Final answer:
The converging airplane not showing any relative motion but getting larger implies it is moving directly toward the observer. This is an application of velocity and relative motion in physics, where the total velocity of an object determines its actual path of movement.
Step-by-step explanation:
If a converging airplane appears to not show any relative motion and is becoming larger, it likely indicates that the plane is moving directly toward the observer. In this case, the observer may observe that the plane is enlarging in size because of the decreasing distance between them, but without any lateral or vertical movement in the line of sight. This situation is related to the concepts of velocity and relative motion.
As the airplane approaches, there is no relative motion horizontally or vertically; rather it's the motion directly along the observer's line of sight that plays the key role. The observer's brain interprets the lack of side-to-side or up-and-down relative motion combined with the increase in the plane's apparent size as an object coming directly towards them. This is similar to the example of an airplane heading north being affected by wind where its total velocity determines its actual motion across the ground.
If we consider the motion of two objects in a system, even if one object (the airplane) is accelerating or moving along one axis (say, the horizontal direction), if the second object (the observer) is also moving along that same axis and at the same rate, then relative to each other, there will be no observed motion - although both may still be changing position with respect to the ground or other reference points.