Final answer:
An action/reaction pair of forces are equal and opposite, as per Newton's third law, and do not cancel as they act on different systems. For instance, a swimmer pushing off a pool wall feels only the reaction force, propelling them forward.
Step-by-step explanation:
An action/reaction pair of forces are always equal in magnitude and opposite in direction, as described by Newton's third law of motion. This law illustrates a symmetry in nature, indicating that any action in one direction will have a corresponding reaction in the opposite direction. However, these forces do not cancel out as they act on different systems. For example, a swimmer pushing off from the pool wall produces a force on the wall (action), while the wall exerts an equal and opposite force on the swimmer (reaction), propelling the swimmer forward. The swimmer feels only the reaction force, which is the force external to their system, thus affecting their movement. In this case, choosing the swimmer as the 'system of interest' helps us understand why the reaction force does not cancel but instead results in the swimmer's propulsion.