Final answer:
Newton's third law of motion concerns action-reaction force pairs, where each force in the pair acts on different objects. The choice of system of interest changes whether these forces cancel out, as forces internal to the system cancel, while forces external to the system do not.
Step-by-step explanation:
Newton's third law of motion indicates a fundamental symmetry in nature, stating that forces always come in pairs. Whenever an object exerts a force on a second object, it simultaneously experiences a force that is equal in strength but opposite in direction. This principle leads to the action-reaction pair concept, where each force is part of a mutual interaction between two different objects.
When determining the effects of these forces, the system of interest is crucial. The system of interest refers to the object or group of objects that is being considered for analysis. If the system of interest includes both objects involved in the interaction, the forces exerted by the objects on each other will cancel out because they are internal to the system. However, if only one object is considered the system of interest, these forces do not cancel out since the reaction force is external to the system and thus must be considered separately.
In summary, the choice of the system of interest determines whether the action and reaction forces cancel each other out within that system. Disregarding this can lead to incorrect conclusions about the motion and dynamics of the objects in question.