Final answer:
Newton's third law does not apply to external forces acting on a system; it refers to the forces exerted between two interacting objects which are equal in magnitude but opposite in direction and act on different systems. Choosing the system of interest allows us to differentiate between internal and external forces, which influences how forces cancel out in analyzing motion.
Step-by-step explanation:
The student's question pertains to whether Newton's third law applies to external forces acting on a system of interest. The answer to the question is that this is false. Newton's third law states that whenever two objects interact, they exert equal and opposite forces on each other. These forces occur in pairs, and such pairs do not cancel each other out because they act on different bodies. When setting up a physics problem, identifying external forces is important because these forces must be added together to find the net force acting on a system. However, the action-reaction pairs described by Newton's third law do not include forces that a system exerts on itself (internal forces) but rather the forces between two distinct bodies or systems.
For instance, when a person walks, they exert a force on the ground (action), and the ground exerts an equal and opposite force on the person (reaction). These two forces are equal in magnitude but act on different systems—the person and the Earth—and therefore, do not cancel each other within a single system. It's also important to note that just because the two forces are equal and opposite does not mean the net force on the person is zero; this is because the force from the ground (being external) does not cancel the person's own force (being internal to them as a system).
The choice of the system of interest affects whether a pair of forces cancels because it determines what forces are considered internal and what forces are considered external. Forces internal to the system of interest have the potential to cancel, while forces external to the system do not. Thus, when analyzing motion and applying Newton's third law, it is crucial to correctly identify the system of interest and distinguish between action-reaction forces and the net external force acting on the system.