Final answer:
Newton's third law force pairs act on different systems, point in opposite directions, and have the same magnitudes. They only cancel out when both forces act within the same system of interest, and their action-reaction nature is fundamental to understanding the interactions between objects.
Step-by-step explanation:
The true statements about Newton's third law force pairs are: if there is a force acting on a system, there must be a paired force acting on a different system; force pairs point in opposite directions; and force pairs have the same magnitudes. Newton's third law represents a fundamental symmetry in nature, where forces always occur in pairs with equal magnitude but in opposite directions. This principle is sometimes referred to as "action-reaction." The forces in a Newton's third law pair do not cancel each other out because they are exerted on different objects or systems. The choice of the system of interest is crucial; if both forces in the pair act within the system chosen for analysis, they cancel each other and do not affect the net force on that system. However, if one force in the pair acts on an object outside the system, it is considered an external force and does not cancel.
An example of Newton's third law in action is when a person pushes against a wall. The wall exerts an equal and opposite force back on the person, but there is no movement in the wall or the person, assuming they push at a right angle and the wall is stable. The paired forces are interacting with different systems (person and wall), and therefore, they do not result in any acceleration of either system. This showcases how the action-reaction forces do not cancel out when considering the individual systems involved.