes us forward.
In short, the forces exist in pairs. The forces are a way to represent the INTERACTION between bodies. It does not make sense to say that someone has a FORCE: this only exists while there is an interaction (an action, to which a reaction will correspond).