Final answer:
Internal forces within a body are neglected when applying Newton's second law of motion because they cancel each other out, having no net effect on the motion, thus only external forces are considered.
Step-by-step explanation:
We can neglect forces such as those holding a body together when we apply Newton's second law of motion because internal forces do not change the motion of the body. Newton's second law pertains to external forces, as internal forces cancel each other out.
This means that the internal forces within a system have no net effect on the system's motion, allowing us to focus solely on the external forces when using this law to predict motion.
When a body is analyzed in mechanics, the system of interest is defined to isolate the external forces acting on it from the internal forces. This helps to simplify equations and solvable models. In the case of internal forces, by Newton's third law, for every action, there is an equal and opposite reaction, meaning these forces come in pairs that exert equal and opposite effects, thereby canceling each other out within the system.