Final answer:
The second force in a pair according to Newton's third law of motion is known as the reaction force. It describes an equal and opposite force exerted in response to an action force, and together, they demonstrate the action-reaction principle without cancelling each other.
Step-by-step explanation:
In accordance with Newton's third law of motion, forces indeed occur in pairs. The first force in the pair is identified as the action force, and the second is known as the reaction force. This principle asserts that when one object exerts a force on a second object, the second object exerts an equal and opposite force back on the first object. This interaction exemplifies the concept of action-reaction pairs. For example, when a person sits on a chair, there is an action force exerted by the person onto the chair, and simultaneously, there is a reaction force exerted by the chair onto the person. These forces are equal in magnitude and opposite in direction, but they act on different bodies and, therefore, do not cancel each other out.