In our example of Figure 4-2, we have tacitly assumed so far that this point, which is first located at A and later at B, is, in fact, the same particle, moving within the refer-ence frame. It could be, for example, one automobile moving along the road from A to B. With that assumption, it is conventional to refer to the vector RBA as a position differ-ence. There is, however, another situation which leads to the same diagram and equa-tion but needs a different name. Assume now that points A and B in Figure 4-2b repre-sent not the same particle but two independent particles moving in the same reference frame, as perhaps two automobiles traveling on the same road. The vector equations 4.1 and the diagram in Figure 4-2b still are valid, but we now refer to RBA as a relative po-sition, or apparent position. We will use the relative position term here. A more for-mal way to distinguish between these two cases is as follows: