The force acting in the opposite direction of an action force is called reaction force. This is a fundamental concept in physics, described by Newton's Third Law of Motion.

Newton's Third Law of Motion states:

* For every action, there is an equal and opposite reaction.

This means that whenever one object exerts a force on another object (the action force), the second object exerts an equal and opposite force back on the first object (the reaction force).

Here are some examples:

* Walking: When you push your foot against the ground (action force), the ground pushes back on your foot with an equal and opposite force, propelling you forward.

* Swimming: When you push water backward with your arms and legs (action force), the water pushes you forward with an equal and opposite force.

* Jumping: When you jump up, you push down on the ground (action force), and the ground pushes back up on you (reaction force), launching you into the air.

Important points to remember:

* Action and reaction forces always act on different objects.

* The forces are equal in magnitude but opposite in direction.

* Action and reaction forces occur simultaneously.

Understanding action-reaction forces is crucial for understanding how forces interact in the world around us.