Motion Referred to a Moving Reference Frame and Relative Motion

Thus far in our studies, the motion of a material point has been referred mainly to an assigned reference frame that often we thought of as being “fixed in space.” We have seen, however, that it is sometimes more convenient to refer the motion to a reference frame which itself is in arbitrary motion relative to some (possibly moving) assigned frame. To trace the motion of a long-range ballistic missile or space rocket, for example, it is essential that the motion of the earth be taken into account. In such cases it becomes necessary to refer the body’s motion to a moving reference system imbedded in the earth; and, in this instance, a reference frame fixed in the distant “fixed stars” may be chosen as a suitable assigned reference frame with respect to which the earth’s motion is known.