Final answer:
In a ball/pendulum system, external nonconservative forces like air resistance and pivot point friction can do work on the system, potentially altering momentum. Momentum conservation requires no net external force acting on the system. Forces acting on different systems, like a swimmer and a pool wall, illustrate Newton's third law without canceling each other.
Step-by-step explanation:
In a ball/pendulum system, external nonconservative forces such as air resistance can do work on the system, changing its energy and potentially its momentum. Another example could be friction at the pivot point in a pendulum. These forces act externally and are not part of the internal interactions within the system. Momentum is conserved in a system when no net external force acts on it. However, if external forces are present and exert a net force on the system, then conservation of momentum can be violated. This would occur if the forces cause the center of mass of the system to accelerate.
For instance, in the case of a swimmer pushing off from the side of a pool, the force from the wall on the swimmer's feet is an external force, which changes the momentum of the swimmer, illustrating Newton's third law. Here, the forces between the swimmer and the wall do not cancel out because they are acting on different systems. In contrast, internal forces, like the swimmer pulling on their own arms, do not change the system's total momentum because they cancel each other within the system.