Which principle explains that for every action, there is an equal and opposite reaction?

The principle that states for every action, there is an equal and opposite reaction is rooted in Newton's 3rd Law of Motion. This law indicates that forces always occur in pairs. When one object exerts a force on a second object, the second object exerts a force of equal magnitude and opposite direction back on the first object. This principle highlights the interaction between two bodies and emphasizes that forces are mutual; they cannot exist in isolation.

For example, if you push against a wall, your hand applies a force to the wall, and the wall applies an equal and opposite force back against your hand. This law is fundamental to understanding how objects interact in various physical systems, whether in mechanics or even in more complex scenarios involving fluids and gases.

In contrast, the first and second laws of motion focus on inertia and the relationship between force, mass, and acceleration, while the Law of Universal Gravitation specifically addresses the attraction between two masses. Understanding these concepts individually helps to build a comprehensive view of physical laws but does not replace the foundational principle described by Newton's 3rd Law.