Final answer:
Action and reaction force pairs do not cancel each other out because they act on different systems, which is explained by Newton's third law of motion. Balanced forces, however, act on the same object and can cancel out, leading to no change in motion. Distinguishing between systems is crucial for correctly applying Newton's third law and understanding the movement.
Step-by-step explanation:
Your friend is incorrect because action and reaction force pairs do not cancel each other since they act on different bodies. An example of this is when you sit on a chair; your body exerts a downward force on the chair (action), and the chair exerts an upward force on your body (reaction). These forces are equal and opposite, but they do not cancel out each other's effects because they are acting on different objects. The force you exert on the chair affects the chair, and the force the chair exerts on you affects you; they don't directly interact with each other on the same body.
According to Newton's third law of motion, every action has an equal and opposite reaction, but the law also implies that these forces are acting on different bodies. This contrasts with balanced forces, which act on the same object and do cancel each other out, resulting in no net force and, thus, no change in motion.
When considering different systems, such as a swimmer pushing against a wall, the wall exerts an equal and opposite force on the swimmer. The swimmer moves through the water because the forces are not acting on the same system: the swimmer and the wall are two distinct systems.
Choosing a system helps us analyze problems and understand the forces at play. For example, when a rocket launches, the gases expelled downwards (action) exert an upward force on the rocket (reaction), and the rocket accelerates upwards. The action and reaction occur on different systems, the gases and the rocket respectively, and so they don't cancel out.